AD7893 Datasheet by Analog Devices Inc.

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FUNCTIONAL BLOCK DIAGRAM
AGND
V
DD
DGND SCLK
CONVST
AD7893
V
IN
SDATA
REF IN
TRACK/
HOLD
*AD7893-5, AD7893-10, AD7893-3
SIGNAL
SCALING* 12-BIT
ADC
OUTPUT
REGISTER
REV. E
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
a
LC
2
MOS 12-Bit, Serial 6 ms
ADC in 8-Pin Package
AD7893
FEATURES
Fast 12-Bit ADC with 6 ms Conversion Time
8-Pin Mini-DlP and SOIC
Single Supply Operation
High Speed, Easy-to-Use, Serial Interface
On-Chip Track/Hold Amplifier
Selection of Input Ranges
610 V for AD7893-10
62.5 V for AD7893-3
0 V to +2.5 V for AD7893-2
0 V to +5 V for AD7893-5
Low Power: 25 mW typ
GENERAL DESCRIPTION
The AD7893 is a fast, 12-bit ADC that operates from a single
+5 V supply and is housed in a small 8-pin mini-DIP and 8-pin
SOIC. The part contains a 6 µs successive approximation A/D
converter, an on-chip track/hold amplifier, an on-chip clock and
a high speed serial interface.
Output data from the AD7893 is provided via a high speed,
serial interface port. This two-wire serial interface has a serial
clock input and a serial data output with the external serial clock
accessing the serial data from the part.
In addition to traditional dc accuracy specifications such as lin-
earity, full-scale and offset errors, the AD7893 is also specified
for dynamic performance parameters, including harmonic dis-
tortion and signal-to-noise ratio.
The part accepts an analog input range of ±10 V (AD7893-10),
±2.5 V (AD7893-3), 0 V to +5 V (AD7893-5) or 0 V to +2.5 V
(AD7893-2) and operates from a single +5 V supply, consuming
only 25 mW typical.
The AD7893 is fabricated in Analog Devices’ Linear Compat-
ible CMOS (LC
2
MOS) process, a mixed technology process
that combines precision bipolar circuits with low power CMOS
logic. The part is available in a small, 8-pin, 0.3" wide, plastic or
hermetic dual-in-line package (mini-DIP) and in an 8-pin, small
outline IC (SOIC).
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700 World Wide Web Site: http://www.analog.com
Fax: 617/326-8703 © Analog Devices, Inc., 1997
PRODUCT HIGHLIGHTS
1. Fast, 12-Bit ADC in 8-Pin Package
The AD7893 contains a 6 µs ADC, a track/hold amplifier,
control logic and a high speed serial interface, all in an 8-pin
package. This offers considerable space saving over alterna-
tive solutions.
2. Low Power, Single Supply Operation
The AD7893 operates from a single +5 V supply and con-
sumes only 25 mW. This low power, single supply operation
makes it ideal for battery powered or portable applications.
3. High Speed Serial Interface
The part provides high speed serial data and serial clock lines,
allowing for an easy, two-wire serial interface arrangement.
AD7893–SPECIFICATIONS
ABS
Parameter Versions
l
Versions Version Units Test Conditions/Comments
DYNAMIC PERFORMANCE
Signal to (Noise + Distortion) Ratio
2
@ +25°C 70 70 70 dB min f
IN
= 10 kHz Sine Wave, f
SAMPLE
= 117 kHz
Total Harmonic Distortion (THD)
2
–80 –80 –80 dB max f
IN
= 10 kHz Sine Wave, f
SAMPLE
= 117 kHz
Peak Harmonic or Spurious Noise
2
–80 –80 –80 dB max f
IN
= 10 kHz Sine Wave, f
SAMPLE
= 117 kHz
Intermodulation Distortion (IMD)
2
fa = 9 kHz, fb = 9.5 kHz, f
SAMPLE
= 117 kHz
2nd Order Terms –80 –80 –80 dB max
3rd Order Terms –80 –80 –80 dB max
DC ACCURACY
Resolution 12 12 12 Bits
Minimum Resolution for which
No Missing Codes are Guaranteed 12 12 12 Bits
Relative Accuracy
2
±1±1/2 ±1 LSB max
Differential Nonlinearity
2
±1±1±1 LSB max
Positive Full-Scale Error
2
±3±1.5 ±3 LSB max
AD7893-2, AD7893-5
Unipolar Offset Error ±4±3±4 LSB max
AD7893-10, AD7893-3
Negative Full-Scale Error
2
±3±1.5 ±3 LSB max
Bipolar Zero Error ±4±2±4 LSB max
ANALOG INPUT
AD7893-10
Input Voltage Range ±10 ±10 ±10 Volts
Input Resistance 16 16 16 k min
AD7893-3
Input Voltage Range ±2.5 ±2.5 ±2.5 Volts
Input Resistance 4 4 4 k min
AD7893-5
Input Voltage Range 0 to +5 0 to +5 0 to +5 Volts
Input Resistance 9 9 9 k min
AD7893-2
Input Voltage Range 0 to +2.5 0 to +2.5 0 to +2.5 Volts
Input Current 500 500 500 nA max
REFERENCE INPUT
REF IN Input Voltage Range 2.375/2.625 2.375/2.625 2.375/2.625 V min/V max 2.5 V ± 5%
Input Current 2 2 10 µA max
Input Capacitance
3
10 10 10 pF max
LOGIC INPUTS
Input High Voltage, V
INH
2.4 2.4 2.4 V min V
DD
= 5 V ± 5%
Input Low Voltage, V
INL
0.8 0.8 0.8 V max V
DD
= 5 V ± 5%
Input Current, I
IN
±10 ±10 ±10 µA max V
IN
= 0 V to V
DD
Input Capacitance, C
IN3
10 10 10 pF max
LOGIC OUTPUTS
Output High Voltage, V
OH
4.0 4.0 4.0 V min I
SOURCE
= 200 µA
Output Low Voltage, V
OL
0.4 0.4 0.4 V max I
SINK
= 1.6 mA
Output Coding
AD7893-10, AD7893-3 2s Complement
AD7893-2, AD7893-5 Straight (Natural) Binary
CONVERSION RATE
Conversion Time 6 6 6 µs max
Track/Hold Acquisition Time
2
1.5 1.5 1.5 µs max
POWER REQUIREMENTS
V
DD
+5 +5 +5 V nom ±5% for Specified Performance
I
DD
9 9 9 mA max
Power Dissipation 45 45 45 mW max Typically 25 mW
NOTES
1
Temperature Ranges are as follows: A, B Versions: –40°C to +85°C, S Version: –55°C to +125°C.
2
See Terminology.
3
Sample tested @ +25°C to ensure compliance.
Specifications subject to change without notice.
REV. E
–2–
(V
DD
= +5 V, AGND = DGND = 0 V, REF IN = +2.5 V. All specifications T
MIN
to T
MAX
unless
otherwise noted.)
AD7893
–3–
REV. E
TIMING CHARACTERISTICS
1, 2
A, B S
Parameter Versions Version Units Test Conditions/Comments
t
1
50 50 ns min CONVST Pulse Width
t
2
60 70 ns min SCLK High Pulse Width
t
3
30 40 ns min SCLK Low Pulse Width
t
43
50 60 ns max SCLK Rising Edge to Data Valid Delay
t
54
10 10 ns min Bus Relinquish Time after Falling Edge of SCLK
100 100 ns max
NOTES
1
Sample tested at +25°C to ensure compliance. All input signals are measured with tr = tf = 1 ns (10% to 90% of +5 V) and timed from a voltage level of +1.6 V.
2
See Figure 5.
3
Measured with the load circuit of Figure 1 and defined as the time required for an output to cross 0.8 V or 2.4 V.
4
Derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 1. The measured number is then extrapolated back
to remove the effects of charging or discharging the 50 pF capacitor. This means that the time, t
5
, quoted in the timing characteristics is the true bus relinquish time
of the part and, as such, is independent of external bus loading capacitances.
(VDD = +5 V, AGND = DGND = 0 V, REF IN = +2.5 V)
ABSOLUTE MAXIMUM RATINGS*
(T
A
= +25°C unless otherwise noted)
V
DD
to AGND . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V
V
DD
to DGND . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V
Analog Input Voltage to AGND
AD7893-10, AD7893-5 . . . . . . . . . . . . . . . . . . . . . . . ±17 V
AD7893-2, AD7893-3 . . . . . . . . . . . . . . . . . . . –5 V, +10 V
Reference Input Voltage to AGND . . . –0.3 V to V
DD
+ 0.3 V
Digital Input Voltage to DGND . . . . . –0.3 V to V
DD
+ 0.3 V
Digital Output Voltage to DGND . . . . –0.3 V to V
DD
+ 0.3 V
Operating Temperature Range
Commercial (A, B Versions) . . . . . . . . . . . –40°C to +85°C
Extended (S Version) . . . . . . . . . . . . . . . . –55°C to +125°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . +150°C
Plastic DIP Package, Power Dissipation . . . . . . . . . . 450 mW
θ
JA
Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 130°C/W
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . +260°C
Cerdip Package, Power Dissipation . . . . . . . . . . . . . . 450 mW
θ
JA
Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 125°C/W
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . +300°C
SOIC Package, Power Dissipation . . . . . . . . . . . . . . . 450 mW
θ
JA
Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 170°C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . +215°C
Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . +220°C
*Stresses above those listed under “Absolute Maximum Ratings” may cause
permanent damage to the device. This is a stress rating only; functional operation
of the device at these or any other conditions above those listed in the operational
sections of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
TO
OUTPUT
PIN +2.1V
1.6mA
200µA
50pF
Figure 1. Load Circuit for Access Time and Bus
Relinquish Time
WARNING!
ESD SENSITIVE DEVICE
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although
the AD7893 features proprietary ESD protection circuitry, permanent damage may occur on devices
subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are
recommended to avoid performance degradation or loss of functionality.
_l:l__l_j E_|__|=L
AD7893
REV. E
–4–
PIN FUNCTION DESCRIPTION
Pin Pin
No. Mnemonic Description
1 REF IN Voltage Reference Input. An external reference source should be connected to this pin to provide the refer-
ence voltage for the AD7893’s conversion process. The REF IN input is buffered on-chip. The nominal ref-
erence voltage for correct operation of the AD7893 is +2.5 V.
2V
IN
Analog Input Channel. The analog input range is ±10 V (AD7893-10), ±2.5 V (AD7893-3), 0 V to +5 V
(AD7893-5) and 0 V to +2.5 V (AD7893-2).
3 AGND Analog Ground. Ground reference for track/hold, comparator and DAC.
4 SCLK Serial Clock Input. An external serial clock is applied to this input to obtain serial data from the AD7893. A
new serial data bit is clocked out on the rising edge of this serial clock, and data is valid on the falling edge.
The serial clock input should be taken low at the end of the serial data transmission.
5 SDATA Serial Data Output. Serial data from the AD7893 is provided at this output. The serial data is clocked out by
the rising edge of SCLK and is valid on the falling edge of SCLK. Sixteen bits of serial data are provided
with four leading zeros followed by the 12 bits of conversion data. On the sixteenth falling edge of SCLK, the
SDATA line is disabled (three-stated). Output data coding is twos complement for the AD7893-10 and
AD7893-3, straight binary for the AD7893-2 and AD7893-5.
6 DGND Digital Ground. Ground reference for digital circuitry.
7CONVST Convert Start. Edge-triggered logic input. On the falling edge of this input, the serial clock counter is reset to
zero. On the rising edge of this input, the track/hold goes into its hold mode and conversion is initiated.
8V
DD
Positive supply voltage, +5 V ± 5%.
PIN CONFIGURATION
DIP and SOIC
1
2
3
4
8
7
6
5
REF IN
VIN
AGND
SCLK
DGND
SDATA
VDD
CONVST
AD7893
TOP VIEW
(NOT TO SCALE)
ORDERING GUIDE
Temperature Linearity Package
Model Range Error SNR Options*
AD7893AN-2 –40°C to +85°C±1 LSB 70 dB N-8
AD7893BN-2 –40°C to +85°C±1/2 LSB 72 dB N-8
AD7893AR-2 –40°C to +85°C±1 LSB 70 dB SO-8
AD7893BR-2 –40°C to +85°C±1/2 LSB 72 dB SO-8
AD7893SQ-2 –55°C to +125°C±1 LSB 70 dB Q-8
AD7893AN-5 –40°C to +85°C±1 LSB 70 dB N-8
AD7893BN-5 –40°C to +85°C±1/2 LSB 72 dB N-8
AD7893AR-5 –40°C to +85°C±1 LSB 70 dB SO-8
AD7893BR-5 –40°C to +85°C±1/2 LSB 72 dB SO-8
AD7893SQ-5 –55°C to +125°C±1 LSB 70 dB Q-8
AD7893AN-10 –40°C to +85°C±1 LSB 70 dB N-8
AD7893BN-10 –40°C to +85°C±1/2 LSB 72 dB N-8
AD7893AR-10 –40°C to +85°C±1 LSB 70 dB SO-8
AD7893BR-10 –40°C to +85°C±1/2 LSB 72 dB SO-8
AD7893SQ-10 –55°C to +125°C±1 LSB 70 dB Q-8
AD7893AR-3 –40°C to +85°C±1 LSB 70 dB SO-8
*N = Plastic DIP, Q = Cerdip, SO = SOIC.
AD7893
–5–
REV. E
Relative Accuracy
Relative accuracy or endpoint nonlinearity is the maximum
deviation from a straight line passing through the endpoints of
the ADC transfer function.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Positive Full-Scale Error (AD7893-10)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal 4 × REF IN – 1 LSB (AD7893-10
±10 V range) after the Bipolar Zero Error has been adjusted out.
Positive Full-Scale Error (AD7893-3)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal (REF IN – 1 LSB) after the
Bipolar Zero Error has been adjusted out.
Positive Full-Scale Error (AD7893-5)
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (2 × REF IN – 1 LSB) after the Uni-
polar Offset Error has been adjusted out.
Positive Full-Scale Error (AD7893-2)
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (REF IN – 1 LSB) after the Unipolar
Offset Error has been adjusted out.
Bipolar Zero Error (AD7893-10, 610 V; AD7893-3, 62.5 V)
This is the deviation of the midscale transition (all 0s to all 1s)
from the ideal 0 V (AGND).
Unipolar Offset Error (AD7893-2, AD7893-5)
This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal 1 LSB.
Negative Full-Scale Error (AD7893-10)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal –4 × REF IN + 1 LSB (AD7893-10
±10 V range) after Bipolar Zero Error has been adjusted out.
Negative Full-Scale Error (AD7893-3)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal (–REF IN + 1 LSB) after Bipolar
Zero Error has been adjusted out.
Track/Hold Acquisition Time
Track/Hold acquisition time is the time required for the output
of the track/hold amplifier to reach its final value, within
±1/2 LSB, after the end of conversion (the point at which the
track/hold returns to track mode). It also applies to situations
where there is a step input change on the input voltage applied
to the V
IN
input of the AD7893. This means that the user must
wait for the duration of the track/hold acquisition time after the
end of conversion or after a step input change to V
IN
before
starting another conversion, to ensure that the part operates to
specification.
TERMINOLOGY
Signal to (Noise + Distortion) Ratio
This is the measured ratio of signal to (noise + distortion) at the
output of the A/D converter. The signal is the rms amplitude of
the fundamental. Noise is the rms sum of all nonfundamental
signals up to half the sampling frequency (f
S
/2), excluding dc.
The ratio is dependent upon the number of quantization levels
in the digitization process; the more levels, the smaller the quan-
tization noise. The theoretical signal to (noise + distortion) ratio
for an ideal N-bit converter with a sine wave input is given by:
Signal to (Noise + Distortion) = (6.02 N + 1.76) dB
Thus for a 12-bit converter, this is 74 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7893, it is defined as:
where V
1
is the rms amplitude of the fundamental and V
2
, V
3
,
V
4
, V
5
and V
6
are the rms amplitudes of the second through the
sixth harmonics.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of the
rms value of the next largest component in the ADC output
spectrum (up to f
S
/2 and excluding dc) to the rms value of the
fundamental. Normally, the value of this specification is deter-
mined by the largest harmonic in the spectrum, but for parts
where the harmonics are buried in the noise floor, it will be a
noise peak.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities will create distortion
products at sum and difference frequencies of mfa ± nfb where
m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for
which neither m nor n are equal to zero. For example, the second
order terms include (fa + fb) and (fa – fb), while the third order
terms include (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb).
The AD7893 is tested using the CCIF standard where two
input frequencies near the top end of the input bandwidth are
used. In this case, the second and third order terms are of differ-
ent significance. The second order terms are usually distanced
in frequency from the original sine waves, while the third order
terms are usually at a frequency close to the input frequencies.
As a result, the second and third order terms are specified sepa-
rately. The calculation of the intermodulation distortion is per
the THD specification where it is the ratio of the rms sum of the
individual distortion products to the rms amplitude of the fun-
damental expressed in dBs.
THD(dB)=20 log V
2
2
+V
3
2
+V
4
2
+V
5
2
+V
6
2
V
1
AD7893
REV. E
–6–
CONVERTER DETAILS
The AD7893 is a fast, 12-bit single supply A/D converter. It
provides the user with signal scaling (AD7893-10), track/hold,
A/D converter and serial interface logic functions on a single
chip. The A/D converter section of the AD7893 consists of a
conventional successive-approximation converter based on an
R-2R ladder structure. The signal scaling on the AD7893-10,
AD7893-5 and AD7893-3 allows the part to handle ±10 V, 0 V
to +5 V and ±2.5 V input signals, respectively, while operating
from a single +5 V supply. The AD7893-2 accepts an analog in-
put range of 0 V to +2.5 V. The part requires an external +2.5 V
reference. The reference input to the part is buffered on-chip.
A major advantage of the AD7893 is that it provides all of the
above functions in an 8-pin package, either 8-pin mini-DIP or
SOIC. This offers the user considerable space saving advantages
over alternative solutions. The AD7893 typically consumes only
25 mW, making it ideal for battery-powered applications.
Conversion is initiated on the AD7893 by pulsing the CONVST
input. On the rising edge of CONVST, the on-chip track/hold
goes from track-to-hold mode and the conversion sequence is
started. The conversion clock for the part is generated internally
using a laser-trimmed clock oscillator circuit. Conversion time
for the AD7893 is 6 µs, and the track/hold acquisition time is
1.5 µs. To obtain optimum performance from the part, the read
operation should not occur during the conversion or during
600 ns prior to the next conversion. This allows the part to op-
erate at throughput rates up to 117 kHz and to achieve data
sheet specifications. The part can operate at higher throughput
rates (up to 133 kHz) with slightly degraded performance (see
Timing and Control section).
CIRCUIT DESCRIPTION
Analog Input Section
The AD7893 is offered as four part types: the AD7893-10,
which handles a ±10 V input voltage range; the AD7893-3,
which handles a ±2.5 V input voltage range; the AD7893-5,
which handles a 0 V to +5 V input range; and the AD7893-2,
which handles a 0 V to +2.5 V input voltage range.
Figure 2 shows the analog input section for the AD7893-10,
AD7893-5 and AD7893-3. The analog input range of the
AD7893-10 is ±10 V into an input resistance of typically 33 k.
The analog input range of the AD7893-3 is ±2.5 V into an input
resistance of typically 12 k. The input range on the AD7893-5 is
0 V to +5 V into an input resistance of typically 11 k. This in-
put is benign with no dynamic charging currents, as the resistor
stage is followed by a high input impedance stage of the track/hold
AGND
AD7893-10/AD7893-5
V
IN
REF IN
TRACK/
HOLD
TO ADC
REFERENCE
CIRCUITRY
TO INTERNAL
COMPARATOR
R3
R2
R1
Figure 2. AD7893-10/AD7893-3/AD7893-5 Analog Input
Structure
amplifier. For the AD7893-10, R1 = 30 k; R2 = 7.5 k and
R3 = 10 k. For the AD7893-3, R1 = R2 = 6.5 k, and R3
is open circuit. For the AD7893-5, R1 and R3 = 5 k while
R2 is open-circuit.
For the AD7893-10 and AD7893-3, the designed code transi-
tions occur on successive integer LSB values (i.e., 1 LSB, 2 LSBs,
3 LSBs . . .). Output coding is twos complement binary with
1 LSB = FS/4096. The ideal input/output transfer function for
the AD7893-10 and AD7893-3 is shown in Table I.
Table I. Ideal Input/Output Code Table for the AD7893-10/
AD7893-3
Digital Output
Analog Input
1
Code Transition
+FSR/2 – 1 LSB
2
011 . . . 110 to 011 . . . 111
+FSR/2 – 2 LSBs 011 . . . 101 to 011 . . . 110
+FSR/2 – 3 LSBs 011 . . . 100 to 011 . . . 101
AGND + 1 LSB 000 . . . 000 to 000 . . . 001
AGND 111 . . . 111 to 000 . . . 000
AGND – 1 LSB 111 . . . 110 to 111 . . . 111
–FSR/2 + 3 LSBs 100 . . . 010 to 100 . . . 011
–FSR/2 + 2 LSBs 100 . . . 001 to 100 . . . 010
–FSR/2 + 1 LSB 100 . . . 000 to 100 . . . 001
NOTES
1
FSR is full-scale range and is 20 V (AD7893-10) and = 5 V (AD7893-3) with
REF IN = +2.5 V.
2
1 LSB = FSR/4096 = 4.883 mV (AD7893-10) and 1.22 mV (AD7893-3) with
REF IN = +2.5 V.
For the AD7893-5, the designed code transitions occur again on
successive integer LSB values. Output coding is straight (natural)
binary with 1 LSB = FS/4096 = 5 V/4096 = 1.22 mV. The ideal
input/output transfer function for the AD7893-5 is shown in
Table II.
The analog input section for the AD7893-2 contains no biasing
resistors, and the V
IN
pin drives the input directly to the track/
hold amplifier. The analog input range is 0 V to +2.5 V into a
high impedance stage, with an input current of less than
500 nA. This input is benign, with no dynamic charging cur-
rents. Once again, the designed code transitions occur on suc-
cessive integer LSB values. Output coding is straight (natural)
binary with 1 LSB = FS/4096 = 2.5 V/4096 = 0.61 mV. Table
II also shows the ideal input/output transfer function for the
AD7893-2.
Table II. Ideal Input/Output Code Table for
AD7893-2/AD7893-5
Digital Output
Analog Input
1
Code Transition
+FSR – 1 LSB
2
111 . . . 110 to 111 . . . 111
+FSR – 2 LSB 111 . . . 101 to 111 . . . 110
+FSR – 3 LSB 111 . . . 100 to 111 . . . 101
AGND + 3 LSB 000 . . . 010 to 000 . . . 011
AGND + 2 LSB 000 . . . 001 to 000 . . . 010
AGND + 1 LSB 000 . . . 000 to 000 . . . 001
NOTES
1
FSR is Full-Scale Range and is 5 V for AD7893-5 and 2.5 V for AD7893-2
with REF IN = +2.5 V.
2
1 LSB = FSR/4096 and is 1.22 mV for AD7893-5 and 0.61 mV for AD7893-2
with REF IN = +2.5 V.
AD7893
–7–
REV. E
Track/Hold Section
The track/hold amplifier on the analog input of the AD7893
allows the ADC to accurately convert an input sine wave of full-
scale amplitude to 12-bit accuracy. The input bandwidth of the
track/hold is greater than the Nyquist rate of the ADC, even
when the ADC is operated at its maximum throughput rate of
117 kHz (i.e., the track/hold can handle input frequencies in
excess of 58 kHz).
The track/hold amplifier acquires an input signal to 12-bit accu-
racy in less than 1.5 µs. The operation of the track/hold is essen-
tially transparent to the user. The track/hold amplifier goes from
its tracking mode to its hold mode at the start of conversion
(i.e., the rising edge of CONVST). The aperture time for the
track/hold (i.e., the delay time between the external CONVST
signal and the track/hold actually going into hold) is typically
15 ns. At the end of conversion (6 µs after the rising edge of
CONVST) the part returns to its tracking mode. The acquisi-
tion time of the track/hold amplifier begins at this point.
Reference Input
The reference input to the AD7893 is a buffered on-chip with a
maximum reference input current of 1 µA. The part is specified
with a +2.5 V reference input voltage. Errors in the reference
source will result in gain errors in the AD7893’s transfer func-
tion and will add to the specified full-scale errors on the part.
On the AD7893-10 it will also result in an offset error injected
in the attenuator stage. Suitable reference sources for the
AD7893 include the AD780 and AD680 precision +2.5 V
references.
Timing and Control Section
Figure 3 shows the timing and control sequence required to ob-
tain optimum performance from the AD7893. In the sequence
shown, conversion is initiated on the rising edge of CONVST,
and new data from this conversion is available in the output reg-
ister of the AD7893 6 µs later. Once the read operation has
taken place, a further 600 ns should be allowed before the next
rising edge of CONVST to optimize the settling of the track/
hold amplifier before the next conversion is initiated. With the
serial clock frequency at its maximum of 8.33 MHz, the achiev-
able throughput rate for the part is 6 µs (conversion time) plus
1.92 µs (read time) plus 0.6 µs (acquisition time). This results in
a minimum throughput time of 8.52 µs (equivalent to a through-
put rate of 117 kHz).
The read operation consists of sixteen serial clock pulses to the
output shift register of the AD7893. After sixteen serial clock
pulses the shift register is reset and the SDATA line is three-
stated. If there are more serial clock pulses after the sixteenth
clock, the shift register will be moved on past its reset state;
however, the shift register will be reset again on the falling edge
of the CONVST signal to ensure that the part returns to a
known state every conversion cycle. As a result, a read operation
from the output register should not straddle across the falling
edge of CONVST as the output shift register will be reset in the
middle of the read operation, and the data read back into the
microprocessor will appear invalid.
The throughput rate of the part can be increased by reading
data during conversion. If the data is read during conversion,
a throughput time of 6 µs (conversion time) plus 1.5 µs is
achieved. This minimum throughput time of 7.5 µs is achieved
with a slight reduction in performance from the AD7893. The
signal to (noise + distortion) number is likely to degrade by ap-
proximately 1.5 dB while the code flicker from the part will also
increase (see AD7893 PERFORMANCE section).
Because the AD7893 is provided in an 8-pin package to mini-
mize board space, the number of pins available for interfacing is
very limited. As a result, no status signal is provided from the
AD7893 to indicate when conversion is complete. In many
applications, this will not be a problem as the data can be read
from the AD7893 during conversion or after conversion; how-
ever, applications that want to achieve optimum performance
from the AD7893 will have to ensure that the data read does not
occur during conversion or during 600 ns prior to the rising
edge of CONVST. This can be achieved in two ways. The first
is to ensure in software that the read operation is not initiated
until 6 µs after the rising edge of CONVST. This will only be
possible if the software knows when the CONVST command is
issued. The second scheme would be to use the CONVST sig-
nal as both the conversion start signal and an interrupt signal.
The simplest way to do this would be to generate a square wave
signal for CONVST with high and low times of 6 µs (see Figure
4). Conversion is initiated on the rising edge of CONVST. The
falling edge of CONVST occurs 6 µs later and can be used as ei-
ther an active low or falling, edge-triggered interrupt signal to
tell the processor to read the data from the AD7893. Provided
that the read operation is completed 600 ns before the rising
edge of CONVST, the AD7893 will operate to specification.
CONVST
SCLK
CONVERSION IS INITIATED
AND TRACK/HOLD GOES
INTO HOLD
t1
CONVERSION ENDS
6µs LATER SERIAL READ
OPERATION OUTPUT SERIAL
SHIFT REGISTER IS
RESET
600ns MIN
tCONVERT
READ OPERATION
SHOULD END 600ns
PRIOR TO NEXT
RISING EDGE OF
CONVST
Figure 3. Timing Sequence for Optimum Performance from the AD7893
AD7893
REV. E
–8–
This scheme limits the throughput rate to 12 µs minimum; how-
ever, depending on the response time of the microprocessor to
the interrupt signal and the time taken by the processor to read
the data, this may be the fastest the system could have operated.
In any case, the CONVST signal does not have to have a 50:50
duty cycle. This can be tailored to optimize the throughput rate
of the part for a given system.
Alternatively, the CONVST signal can be used as a normal narrow
pulse width. The rising edge of CONVST can be used as an active
high or rising edge-triggered interrupt. A software delay of 6 µs can
then be implemented before data is read from the part.
Serial Interface
The serial interface to the AD7893 consists of just two wires, a
serial clock input (SCLK) and the serial data output (SDATA).
This allows for an easy to use interface to most microcontrollers,
DSP processors and shift registers.
Figure 5 shows the timing diagram for the read operation to the
AD7893. The serial clock input (SCLK) provides the clock
source for the serial interface. Serial data is clocked out from the
SDATA line on the rising edge of this clock and is valid on the
falling edge of SCLK. Sixteen clock pulses must be provided to
the part to access to full conversion result. The AD7893 pro-
vides four leading zeros followed by the 12-bit conversion result
starting with the MSB (DB11). The last data bit to be clocked
out on the final rising clock edge is the LSB (DB0). On the six-
teenth falling edge of SCLK, the SDATA line is disabled (three-
stated). After this last bit has been clocked out, the SCLK input
should return low and remain low until the next serial data read
operation. If there are extra clock pulses after the sixteenth
clock, the AD7893 will start over again with outputting data
from its output register, and the data bus will no longer be
three-stated even when the clock stops. Provided that the serial
clock has stopped before the next falling edge of CONVST, the
AD7893 will continue to operate correctly with the output shift
register being reset on the falling edge of CONVST; however,
the SCLK line must be low when CONVST goes low in order
to reset the output shift register correctly.
The serial clock input does not have to be continuous during the
serial read operation. The sixteen bits of data (four leading zeros
and 12 bit conversion result) can be read from the AD7893 in a
number of bytes; however, the SCLR input must remain low be-
tween the two bytes.
Normally, the output register is updated at the end of conver-
sion. If a serial read from the output register is in progress when
conversion is complete; however, the updating of the output
register is deferred. In this case, the output register is updated
when the serial read is completed. If the serial read has not been
completed before the next falling edge of CONVST, the output
register will be updated on the falling edge of CONVST, and
the output shift register count is reset. In applications where the
data read has been started and not completed before the falling
edge of CONVST, the user must provide a CONVST pulse
width of greater than 1.5 µs to ensure correct setup of the AD7893
before the next conversion is initiated. In applications where the
output update takes place either at the end of conversion or at
the end of a serial read that is completed 1.5 µs before the rising
edge of CONVST, the normal pulse width of 50 ns minimum
applies to CONVST.
CONVST
SCLK
CONVERSION IS INITIATED
AND TRACK/HOLD GOES
INTO HOLD
CONVST INDICATES
TO µP THAT
CONVERSION IS
COMPLETE
t
CONVERT
SERIAL READ
OPERATION
µP INT SERVICE
OR POLLING
ROUTINE
600ns MIN
READ OPERATION
SHOULD END 600ns
PRIOR TO NEXT
RISING EDGE OF
CONVST
Figure 4.
CONVST
Used as Status Signal
SDATA (O)
SCLK (I)
FOUR LEADING ZEROS DB11 DB10
THREE-STATE THREE-STATE
DB0
t
5
t
4
t
3
t
2
Figure 5. Data Read Operation
AD7893
–9–
REV. E
The AD7893 counts the serial clock edges to know which bit
from the output register should be placed on the SDATA out-
put. To ensure that the part does not lose synchronization, the
serial clock counter is reset on the falling edge of the CONVST
input, provided the SCLR line is low. The user should ensure
that a falling edge on the CONVST input does not occur while
a serial data read operation is in progress.
MICROPROCESSOR/MICROCONTROLLER INTERFACE
The AD7893 provides a two-wire serial interface that can be
used for connection to the serial ports of DSP processors and
microcontrollers. Figures 6 through 9 show the AD7893 inter-
faced to a number of different microcontrollers and DSP pro-
cessors. The AD7893 accepts an external serial clock and, as a
result, in all interfaces shown here, the processor/controller is
configured as the master, providing the serial clock with the
AD7893 configured as the slave in the system.
AD7893-8051 Interface
Figure 6 shows an interface between the AD7893 and the
8XC51 microcontroller. The 8XC51 is configured for its Mode
0 serial interface mode. The diagram shows the simplest form of
the interface where the AD7893 is the only part connected to
the serial port of the 8XC51 and, therefore, no decoding of the
serial read operations is required. It also makes no provisions for
monitoring when conversion is complete on the AD7893.
Either of these two tasks can readily be accomplished with minor
modifications to the interface. To chip select the AD7893 in
systems where more than one device is connected to the 8XC51’s
serial port, a port bit configured as an output from one of the
8XC51’s parallel ports can be used to gate on or off the serial
clock to the AD7893. A simple AND function on this port bit
and the serial clock from the 8XC51 will provide this function.
The port bit should be high to select the AD7893 and low when
it is not selected.
To monitor the conversion time on the AD7893, a scheme such
as previously outlined with CONVST can be used. This can be
implemented in two ways. One is to connect the CONVST line
to another parallel port bit that is configured as an input. This
port bit can then be polled to determine when conversion is
complete. An alternative is to use an interrupt driven system, in
which case the CONVST line should be connected to the INT1
input of the 8XC51.
The serial clock rate from the 8XC51 is limited to significantly
less than the allowable input serial clock frequency with which
the AD7893 can operate. As a result, the time to read data from
the part will actually be longer than the conversion time of the
part. This means that the AD7893 cannot run at its maximum
throughput rate when used with the 8XC51.
AD7893-68HC11 Interface
An interface circuit between the AD7893 and the 68HC11
microcontroller is shown in Figure 7. For the interface shown,
the 68HC11 SPI port is used, and the 68HC11 is configured in
its single-chip mode. The 68HC11 is configured in the master
mode with its CPOL bit set to a logic zero and its CPHA bit set
to a logic one. As with the previous interface, the diagram shows
the simplest form of the interface where the AD7893 is the only
part connected to the serial port of the 68HC11 and, therefore,
no decoding of the serial read operations is required. It also
makes no provisions for monitoring when conversion is com-
plete on the AD7893.
Once again, either of these two tasks can readily be accom-
plished with minor modifications to the interface. To chip select
the AD7893 in systems where more than one device is con-
nected to the 68HC11’s serial port, a port bit, configured as an
output from one of the 68HC11’s parallel ports, can be used to
gate on or off the serial clock to the AD7893. A simple AND
function on this port bit and the serial clock from the 68HC11
will provide this function. The port bit should be high to select
the AD7893 and low when it is not selected.
To monitor the conversion time on the AD7893, a scheme such
as outlined in the previous interface with CONVST can be
used. This can be implemented in two ways. One is to connect
the CONVST line to another parallel port bit that is configured
as an input. This port bit can then be polled to determine when
conversion is complete. An alternative is to use an interrupt
driven system, in which case the CONVST line should be con-
nected to the IRQ input of the 68HC11.
The serial clock rate from the 68HC11 is limited to significantly
less than the allowable input serial clock frequency with which
the AD7893 can operate. As a result, the time to read data from
the part will actually be longer than the conversion time of the
part. This means that the AD7893 cannot run at its maximum
throughput rate when used with the 68HC11.
AD7893
SDATA
SCLK
8XC51
P3.0
P3.1
Figure 6. AD7893 to 8XC51 Interface
AD7893
SDATA
SCLK
68HC11
SCK
MISO
Figure 7. AD7893 to 68HC11 Interface
AD7893
REV. E
–10–
AD7893–ADSP-2105 Interface
An interface circuit between the AD7893 and the ADSP-2105
DSP processor is shown in Figure 8. In the interface shown, the
RFS1 output from the ADSP-2105’s SPORT1 serial port is
used to gate the serial clock (SCLK1) of the ADSP-2105 before
it is applied to the SCLK input of the AD7893. The RFS1 out-
put is configured for active high operation. The interface
ensures a noncontinuous clock for the AD7893’s serial clock
input with only sixteen serial clock pulses provided, and the
serial clock line of the AD7893 remaining low between data
transfers. The SDATA line from the AD7893 is connected to
the DR1 line of the ADSP-2105’s serial port.
AD7893
SDATA
SCLK
ADSP-2105
DR1
RFS1
SCLK1
Figure 8. AD7893 to ADSP-2105 Interface
The timing relationship between the SCLK1 and RFS1 outputs
of the ADSP-2105 are such that the delay between the rising
edge of the SCLK1 and the rising edge of an active high RFS1
is up to 25 ns. There is also a requirement that data must be set
up 10 ns prior to the falling edge of the SCLK1 to be read cor-
rectly by the ADSP-2105. The data access time for the AD7893
is 50 ns from the rising edge of its SCLK input. Assuming a
10 ns propagation delay through the external AND gate, the
high time of the SCLK1 output of the ADSP-2105 must be
(50 + 25 + 10 + 10) ns, i.e., 95 ns. This means that the
serial clock frequency with which the interface of Figure 13 can
work with is limited to 5.26 MHz.
An alternative scheme is to configure the ADSP-2105 to accept
an external serial clock. In this case, an external noncontinuous
serial clock that drives the serial clock inputs of both the ADSP-
2105 and the AD7893 is provided. In this scheme, the serial
clock frequency is limited to 5 MHz by the ADSP-2105.
To monitor the conversion time on the AD7893, a scheme such
as outlined in previous interfaces with CONVST can be used.
This can be implemented by connecting the CONVST line
directly to the IRQ2 input of the ADSP-2105.
AD7893–DSP56000 Interface
Figure 9 shows an interface circuit between the AD7893 and the
DSP56000 DSP processor. The DSP5600 is configured for nor-
mal mode asynchronous operation with gated clock. It is also set
up for a 16-bit word with the gated serial clock being generated
by the DSP56000 and appears on the SC0 pin. The SC0 pin
should be configured as an output by setting bit SCD0 to 1. In
this mode, the DSP56000 provides sixteen serial clock pulses to
the AD7893 in a serial read operation. The DSP56000 assumes
valid data on the first falling edge of SCK, so the interface is
simply two-wire as shown in Figure 9.
To monitor the conversion time on the AD7893, a scheme such
as outlined in previous interface examples with CONVST can
be used. This can be implemented by connecting the CONVST
line directly to the IRQA input of the DSP56000.
AD7893
SDATA
SCLK
DSP56000
SC0
SRD
Figure 9. AD7893 to DSP56000 Interface
AD7893 PERFORMANCE
Linearity
The linearity of the AD7893 is determined by the on-chip 12-bit
D/A converter. This is a segmented DAC that is laser trimmed
for 12-bit integral linearity and differential linearity. Typical
relative numbers for the part are ±1/4 LSB, while the typical
DNL errors are ±1/2 LSB.
Noise
In an A/D converter, noise exhibits itself as code uncertainty in
dc applications and as the noise floor (in an FFT, for example)
in ac applications. In a sampling A/D converter like the AD7893,
all information about the analog input appears in the baseband
from dc to 1/2 the sampling frequency. The input bandwidth of
the track/hold exceeds the Nyquist bandwidth; therefore, an
antialiasing filter should be used to remove unwanted signals
above f
S
/2 in the input signal in applications where such signals
exist.
Figure 10 shows a histogram plot for 8192 conversions of a dc
input using the AD7893. The analog input was set at the center
of a code transition. The timing and control sequence used was
per Figure 3 where the optimum performance of the ADC was
achieved. It can be seen that almost all the codes appear in the
one output bin, indicating very good noise performance from
the ADC. The rms noise performance for the AD7893-2 for the
above plot was 87 µV. Since the analog input range, and hence
LSB size, on the AD7893-10 is eight times what it is for the
AD7893-2, the same output code distribution results in an out-
put rms noise of 700 µV for the AD7893-10.
CODE
9000
1000
(X–4) (X–3)
OCCURRENCES OF CODE
(X–2) (X–1) X (X+1) (X+2) (X+3) (X+4)
8000
5000
4000
3000
2000
7000
6000
0
SAMPLING FREQUENCY = 102.4kHz
T
A
= +25
°
C
Figure 10. Histogram of 8192 Conversions of a DC Input
AD7893
–11–
REV. E
Effective Number of Bits
The formula for signal to (noise + distortion) ratio (see Termi-
nology section) is related to the resolution or number of bits in
the converter. Rewriting the formula gives a measure of perfor-
mance expressed in effective number of bits (N):
N = (SNR – 1.76) / 6.02
where SNR is Signal to (Noise + Distortion) Ratio.
The effective number of bits for a device can be calculated from
its measured signal to (noise + distortion) ratio. Figure 13 shows
a typical plot of effective number of bits versus frequency for the
AD7893-2 from dc to f
SAMPLING
/2. The sampling frequency is
102.4 kHz. The plot shows that the AD7893 converts an input
sine wave of 51.2 kHz to an effective numbers of bits of 11,
which equates to a signal to (noise + distortion) level of 68 dB.
INPUT FREQUENCY – kHz
12.0
11.5
10.0 0 51.225.6
EFFECTIVE NUMBER OF BITS
11.0
10.5
Figure 13. Effective Number of Bits vs. Frequency
The same data is presented in Figure 11 as in Figure 10 except
that, in this case, the output data read for the device occurs dur-
ing conversion. This has the effect of injecting noise onto the die
while bit decisions are being made; this increases the noise gen-
erated by the AD7893. The histogram plot for 8192 conversions
of the same dc input now shows a larger spread of codes with
the rms noise for the AD7893-2 increasing to 210 µV. This ef-
fect will vary depending on where the serial clock edges appear
with respect to the bit trials of the conversion process. It is pos-
sible to achieve the same level of performance when reading
during conversion as when reading after conversion, depending
on the relationship of the serial dock edges to the bit trial points.
CODE
7500
(X–4) (X–3)
OCCURRENCES OF CODE
7000
6500
6000
0
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
(X–2) (X–1) (X+1) (X+2) (X+3) (X+4)X
SAMPLING
FREQUENCY = 102.4kHz
T
A
= +25
°
C
Figure 11. Histogram of 8192 Conversions with Read Dur-
ing Conversion
Dynamic Performance
With a combined conversion and acquisition time of 7.5 µs, the
AD7893 is ideal for wide bandwidth signal processing applica-
tions. These applications require information on the ADC’s
effect on the spectral content of the input signal. Signal to (noise
+ distortion) ratio, total harmonic distortion, peak harmonic or
spurious noise, and intermodulation distortion are all specified.
Figure 12 shows a typical FFT plot of a 10 kHz, 0 V to +2.5 V
input after being digitized by the AD7893-2, operating at a
102.4 kHz sampling rate. The signal to (noise + distortion)
ratio is 71.5 dB, and the total harmonic distortion is –83 dB.
FREQUENCY – kHz
SNR IS SIGNAL TO (NOISE AND DISTORTION) RATIO
0
–30
–180 0 51.225.6
SIGNAL AMPLITUDE – dB
–60
–80
–120
SAMPLE RATE = 102.4kHz
INPUT FREQUENCY = 10kHz
SNR = 71.5dB
T
A
= +25°C
Figure 12. AD7893 FFT Plot
AD7893
REV. E
–12–
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
PRINTED IN U.S.A. C1787c–2–1/97
Plastic DIP (N-8)
PIN 1 0.280 (7.11)
0.240 (6.10)
4
5
8
1
SEATING
PLANE
0.060 (1.52)
0.015 (0.38)
0.130
(3.30)
MIN
0.210
(5.33)
MAX
0.160 (4.06)
0.115 (2.93)
0.430 (10.92)
0.348 (8.84)
0.022 (0.558)
0.014 (0.356)
0.070 (1.77)
0.045 (1.15)
0.100
(2.54)
BSC
0.325 (8.25)
0.300 (7.62)
0.015 (0.381)
0.008 (0.204)
0.195 (4.95)
0.115 (2.93)
Cerdip (Q-8)
0.320 (8.13)
0.290 (7.37)
0.015 (0.38)
0.008 (0.20)
15
°
0
°
0.005 (0.13) MIN 0.055 (1.4) MAX
1
PIN 1
4
58
0.310 (7.87)
0.220 (5.59)
0.405 (10.29) MAX
0.200
(5.08)
MAX
SEATING
PLANE
0.023 (0.58)
0.014 (0.36)
0.070 (1.78)
0.030 (0.76)
0.060 (1.52)
0.015 (0.38)
0.150
(3.81)
MIN
0.200 (5.08)
0.125 (3.18)
0.100
(2.54)
BSC
SOIC (SO-8)
0.0098 (0.25)
0.0075 (0.19)
0.0500 (1.27)
0.0160 (0.41)
8
°
0
°
0.0196 (0.50)
0.0099 (0.25) x 45
°
PIN 1
0.1574 (4.00)
0.1497 (3.80)
0.2440 (6.20)
0.2284 (5.80)
4
5
1
8
0.0192 (0.49)
0.0138 (0.35)
0.0500
(1.27)
BSC
0.0688 (1.75)
0.0532 (1.35)
0.0098 (0.25)
0.0040 (0.10)
0.1968 (5.00)
0.1890 (4.80)

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IC ADC 12BIT 8CERDIP
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IC ADC 12BIT 8SOIC
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ADC, SUCCESSIVE APPROXIMATION, 1
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