CN0326 Datasheet, PDF (5/7 Pages) Analog Devices – Devices Connected

Circuit Note
System Noise Considerations
For an output data rate of 16.7 Hz and a gain of 1, the rms noise
of the AD7793 equals 1.96 μV (noise is referred to input, taken
from AD7793 data sheet). The peak-to-peak noise is
6.6 × RMS Noise = 6.6 × 1.96 µV = 12.936 µV
If the pH meter has a sensitivity of 59 mV/pH, the pH meter
should measure the pH level to a noise-free resolution of
12.936 μV / (59 mV/pH) = 0.000219 pH
This includes only the noise contribution of the AD7793. The
actual system results are presented in the next section.
Test Data and Results
All data capture was performed using the CN0326 LabVIEW
evaluation software. A Yokogawa GS200 precision voltage
source was used to simulate the input of a pH sensor.
By sweeping the precision voltage from −420 mV to +420 mV
in 1 mV increments, the EVAL-CN0326-PMDZ was able to
capture the data according to the user defined calibration option.
The peak-to-peak noise of the AD8603 buffer and the AD7793
in the actual system was determined by shorting the input pH
probe BNC connector and acquiring 1000 samples. As seen by
the histogram in Figure 5, the code spread is approximately 500
codes, which translates to a peak-to-peak noise of 31.3 µV, with
an equivalent pH reading spread of 0.00053 pH peak-to-peak.
35
30
25
20
15
10
5
0
6FB864
6FB900
6FB980
ADC CODE
6FBA00
6FBA58
Figure 5. Histogram Showing Output Code Spread with AD7793 Input Pins
Shorted Together
The system was tested with three different resistors in series
with the ADC input to simulate the different impedances of the
high impedance glass electrode. The system was also calibrated
to give 60 mV/pH. According to Figure 6, the linearity error
increases with the increase of simulated glass electrode
impedance. Figure 6 also shows that over the entire simulated
pH output voltage range, the linearity error is less than 0.5%
with for a 200 MΩ pH probe impedance.
CN-0326
0.5
0.40
0.4
200MΩ
0.35
0.3
0.03
0.2
100MΩ
0.1
1MΩ
0.25
0
0.20
–0.1
0.15
–0.2
SIMULATED pH
OUTPUT VOLTAGE
0.10
–0.3
–0.4
0.05
–0.5
0
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
ADC OUTPUT pH READING (pH)
Figure 6. pH Sensor Simulated Output Voltage (with Associated Linearity
Error Plot) vs. ADC Output pH Reading (Shown for Probe Resistance of
1 MΩ, 100 MΩ, and 200 MΩ)
The test data was taken using the board shown in Figure 7.
Complete documentation for the system can be found in the
CN-0326 Design Support package.
Figure 7. Photo of EVAL-CN0326-PMDZ Board
COMMON VARIATIONS
Other suitable ADCs are the AD7792 and AD7785. Both parts
have the same feature set as the AD7793. However, the AD7792
is a 16-bit ADC while the AD7785 is a 20-bit ADC.
The AD8607 buffer amplifier is available in an 8-lead MSOP
package . It is a dual micropower rail-to-rail input/output
amplifier which is in the same family as the AD8603.
Other families of ADuM5401 includes a variety of channel
configuration such as the ADuM5402/ADuM5403/ADuM5404
which also provides four independent isolation channels.
CIRCUIT EVALUATION AND TEST
This circuit uses the EVAL-CN0326-PMDZ circuit board, the
EVAL-SDP-CB1Z System Demonstration Platform (SDP)
evaluation board and the SDP-PMD-IB1Z, a PMOD interposer
board for the EVAL-SDP-CB1Z. The SDP and the SDP-PMD-
IB1Z boards have 120-pin mating connectors, allowing the
quick setup and evaluation of the circuit’s performance. In order
to evaluate the EVAL-CN0326-PMDZ board using the SDP-
PMD-IB1Z and the SDP, the EVAL-CN0326-PMDZ is
connected to the SDP-PMD-IB1Z by a standard 100 mil-
spaced, 25 mil square, right angle pin-header connector.
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