CN0326: Isolated Low Power pH Monitor with Temperature Compensation PDF

Circuit Note
CN-0326
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0326.
AD7793
3-Channel, Low Noise, Low Power,
24-Bit Sigma Delta ADC
ADuM5401
Quad-Channel Isolators with
Integrated DC/DC Converter
AD8603
MicroPower RRIO Low Noise
Precision Single CMOS Op Amp
Isolated Low Power pH Monitor with Temperature Compensation
EVALUATION AND DESIGN SUPPORT
The circuit gives 0.5% accurate readings for pH values from 0 to
14 with greater than 14-bits of noise-free code resolution and is
suitable for a variety of industrial applications such as chemical,
food processing, water, and wastewater analysis.
Circuit Evaluation Boards
CN0326 Evaluation Board (EVAL-CN0326-PMDZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
SDP PMOD Interposer Board (SDP-PMD-IB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
This circuit supports a wide variety of pH sensors that have very
high internal resistance that can range from 1 MΩ to several
GΩ, and digital signal and power isolation provides immunity
to noise and transient voltages often encountered in harsh
industrial environments.
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 is a completely isolated low power
pH sensor signal conditioner and digitizer with automatic
temperature compensation for high accuracy.
3.3VISO
FERRITE BEAD:
MURATA BLM21PG331SN1D
BEAD
3.3VISO
AVDD
210µA
3.3V
DVDD
IOUT2
10kΩ
VDD1
CS
VOA
VIA
CS
SCLK
VOB
VIB
SCLK
DIN
VOC
VIC
DIN
AIN2(–)
DOUT/
RDY
RFIN(+)/AIN3(+)
VID
VOD
RFIN(–)/AIN3(–)
GNDISO
1µF
pH SENSOR
J1
3.3VISO
1MΩ
AD8603
AD7793
10kΩ
AIN1(+)
1µF
AIN1(–)
10kΩ
5kΩ
1µF
GND
DOUT/RDY
GND1
ADUM5401
GNDISO
11821-001
TO
Pt1000
RTD
P1
VISO
AIN2(+)
Figure 1. pH Sensor Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. 0
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CN-0326
Circuit Note
CIRCUIT DESCRIPTION
Circuit Details
Fundamentals of pH Measurements
The design provides a complete solution for pH sensor with
temperature compensation. The circuit has three critical stages:
the pH probe buffer, the ADC, and the digital and power
isolator as shown in Figure 1.
The pH value is a measure of the relative amount of hydrogen
and hydroxide ions in an aqueous solution. In terms of molar
concentrations, water at 25°C contains 1 × 10−7 moles/liter of
hydrogen ions and the same concentration of hydroxide ions. A
neutral solution is one in which the hydrogen ion concentration
exactly equals the hydroxide ion concentration. pH is another
way of expressing the hydrogen ion concentration and is
defined as follows:
pH  – log( H  )
Therefore, if the hydrogen ion concentration is 1.0 × 10−2
moles/liter, the pH is 2.00.
The pH electrodes are electrochemical sensors used by many
industries but are of particular importance to the water and wastewater industry. The pH probe consists of a glass measuring
electrode and a reference electrode, which is analogous to a battery.
When the probe is place in a solution, the measuring electrode
generates a voltage depending on the hydrogen activity of the
solution, which is compared to the potential of the reference
electrode. As the solution becomes more acidic (lower pH) the
potential of the glass electrode becomes more positive (+mV) in
comparison to the reference electrode; and as the solution becomes
more alkaline (higher pH) the potential of the glass electrode
becomes more negative (−mV) in comparison to the reference
electrode. The difference between these two electrodes is the measured potential. A typical pH probe ideally produces 59.154 mV/pH
units at 25oC. This is expressed in the Nernst equation as follows
  pH – pH ISO

nF
where:
E = voltage of the hydrogen electrode with unknown activity
ܽ = ±30 mV, zero point tolerance
T = ambient temperature in oC
n = 1 at 25 oC, valence (number of charges on ion)
F = 96485 coulombs/mol, Faraday constant
R = 8.314 volt-coulombs /°K mol, Avogadro's number
pH = hydrogen ion concentration of an unknown solution
pHISO = 7, reference hydrogen ion concentration
The ADuM5401, quad-channel digital isolator with an
integrated dc-to-dc converter provides the digital signal and
power isolation between the microcontroller and the AD7793
digital lines. The iCoupler chip-scale transformer technology is
used to isolate the logic signals and the power feedback path in
the dc-to-dc converter.
Buffer for pH Sensor Interface
The electrode of a typical pH probe is made up of glass that
creates an extremely high resistance that can range from 1 MΩ
to 1 GΩ and acts as a resistance in series with the pH voltage
source as shown in Figure 2.
210µA
pH SENSOR
1GΩ
IOUT2
AD7793
3.3VISO
IBIAS
J1
1MΩ AD8603
pH
VOUT
10kΩ
AIN1(+)
1µF
AIN1(–)
AIN2(–)
10kΩ
+1.05V
5kΩ
RFIN(+)/AIN3(+)
1µF
RFIN(–)/AIN3(–)
GND
The equation shows that the voltage generated is dependent on
the acidity or alkalinity of the solution and varies with the hydrogen
ion activity in a known manner. The change in temperature of
the solution changes the activity of its hydrogen ions. When the
solution is heated, the hydrogen ions move faster which result
in an increase in potential difference across the two electrodes.
In addition, when the solution is cooled, the hydrogen activity
decreases causing a decrease in the potential difference. Electrodes
are designed ideally to produce a zero volt potential when
placed in a buffer solution with a pH of 7.
A good reference on the theory of pH is pH Theory and Practice,
Radiometer Analytical SAS, Villeurbanne Cedex, France.
The pH sensing and temperature compensation system is based on
the AD7793, 24-bit sigma-delta (Σ-Δ) with. It has three differential
analog inputs and has an on-chip, low noise, programmable gain
amplifier (PGA) that ranges from unity gain to 128. The AD7793
consumes only a maximum of 500 μA making it suitable for any
low power applications. It has a low noise, low drift internal band
gap reference and can accept external differential reference. The
output data rate from the part is software programmable and
can be varied from 4.17 Hz to 470 Hz.
Figure 2. pH Sensor and Buffer Interface to ADC (Simplified Schematic: All
Connections, RTD, and Decoupling Not Shown.)
The buffer amplifier bias current flowing through this series
resistance introduces an offset error in the system. To isolate the
circuit from this high source resistance, a buffer amplifier with
high input impedance and very low input bias current is needed
for this application. The AD8603 is used as a buffer amplifier for
this application as shown in Figure 2. The low input current of
the AD8603 minimizes the voltage error produced by the bias
current flowing through the electrode resistance.
Rev. 0 | Page 2 of 7
11821-002
Ea–
2.303 R  T  273.1 
The AD8603, a precision micro power (50 μA maximum) and
low noise (22 nV/√Hz) CMOS operational amplifier configured
as a buffer to the input of one of the channels of the AD7793.
The AD8603 has a typical input bias current of 200 fA that
provides an effective solution to the pH probe that has high
internal resistance.
Circuit Note
CN-0326
For 200 fA typical input bias current, the offset error is 0.2 mV
(0.0037 pH) for a pH probe that has 1 GΩ series resistance at
25oC. Even at the maximum input bias current of 1 pA, the
error is only 1 mV.
Table 2. Standard RTD Accuracy for DIN-43760
Class
DIN 43760 Class A
DIN 43760 Class B
Tolerance
±0.06% @ 0°C
±0.12% @ 0°C
The cut-off frequency of the 10 kΩ/1 µF low pass noise filter for
the buffer amplifier output is given by f= 1/2πRC, or 16 Hz.
Table 3. Standard RTD Accuracy for ASTM E-1137
Guarding, shielding, high insulation resistance standoffs, and
other such standard picoamp methods must be used to minimize
leakage at the high impedance input of the AD8603 buffer.
Grade
ASTM E-1137 Grade A
ASTM E-1137 Grade B
ADC Channel 1 Configuration, pH sensor
This stage involves measuring the small voltage generated by
the pH electrode. Table 1 shows the specifications of a typical
pH probe. Based on the Nernst equation, the full range voltage
from the probe can range from ±414 mV (±59.14 mV/pH) at
25°C to ±490 mV (±70 mV/pH) at 80°C.
Table 1. Specifications of a Typical pH Probe
Measurement Range
pH at zero voltage
Accuracy
Resolution
Operating Temperature
Reaction time
pH 0 to pH 14
pH 7.00 ± 0.25
pH 0.05 in the range from 20°C to
25°C
pH 0.01
0.1 mV
Maximum 80°C
≤ 1 sec for 95% of final value
Tolerance
±0.05% @ 0°C
±0.10% @ 0°C
The RTD resistance value can be computed as
RTD Resistance = RTD 0 (1 + T α )
where:
RTD Resistance = Resistance value at T
RTD0 = Resistance value at 0°C
T = ambient temperature
α = 0.00385 Ω/Ω/°C, temperature coefficient defined by DIN
Std. 43760-1980 and IEC 751-1983
The RTD resistance varies from 0°C (1000 Ω) to 100°C (1385 Ω),
producing a voltage signal range of 210 mV to 290 mV with
210 µA excitation current.
When reading the pH probe output voltage, the ADC uses the
external 1.05 V reference and is configured with a gain of 1. The
full-scale input range is ±VREF/G = ±1.05 V, and the maximum
signal from the pH probe is ±490 mV at 80°C.
Because the output of the sensor is bipolar, and the AD7793
operates from a single power supply, the signal generated by the
pH probe should be biased above ground so that it is within the
acceptable common-mode range of the ADC. This bias voltage
is generated by injecting the 210 µA IOUT2 current into the
5 kΩ, 0.1% resistor as shown in Figure 2. This generates the
1.05 V common-mode bias voltage that also serves as the ADC
reference voltage.
ADC Channel 2 Configuration, RTD
The second channel of the ADC monitors the voltage generated
across an RTD being driven by the IOUT2 current output pins
of the AD7793. The 210 μA excitation current drives the series
combination of the RTD and precision resistor (5 kΩ, 0.1%).
(See Figure 1).
The precision 5 kΩ resistor generates the 1.05 V used as an
external reference. With a gain of one, the analog input range is
±1.05 V (±VREF/G). This architecture gives a ratiometric
configuration. Changes in the value of the excitation current do
not affect the accuracy of the system.
Although 100 Ω Pt RTDs are popular, other resistances (200 Ω,
500 Ω, 1000 Ω, etc.) and materials (Nickel, Copper, Nickel Iron)
can be specified. This application uses a 1 kΩ DIN 43760 Class A
RTD for temperature compensation of the pH sensor. A 1000 Ω
RTD is less sensitive to wiring resistance errors than a 100 Ω RTD.
A 2-wire connection is made as shown in Figure 3. A constant
current is applied through the leads of the RTD, and the voltage
across the RTD itself is measured. The measuring device is the
AD7793 that exhibits high input impedance and low input bias
current. The sources of errors in this scheme are lead resistance,
the stability of the constant current source produced by AD7793,
and the input impedance and/or bias current in the input
amplifier, and the associated drift.
The temperature coefficient for pure platinum is 0.003926 Ω/Ω/°C.
The normal coefficient for industrial RTDs is 0.00385 Ω/Ω/°C
per the DIN Std. 43760-1980 and IEC 751-1983. The accuracy
of an RTD is usually stated at 0°C. The DIN 43760 standard
recognizes two classes as shown in Table 2, and ASTM E–1137
recognizes two grades as shown in Table 3.
Rev. 0 | Page 3 of 7
CN-0326
Circuit Note
210µA
IOUT2
10kΩ
AIN2(+)
1µF
RLEAD
Pt1000
RTD
DIN 43760
CLASS A
AD7793
P1
RLEAD
AIN2(–)
10kΩ
210µA
RFIN(+)/AIN3(+)
1µF
RFIN(–)/AIN3(–)
GND
11821-003
5kΩ, 0.1%
Figure 3. 2-Wire Pt RTD Connections (Simplified Schematic: All Connections
and Decoupling Not Shown)
11821-004
Another possibility for eliminating wiring resistance errors is
the 3-wire RTD configuration that is described in detail in
Circuit Note CN-0287.
Figure 4. Evaluation Software Calibration Settings Window
Output Coding
The user is required to use a minimum of two buffer solutions,
where a neutral pH buffer with a value of pH-7 should be used
to remove the offset introduced by the pH probe and by the system.
The neutral buffer solution can be used to set the first point for
calibration. The pH of the second buffer solution depends on
the pH of the solution to be measured. A pH-10 buffer solution
can be used when measuring alkaline base solutions, and a pH4 buffer can be used when measuring acidic solutions. For more
precise measurement, a three point calibration can be
performed. This can be done by using two different sets of
buffer solutions in Step 2 and in Step 3 as shown in Figure 4,
where the pH-7 solution is used to remove the offset.
The output code for an input voltage on either channel is

 AIN × GAIN
+1 
Code = 2 N –1 


V REF


where:
AIN is the analog input voltage.
GAIN is the in-amp setting.
N = 24
The EVAL-SDP-CB1Z system demonstration platform board
and the PC processes the data output from the AD7793.
Digital and Power Isolation
The ADuM5401 isolates the ADC digital signals and also
supplies isolated regulated 3.3 V power to the circuit. The input
to the ADuM5401 (VDD1) should be between 3.0 V and 3.6 V.
Take care with the layout of the ADuM5401 to minimize
EMI/RFI problems. For more details, please refer to Application
Note AN-1109, Recommendations for Control of Radiated
Emissions with iCoupler Devices.
System Calibration
In order to accurately measure the RTD resistance, the ±5%
variation in the IOUT2 current must be taken into account. The
AIN3(+) input to the AD7793 is used to measure the voltage
dropped across the precision 5 kΩ 0.1% resistor. The exact
IOUT2 current is then determined by dividing this voltage by 5 kΩ.
The RTD resistance is calculated by dividing the voltage across the
RTD by the exact IOUT2 current.
A two-point calibration procedure shown in Figure 4 is used to
calibrate the pH meter in the EVAL-CN0326-PMDZ evaluation
software.
The software includes a list of buffer solution recommended by
the NIST. Each buffer solution described in the list has its own
temperature coefficient from 0°C to 95°C, which can be found
in “pH Theory and Practice” by Radiometer Analytical. The
software uses this table to correlate the mV input from the pH
probe to the correct pH value that correspond to the temperature
read from the RTD sensor using linear interpolation to fill in
the gaps in the table. The user is given an option to enable/
disable the option for continuous temperature compensation by
clicking the green button as shown in Figure 4.
Buffer solutions are commonly found in the market for pH
sensor calibration. Other NIST-certified pH reference can also
be used for calibration. Because of the variety of buffer solutions
available, the software also provides the user an option to use
their desired NIST-certified pH reference for calibration as
shown in Figure 4.
The software also provides the user an option to use other RTD
resistance values, but by default it is set to 1000 Ω.
Rev. 0 | Page 4 of 7
CN-0326
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.
0.4
200MΩ
0.35
0.3
0.03
0.2
100MΩ
1MΩ
0.1
0.25
0.20
0
–0.1
0.15
–0.2
SIMULATED pH
OUTPUT VOLTAGE
0.10
–0.3
0.05
–0.4
–0.5
14
0
13
12
11
10
9
8
7
6
5
4
3
2
0
ADC OUTPUT pH READING (pH)
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.
11821-007
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
Figure 7. Photo of EVAL-CN0326-PMDZ Board
30
COMMON VARIATIONS
25
COUNTS
1
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)
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.
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.
20
15
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.
10
5
6FB900
6FB980
ADC CODE
6FBA00
6FBA58
Other families of ADuM5401 includes a variety of channel
configuration such as the ADuM5402/ADuM5403/ADuM5404
which also provides four independent isolation channels.
11821-005
0
6FB864
LINEARITY ERROR (%)
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
0.40
0.5
11821-006
System Noise Considerations
pH SENSOR SIMULATED OUTPUT VOLTAGE (mV)
Circuit Note
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.
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-PMDIB1Z 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 SDPPMD-IB1Z and the SDP, the EVAL-CN0326-PMDZ is
connected to the SDP-PMD-IB1Z by a standard 100 milspaced, 25 mil square, right angle pin-header connector.
Rev. 0 | Page 5 of 7
CN-0326
Circuit Note
Equipment Required
Functional Block Diagram
The following equipment is needed:
The functional block diagram of the test set-up is shown in
Figure 8. The test set-up should be connected as shown. A
screenshot of the main software window is shown in Figure 9.
A PC with a USB port and Windows® XP and Windows®
Vista (32-bit), or Windows® 7 (32-bit)
•
EVAL-CN0326-PMDZ circuit evaluation board
•
EVAL-SDP-CB1Z circuit evaluation board
•
SDP-PMD-IB1Z SDP interposer board
•
CN0326 Evaluation Software
•
Power supply: 6 V wall wart or equivalent
•
Yokogawa 2000 Precision DC Power Supply or
equivalent
Pt1000
RTD
1MΩ
PRECISION
VOLTAGE
SOURCE
6V SUPPLY
PC
SDP-PMD-IB1Z
EVAL-SDP-CB1Z
P1
EVAL-CN0326-PMDZ
J1
11821-008
•
Figure 8. pH Sensor Block Diagram Test Set-up
Getting Started
Load the evaluation software by placing the CN-0326 Evaluation
Software disc in the CD drive of the PC. Using "My Computer,"
locate the drive that contains the evaluation software disc and
open the Readme file. Follow the instructions contained in the
Readme file for installing and using the evaluation software.
Setup
The CN0326 evaluation kit includes self-installing software on a
CD. The software is compatible with Windows® XP (SP2) and Vista
(32-bit and 64-bit). If the setup file does not run automatically,
you can run the setup.exe file from the CD.
1.
After installation from the CD is complete, power up the
SDP-PMD-IB1Z evaluation board as described in the
Power Supplies Configuration section. Connect the SDP
board (via either Connector A) to the SDP-PMD-IB1Z
evaluation board and then to the USB port of the PC
that will be used for evaluation using the supplied cable.
2.
Connect the 12-pin right angle male pin header of the
EVAL-CN0326-PMDZ to the 12-pin right angle female
pin header of the SDP-PMD-IB1Z.
3.
Before running the program shown in Figure 9, connect
the pH probe at the BNC terminal and the RTD sensor
to the terminal jack of the EVAL-CN0326-PMDZ.
4.
After all the peripherals and power supply are all
connected and turned on, click Connect on the GUI
shown in Figure 9. When the evaluation system is
successfully detected by the PC, the EVAL-CN0326PMDZ circuit board can now be evaluated using the
software shown in Figure 9.
11821-009
Install the evaluation software before connecting the evaluation
board and SDP board to the USB port of the PC to ensure that
the evaluation system is correctly recognized when connected to
the PC.
Figure 9. Evaluation Software Main Window
Power Supply Configuration
The SDP-PMD-IB1Z must be supplied with 6 V dc power
supply and its jumper, JP1, should be set to 3.3 V to give power
to the EVAL-CN0326-PMDZ.
Rev. 0 | Page 6 of 7
Circuit Note
CN-0326
Test
Agilent E3631A and Yokogawa GS200 precision voltage supplies
were used to simulate the sensor output. The negative terminal
of the Yokogawa is connected to the negative terminal of the
ADC for the pH sensor. The positive terminal is in series with
the resistor, which is connected to the positive terminal of the
ADC as shown in Figure 8. The Yokogawa generates the
±420 mV, which then simulates the pH sensor output, and the
series resistor is then varied to simulate the impedance of the
glass electrode of the pH probe as shown in Figure 8.
Kester, Walt. 1999. Temperature Sensors. Section 7. Analog
Devices.
Chen, Baoxing. 2006. iCoupler® Products with isoPower®
Technology: Signal and Power Transfer Across Isolation Barrier
Using Microtransformers. Analog Devices.
Wayne, Scott. 2005. “iCoupler® Digital Isolators Protect RS-232,
RS-485, and CAN Buses in Industrial, Instrumentation, and
Computer Applications.” Analog Dialogue, Volume 39. Analog
Devices (October).
The CN-0326 Evaluation Software is used to capture the data
from the EVAL-CN0326-PMDZ circuit board using the setup
seen in Figure 8.
Brian Kennedy and Mark Cantrell, Recommendations for
Control of Radiated Emissions with iCoupler Devices,
Application Note AN-1109, Analog Devices.
Details regarding the use of the software can be found in the
CN-0326 Software User Guide.
pH Theory and Practice, Radiometer Analytical, SAS,
Villeurbanne Cedex, France.
Data Sheets and Evaluation Boards
LEARN MORE
AD7793 Data Sheet
CN-0326 Design Support Package:
www.analog.com/CN0326-DesignSupport
AD7793 Evaluation Board
MT-004 Tutorial, The Good, the Bad, and the Ugly Aspects of
ADC Input Noise—Is No Noise Good Noise? Analog Devices.
ADUM5401 Data Sheet
ADuM5401 Evaluation Board
MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC
Basics, Analog Devices.
AD8603 Data Sheet
MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC
Advanced Concepts and Applications, Analog Devices.
REVISION HISTORY
9/13—Revision 0: Initial Version
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND", Analog Devices.
MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and
Rail-to-Rail Issues. Analog Devices.
MT-037 Tutorial, Op Amp Input Offset Voltage.
MT-038 Tutorial, Op Amp Input Bias Current
MT-040 Tutorial, Op Amp Input Impedance
MT-095 Tutorial, EMI, RFI, and Shielding Concepts
MT-101 Tutorial, Decoupling Techniques, Analog Devices
Kester, Walt. 1999. High Impedance Sensors. Section 5. Analog
Devices.
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CN11821-0-9/13(0)
Rev. 0 | Page 7 of 7
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