CN-0359: Fully Automatic High Performance Conductivity Measurement System

Circuit Note
CN-0359
Devices Connected/Referenced
Circuits from the Lab® reference
designs 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/CN0359.
AD8253
10 MHz, 20 V/μs, G = 1, 10, 100,
1000, iCMOS Programmable Gain
Instrumentation Amplifier
ADuCM360
Low Power, Precision Analog
Microcontroller with Dual SigmaDelta ADCs, ARM Cortex-M3
ADA4627-1
30 V, High Speed, Low Noise, Low Bias
Current, JFET Operational Amplifier
AD8542
CMOS Rail-to-Rail GeneralPurpose Amplifiers
ADA4000-1
Low Cost, Precision JFET Input
Operational Amplifiers
ADP2300
1.2 A, 20 V, 700 kHz/1.4 MHz,
Nonsynchronous Step-Down
Regulator
ADA4638-1
30 V, Zero-Drift, Rail-to-Rail Output
Precision Amplifier
ADP1613
650 kHz/1.3 MHz, Step-Up PWM
DC-to-DC Switching Converters
ADA4528-2
Precision, Ultralow Noise, RRIO,
Dual, Zero-Drift Op Amp
ADG1211
Low Capacitance, Low Charge
Injection, ±15 V/+12 V, iCMOS
Quad SPST Switches
ADA4077-2
4 MHz, 7 nV/√Hz, Low Offset and
Drift, High Precision Amplifiers
ADG1419
2.1 Ω On Resistance, ±15 V/+12 V/
±5 V, iCMOS SPDT Switch
AD8592
CMOS, Single-Supply, Rail-to-Rail
Input/Output Operational
Amplifiers with Shutdown
ADM3483
3.3 V Slew Rate Limited, Half
Duplex, RS-485/RS-422
Transceivers
Fully Automatic High Performance Conductivity Measurement System
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
CN-0359 Circuit Evaluation Board (EVAL-CN0359-EB1Z)
Design and Integration Files
Schematics, Source Code, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 is a completely self-contained,
microprocessor controlled, highly accurate conductivity
measurement system ideal for measuring the ionic content of
liquids, water quality analysis, industrial quality control, and
chemical analysis.
A carefully selected combination of precision signal conditioning
components yields an accuracy of better than 0.3% over a
conductivity range of 0.1 µS to 10 S (10 MΩ to 0.1 Ω) with
no calibration requirements.
Automatic detection is provided for either 100 Ω or 1000 Ω
platinum (Pt) resistance temperature devices (RTDs), allowing
the conductivity measurement to be referenced to room
temperature.
The system accommodates 2- or 4-wire conductivity cells, and
2-, 3-, or 4-wire RTDs for added accuracy and flexibility.
The circuit generates a precise ac excitation voltage with
minimum dc offset to avoid a damaging polarization voltage on
the conductivity electrodes. The amplitude and frequency of the
ac excitation is user-programmable.
An innovative synchronous sampling technique converts the
peak-to-peak amplitude of the excitation voltage and current
to a dc value for accuracy and ease in processing using the dual,
24-bit Σ-Δ ADC contained within the precision analog
microcontroller.
The intuitive user interface is an LCD display and an encoder
push button. The circuit can communicate with a PC using an
RS-485 interface if desired, and operates on a single 4 V to 7 V
supply.
Rev. A
Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog
Devices engineers. Standard engineering practices have been employed in the design and
construction of each circuit, and their function and performance have been tested and verified in a lab
environment at room temperature. However, you are solely responsible for testing the circuit and
determining its suitability and applicability for your use and application. Accordingly, in no event shall
Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due
toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage)
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Fax: 781.461.3113
©2015 Analog Devices, Inc. All rights reserved.
CN-0359
Circuit Note
+3.3V
J3
1
AIN6
RX
AIN1
PWM1
+15V
ADC2
AIN2
PWM2
−15V
ADC3
AIN3
1kΩ
+0.115V
P0.0
U2WRI
AIN6
P2.0
U4WRI
P1.2
A0
A1
AIN9
R23
+VEXC
–VEXC G1 = 1, 10, 100, 1000
PWM0
+15V AD8253
A0, A1,
−
U2WRI
U15
430Ω
+
REF
2
V1P-P
YX
5
3
ADA4000-1
P2.1
AGND
EP
¼
ADG1211
ADG1419
1kΩ
ADuCM360
AIN5/IEXC
AIN8
36Ω
1
PWM0
AIN7/IEXC
AIN9
J5
−15V
U14
−
CONDUCTIVITY
CELL
+1.65V
−15V
470kΩ
−
1nF
IP-P
U22B
R47,1.0kΩ
G2 × VOUT2
G1 × VOUT1
ADA4638-1
+
C73
4.7µF
R19
7.5kΩ
U10
−
+3.3V
−
+1.65V
½ ADA4528-2
+
R29
ADA4638-1 7.5kΩ
+
−
U13
−
−15V
1k Ω
24Ω
U12A
+1.65V
PWM1
−
10nF
820pF
+15V
C84
4.7µF
ADA4638-1
−
–15V
1k Ω
+1.65V
+15V
ADC1
R52
1.2kΩ
R48
10nF
VOUT2 =
G1 × 0.16 × V2P-P
820pF
ADA4638-1 7.5kΩ
−
U21
−
24Ω
½ ADA4528-2
+
1nF
+
PWM2
+3.3V
−
U20B
R42
430Ω
C93
4.7µF
R37
7.5kΩ
U16
¼
ADG1211
+
U22A
ADC2
½ ADA4528-2
+
R38
1.2kΩ
+
+15V
+1.65V
VOUT1 =
G1 × 0.16 × V1P-P
820pF
+15V
ADC3
10nF
R31
1.2kΩ
1nF
R41
430Ω
G2 = 1, 10, 100, 1000
× 1mS
24Ω
U12B
–15V
1nF
−15V
YX =
820pF
+15V
¼
ADG1211
+15V AD8253
A0, A1,
U19
U4WRI
ADA4627-1
+
V2P-P U18
+
430Ω
−15V
−
REF
−
R34
430Ω
½ AD8542
1nF
+VEXC
R20
1.2kΩ
R36
430Ω
PWM2
POWER
SUPPLIES
+1.65V
−VEXC
¼
ADG1211
+
+3.3V
128 × 64
LCD
MCCOG128064A6S-SPTLY
1k Ω
+
4
J1
+4V TO +7V
DAC
C50
PWM1 4.7µF
+3.3V
+15V
S1
ENCODER
PEC11R-S
AIN0
AIN5/IEXC
4
J4
J-LINK
ADC1
+3.3V
3
J2
ADM3483
RS-485
ADC0
AIN7/IEXC
AIN8
2
AVDD
24Ω
ADC0
U20A
–15V
1k Ω
½ AD8542
+1.65V
½ ADA4528-2
+
10nF
12970-001
100Ω OR
1000Ω
Pt RTD
R13
1.5kΩ
1nF
Figure 1. High Performance Conductivity Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown)
CIRCUIT DESCRIPTION
R14
110kΩ
The +VEXC and −VEXC voltages are generated by the
ADA4077-2 op amps (U9A and U9B), and their amplitudes are
controlled by the DAC output of the ADuCM360, as shown in
Figure 2.
1nF
R15
15kΩ
+15V
−
DAC
FROM
ADuCM360
0V TO +1.2V
R16
13kΩ
G = 8.33
U9A
+
½ ADA4077-2
0V TO +10V
R27
27kΩ
1nF
R24
27kΩ
−
R22
13kΩ
−VEXC
U9B
+
½ ADA4077-2
−15V
Figure 2. Excitation Voltage Sources
Rev. A | Page 2 of 10
+VEXC
0V TO −10V
12970-002
The excitation square wave for the conductivity cell is generated
by switching the ADG1419 between the +VEXC and −VEXC
voltages using the PWM output of the ADuCM360
microcontroller. It is important that the square wave has a
precise 50% duty cycle and a very low dc offset. Even small
dc offsets can damage the cell over a period of time.
Circuit Note
CN-0359
The ADG1419 is a 2.1 Ω, on-resistance SPDT analog switch
with an on-resistance flatness of 50 mΩ over a ±10 V range,
making it ideal for generating a symmetrical square wave from
the ±VEXC voltages. The symmetry error introduced by the
ADG1419 is typically 50 mΩ ÷1 kΩ = 50 ppm. Resistor R23
limits the maximum current through the sensor to 10 V/1 kΩ =
10 mA.
The voltage applied to the cell, V1, is measured with the
AD8253 instrumentation amplifier (U15). The positive input
to U15 is buffered by the ADA4000-1 (U14). The ADA4000-1
is chosen because of its low bias current of 5 pA to minimize
the error in measuring low currents associated with low
conductivities. The negative input of the AD8253 does not
require buffering.
The offset voltages of U14 and U15 are removed by the
synchronous sampling stage and do not affect the measurement
accuracy.
U15 and U18 are AD8253 10 MHz, 20 V/µs, programmable
gain (G = 1, 10, 100, 1000) instrumentation amplifiers with gain
error of less than 0.04%. The AD8253 has a slew rate of 20 V/µs
and a settling time of 1.8 µs to 0.001% for G = 1000. Its
common-mode rejection is typically 120 dB.
The U19 (ADA4627-1) stage is a precision current to voltage
converter that converts the current through the sensor to a
voltage. The ADA4627-1 has an offset voltage of 120 µV
(typical, A grade), a bias current of 1 pA (typical), a slew rate of
40 V/µs, and a 550 ns settling time to 0.01%. The low bias
current and offset voltage make it ideal for this stage. The
symmetry error produced by the 120 µV offset error is only
120 µV/10 V = 12 ppm.
The U22A and U22B (AD8542) buffers supply the 1.65 V
reference to the U18 and U15 instrumentation amplifiers,
respectively.
The following is a description of the remainder of the signal path in
the voltage channel (U17A, U17B, U10, U13, U12A, and U12B).
The operation of the current channel (U17C, U17D, U16, U21,
U20A, and U20B) is identical.
The ADuCM360 generates the PWM0 square wave switching
signal for the ADG1419 switch as well as PWM1 and PWM2
synchronizing signals for the synchronous sampling stages.
The cell voltage and the three timing waveforms are shown in
Figure 3.
+VEXC
CELL VOLTAGE
0
–VEXC
PWM0
PWM1
TRACK −V
TRACK −V
HOLD –V
20µs
500ns
PWM2
TRACK +V
HOLD +V
20µs
HOLD +V
500ns
12970-003
The ADA4077-2 has a typical offset voltage of 15 µV (A grade),
a 0.4 nA bias current, a 0.1 nA offset current, and an output
current of up to ±10 mA, with a dropout voltage of less than 1.2
V. The U9A op amp has a closed-loop gain of 8.33 and converts
the ADuCM360 internal DAC output (0 V to 1.2 V) to the
+VEXC voltage of 0 V to 10 V. The U9B op amp inverts the
+VEXC and generates the −VEXC voltage. R22 is chosen such
that R22 = R24||R27 to achieve first-order bias current
cancellation. The error due to the 15 µV offset voltage of U9A is
approximately (2 × 15 µV) ÷ 10 V = 3 ppm. The primary error
introduced by the inverting stage is therefore the error in the
resistor matching between R24 and R27.
Figure 3. Cell Voltage and Track-and-Hold Timing Signals
The output of the AD8253 in amp (U15) drives two parallel
track-and-hold circuits composed of ADG1211 switches
(U17A/U17B), series resistors (R34/R36), hold capacitors
(C50/C73), and unity-gain buffers (U10/U13).
The ADG1211 is a low charge injection, quad SPST analog
switch, operating on a ±15 V power supply with up to ±10 V
input signals. The maximum charge injection due to switching
is 4 pC, which produces a voltage error of only 4 pC ÷ 4.7 µF =
0.9 µV.
The PWM1 signal causes the U10 track-and-hold buffer to
track the negative cycle of the sensor voltage and then hold it
until the next track cycle. The output of the U10 track-and-hold
buffer is therefore a dc level corresponding to the negative
amplitude of the sensor voltage square wave.
Similarly, the PWM2 signal causes the U13 track-and-hold
buffer to track the positive cycle of the sensor voltage and then
hold it until the next track cycle. The output of the U13 trackand-hold buffer is therefore a dc level corresponding to the
positive amplitude of the sensor voltage square wave.
The bias current of the track-and-hold buffers (ADA4638-1) is
45 pA typical, and the leakage current of the ADG1211 switch
is 20 pA typical. Therefore, the worst-case leakage current on
the 4.7 µF hold capacitors is 65 pA. For a 100 Hz excitation
frequency, the period is 10 ms. The drop voltage over one-half
the period (5 ms) due to the 65 pA leakage current is (65 pA ×
5 ms) ÷ 4.7 µF = 0.07 µV.
The offset voltage of the ADA4638-1 zero-drift amplifier is only
0.5 µV typical and contributes negligible error.
The final stages in the signal chain before the ADC are the
ADA4528-2 inverting attenuators (U12A and U12B) that have
a gain of −0.16 and a common-mode output voltage of +1.65 V.
The ADA4528-2 has an offset voltage of 0.3 µV typical and
therefore contributes negligible error.
Rev. A | Page 3 of 10
CN-0359
Circuit Note
The attenuator stage reduces the ±10 V maximum signal to
±1.6 V with a common-mode voltage of +1.65 V. This range is
compatible with the input range of the ADuCM360 ADC input,
which is 0 V to 3.3 V (1.65 V ± 1.65 V) for an AVDD supply of
3.3 V.
The attenuator stages also provide noise filtering and have a
−3 dB frequency of approximately 198 kHz.
The differential output of the voltage channel, VOUT1, is
applied to the AIN2 and AIN3 inputs of the ADuCM360. The
differential output of the current channel, VOUT2, is applied to
the AIN0 and AIN1 inputs of the ADuCM360.
The equations for the two outputs are given by
VOUT1 = G1 × 0.16 × V1P-P
(1)
VOUT2 = G2 × 0.16 × V2P-P
(2)
The cell current is given by
IP-P = V1P-P × YX
(3)
System Accuracy Measurements
The following four resistors affect the accuracy in the VOUT1
voltage channel: R19, R20, R29, and R31.
The following five resistors affect the accuracy in the VOUT2
current channel: R47, R37, R38, R48, and R52.
Assuming that all nine resistors are 0.1% tolerance, and
including the 0.04% gain error of the AD8253, a worst-case
error analysis yields approximately 0.6%. The analysis is
included in the CN-0359 Design Support Package.
In practice, the resistors are more likely to combine in an RSS
manner, and the RSS error due to the resistor tolerances in the
positive or negative signal chain is √5 × 0.1% = 0.22%.
Accuracy measurements were taken using precision resistors
from 1 Ω to 1 MΩ (1 S to 1 µS) to simulate the conductivity cell.
Figure 4 shows the results, and the maximum error is less than
0.1%.
0.04
The V2P-P voltage is given by
V2P-P = IP-P × R47
0.02
(4)
Solving Equation 4 for IP-P and substituting into Equation 3
yields the following for YX:
V2P − P
YX =
V1P − P × R 47
(5)
ERROR (%)
0
Solving Equation 1 and Equation 2 for V1P-P and V2P-P and
substituting into Equation 5 yields the following:
YX =
–0.06
–0.08
G2 × VOUT2
(6)
G1 × VOUT1 × R 47
G2 × VOUT2
G1 × VOUT1
× 1 mS
(7)
The AD8253 gain error (G1 and G2) is 0.04% maximum, and
R47 is chosen to be a 0.1% tolerance resistor.
From this point, the resistors in the VOUT1 and VOUT2 signal
chain determine the overall system accuracy.
The software sets the gain of each AD8253 as follows:
•
–0.10
1µ
Equation 7 shows that the conductivity measurement depends
on G1, G2, and R47, and the ratio of VOUT2 to VOUT1.
Therefore, a precision reference is not required for the ADCs
within the ADuCM360.
•
–0.04
If the ADC code is over 93.2% of full scale, the gain of the
AD8253 is reduced by a factor of 10 on the next sample.
If the ADC code is less than 9.13% of full scale, the gain of
the AD8253 is increased by a factor of 10 on the next sample.
10µ
0.1m
1m
10m
CONDUCTIVITY (S)
0.1
1
12970-004
YX =
–0.02
Figure 4. System Error (%) vs. Conductivity of 1 µS to 1 S
RTD Measurement
Conductivity measuring system accuracy is only as good as
its temperature compensation. Because common solution
temperature coefficients vary in the order of 1%/°C to 3%/°C
or more, measuring instruments with adjustable temperature
compensation must be used. Solution temperature coefficients
are somewhat nonlinear and usually vary with the actual
conductivity, as well. Therefore, calibration at the actual
measuring temperature yields the best accuracy.
The ADuCM360 contains two matched, software configurable,
excitation current sources. They are individually configurable
to provide a current output of 10 µA to 1 mA, and matching is
better than 0.5%. The current sources allow the ADuCM360
to easily perform 2-wire, 3-wire, or 4-wire measurements for
either Pt100 or Pt1000 RTDs. The software also automatically
detects if the RTD is Pt100 or Pt1000 during the setup procedure.
The following discussion shows simplified schematics of how
the different RTD configurations operate. All mode switching is
accomplished in the software, and there is no need to change
the external jumper settings.
Rev. A | Page 4 of 10
Circuit Note
CN-0359
The 3-wire connection is another popular RTD configuration
that eliminates lead resistance errors, as shown in Figure 6.
Figure 5 shows the configuration for 4-wire RTDs.
IEXC
V7 – V8
TO ADC
– VP +
RP
AIN8
RP
2
AIN6
RX
RP
2
RX
+3.3V
V6 – V5
V7 – V8
+0.115V
– VP +
RP
AIN5/
IEXC
3
1kΩ
36Ω
× 1.5kΩ
RP
RX =
Figure 5. Configuration for 4 Wire RTD Connection
The parasitic resistance in each of the leads to the remote RTD
is shown as RP. The excitation current (IEXC) passes through
a precision 1.5 kΩ resistor and the RTD. The on-chip ADC
measures the voltage across the resistor (V7 − V8).
It is important that the R13 resistor and the IEXC excitation
current value be chosen such that the ADuCM360 maximum
input voltage at AIN7 does not exceed AVDD − 1.1 V; otherwise,
the IEXC current source does not function properly.
The RTD voltage is accurately measured using the two sense
leads that connect to AIN6 and AIN5. The input impedance is
approximately 2 MΩ (unbuffered mode, PGA gain = 1), and the
current flowing through the sense lead resistance produces
minimum error. The ADC then measures the RTD voltage
(V6 − V5).
V8 – V5
V7 – V8
4
V6 − V5
V7 − V8
× 1.5 kΩ
+0.115V
36Ω
Figure 6. Configuration for 3-Wire RTD Connection
The second matched IEXC current source (AIN5/IEXC)
develops a voltage across the lead resistance in series with
Terminal 3 that cancels the voltage dropped across the lead
resistance in series with Terminal 1. The measured V8 − V5
voltage is therefore free of lead resistance error.
Figure 7 shows the 2-wire RTD configuration where there is no
compensation for lead resistance.
RP
100Ω OR
1000Ω
Pt RTD
1
R13
1.5kΩ
0.1%
J3
ADuCM360
AIN7/
IEXC
IEXC
V7 – V8
TO ADC
AIN8
2
AIN6
3
AIN5/
IEXC
RX
V8 – V9
TO ADC
+3.3V
(8)
1kΩ
RP
The measurement is ratiometric and does not depend on an
accurate external reference voltage, only the tolerance of the
1.5 kΩ resistor. In addition, the 4-wire configuration eliminates
the error associated with the lead resistances.
The ADuCM360 has a buffered or unbuffered input option. If
the internal buffer is activated, the input voltage must be greater
than 100 mV. The 1 kΩ/36 Ω resistor divider provides a 115 mV
bias voltage to the RTD that allows buffered operation. In the
unbuffered mode, Terminal 4 of J3 can be grounded and
connected to a grounded shield for noise reduction.
AIN9
× 1.5kΩ
The RTD resistance is then calculated as
RX =
IEXC
+3.3V
AIN9
12970-005
RX =
4
V7 – V8
TO ADC
AIN6
1kΩ
RP
IEXC
V8 – V5
TO ADC
AIN5/
IEXC
3
ADuCM360
AIN7/
IEXC
AIN8
100Ω OR
1000Ω
Pt RTD
V6 – V5
TO ADC
1
R13
1.5kΩ
0.1%
J3
12970-006
1
ADuCM360
AIN7/
IEXC
RX =
V8 – V9
V7 – V8
4
+0.115V
AIN9
36Ω
× 1.5kΩ – 2R P
12970-007
RP
100Ω OR
1000Ω
Pt RTD
R13
1.5kΩ
0.1%
J3
Figure 7. Configuration for 2-Wire RTD Connection
The 2-wire configuration is the lowest cost circuit and is suitable
for less critical applications, short RTD connections, and higher
resistance RTDs such as Pt1000.
Rev. A | Page 5 of 10
CN-0359
Circuit Note
Conductivity Theory
the temperature, a second temperature sensor and compensation
network must be used.
The resistivity, ρ, of a material or liquid is defined as the
resistance of a cube of the material with perfectly conductive
contacts on opposite faces. The resistance, R, for other shapes
can be calculated by
R = ρL/A
(9)
where:
L is the distance between the contacts.
A is the area of the contacts.
Resistivity is measured in units of Ω cm. A 1 Ω cm material has
a resistance of 1 Ω when contacted on opposite faces of a 1 cm ×
1 cm × 1 cm cube.
Conductance is the reciprocal of resistance, and conductivity
is the reciprocal of resistivity. The unit of measurement of
conductance is Siemens (S), and the unit of measurement of
conductivity is S/cm, mS/cm, or µS/cm.
All aqueous solutions conduct electricity to some degree.
Adding electrolytes such as salts, acids, or bases to pure water
increases the conductivity (and decreases resistivity).
For the purposes of this circuit note, Y is the general symbol for
conductivity measured in S/cm, mS/cm, or µS/cm. However, in
many cases, the distance term is dropped for convenience, and
the conductivity is simply expressed as S, mS, or µS.
A conductivity system measures conductivity by means of
electronics connected to a sensor called a conductivity cell
immersed in a solution, as shown in Figure 8.
The contacting-type sensor typically consists of two electrodes
that are insulated from one another. The electrodes, typically
Type 316 stainless steel, titanium palladium alloy, or graphite,
are specifically sized and spaced to provide a known cell
constant. Theoretically, a cell constant of 1.0/cm describes two
electrodes, each sized 1 cm2 in area, and spaced 1 cm apart. Cell
constants must be matched to the measurement system for a
given range of operation. For instance, if a sensor with a cell
constant of 1.0/cm is used in pure water with a conductivity of
1 µS/cm, the cell has a resistance of 1 MΩ. Conversely, the same
cell in seawater has a resistance of 30 Ω. Because the resistance
ratio is so large, it is difficult for ordinary instruments to
accurately measure such extremes with only one cell constant.
When measuring the 1 µS/cm solution, the cell is configured with
large area electrodes spaced a small distance apart. For example,
a cell with a cell constant of 0.01/cm results in a measured cell
resistance of approximately 10,000 Ω rather than 1 MΩ. It is
easier to accurately measure 10,000 Ω than 1 MΩ; therefore, the
measuring instrument can operate over the same range of cell
resistance for both ultrapure water and high conductivity seawater
by using cells with different cell constants.
The cell constant, K, is defined as the ratio of the distance between
the electrodes, L, to the area of the electrodes, A:
K = L/A
(10)
The instrumentation then measures the cell conductance, Y:
Y = I/V
(11)
The conductivity of the liquid, YX, is then calculated:
YX = K × Y
There are two types of conductivity cells: those with two electrodes,
and those with four electrodes, as shown in Figure 9. The
electrodes are often referred to as poles.
TO I/V
CONVERTER
V
2-POLE
EVAL-CN0359-EB1Z
I
A
L
4-POLE
A = AREA OF ELECTRODE SURFACE (cm2)
L = DISTANCE BETWEEN ELECTRODES (cm)
V = EXCITATION VOLTAGE
I = CELL CURRENT
K = L/A = CELL CONSTANT (cm–1)
Y = MEASURED CONDUCTANCE = I/V (S)
YX = WATER CONDUCTIVITY = K × Y (S/cm)
12970-008
A
(12)
The electronic circuitry impresses an alternating voltage on the
sensor and measures the size of the resulting current, which is
related to the conductivity. Because conductivity has a large
temperature coefficient (up to 4%/°C), an integral temperature
sensor is incorporated into the circuitry to adjust the reading to
a standard temperature, usually 25°C (77°F). When measuring
solutions, the temperature coefficient of the conductivity of the
water itself must be considered. To compensate accurately for
12970-009
Figure 8. Interface Between Conductivity Cell and EVAL-CN0359-EB1Z
Figure 9. 2-Pole and 4-Pole Conductivity Cells
The 2-pole sensor is more suitable for low conductivity
measurements, such as purified water, and various biological
and pharmaceutical liquids. The 4-pole sensor is more suitable
the high conductivity measurements, such as waste water and
seawater analysis.
Rev. A | Page 6 of 10
Circuit Note
CN-0359
Power Supply Circuits
The cell constants for 2-pole cells range from approximately
0.1/cm to 1/cm, and the cell constants for 4-pole cells range
from 1/cm to 10/cm.
To simplify system requirements, all the required voltages
(±15 V and +3.3 V) are generated from a single 4 V to 7 V
supply, as shown in Figure 10.
The 4-pole cell eliminates the errors introduced by polarization
of the electrodes and field effects that can interfere with the
measurement.
The ADP2300 buck regulator generates the 3.3 V supply for the
board. The design is based on the downloadable ADP230x Buck
Regulator Design Tool.
The actual configuration of the electrodes can be that of parallel
rings, coaxial conductors, or others, rather than the simple
parallel plates shown in Figure 8.
The ADP1613 boost regulator generates a regulated +15 V
supply and an unregulated −15 V supply. The −15 V supply is
generated with a charge pump. The design is based on the
ADP161x Boost Regulator Design Tool.
Regardless of the type of cell, it is important not to apply a
dc voltage to any electrode, because ions in the liquid will
accumulate on the electrode surface, thereby causing
polarization, measurement errors, and damage to the electrode.
Details regarding the selection and design of power supplies are
available at www.analog.com/ADIsimPower.
Take special care with sensors that have shields, as in the case of
coaxial sensors. The shield must be connected to the same potential
as the metal container holding the liquid. If the container is
grounded, the shield must be connected to Pin 5 of J5, the
circuit board ground.
Use proper layout and grounding techniques to prevent the
switching regulator noise from coupling into the analog circuits.
See the Linear Circuit Design Handbook, the Data Conversion
Handbook, the MT-031 Tutorial, and the MT-101 Tutorial for
further details.
The final precaution is not to exceed the rated excitation voltage
or current for the cell. The CN-0359 circuit allows programmable
excitation voltages from 100 mV to 10 V, and the R23 (1 kΩ)
series resistor limits the maximum cell current to 10 mA.
100nF
+4V TO +7V
BST
VIN
22µH
+3.3V
+3.3V
EMI
FILTER
SW
EN
B120
ADP2300
10kΩ
100nF 10µF
22µF 100nF
FB
+0.8V
GND
7.5kΩ
+1.65V
10kΩ
2.4kΩ
−15V UNREGULATED
½ BAT54S
22µF
−15V
EMI
FILTER
100nF
22µH
10µF
47µF
SW
VIN
100nF
½ BAT54S
1N4148
+15V REGULATED
EMI
FILTER
+15V
EN
200kΩ
ADP1613
FREQ
FB
33nF
SS
COMP
GND
+1.235V
18kΩ
EMI FILTERS:
BNX025H01
56pF
3.3nF
47kΩ
Figure 10. Power Supply Circuits
Rev. A | Page 7 of 10
12970-010
100nF
CN-0359
Circuit Note
Figure 11 shows the LCD backlight driver circuit.
After connecting, the conductivity cell and the RTD the board
are powered up. The LCD screen appears as shown in Figure 12.
+3.3V
Push the encoder knob to enter the setting menu, and then
input the EXC Voltage, EXC Frequency, TEMP Coefficient,
and Cell Constant, as shown in Figure 13.
100nF
36kΩ
+3.3V 6.8Ω
400mV
+
U6B
−
K1
K2
+
U6A
−
½ AD8592
½ AD8592
A1
60mA
+3.3V
6.8Ω +3.3V
400mV
12970-011
+3.3V
A2
LCD DISPLAY
60mA
SETTING SCREEN
Figure 11. LCD Backlight Drivers
Each half of the AD8592 op amp acts as a 60 mA current source
to supply the LCD backlight currents. The AD8592 can source and
sink up to 250 mA, and the 100 nF capacitor ensures a soft startup.
Software Operation and User Interface
The EVAL-CN0359-EB1Z comes preloaded with the code
required to make the conductivity measurements. The code
can be found in the CN-0359 Design Support Package, in the
CN0359-SourceCode.zip file.
The CN-0359 user interface is intuitive and easy to use. All
user inputs are from a dual function push button/rotary
encoder knob. The encoder knob can be turned clockwise or
counterclockwise (no mechanical stop), and can also be used
as a push button.
Figure 12 is a photo of the EVAL-CN0359-EB1Z board that
shows the LCD display and the position of the encoder knob.
SETTING EXCITATION VOLTAGE
Figure 13. LCD Display Screens
Rotating the knob moves the cursor up and down through the
various parameters.
Set the cursor to EXC Voltage and push the knob until it clicks.
Position the cursor over the first digit in the number to be set
by rotating the knob. Push the button, and the cursor blinks.
Change the number by rotating the knob, and push the knob
when the desired number is reached. After setting all the digits
in the number, position the cursor on Save and push the button
to save the setting.
Continue the process and set the EXC Frequency, TEMP
Coefficient, and Cell Constant.
After setting all the constants, select RETURN TO HOME and
push the knob. The system is now ready to make measurements.
If numbers are entered that are outside the allowable range, the
buzzer sounds.
If the conductivity cell is incorrectly connected, the screen
displays Sensor Incorrect.
If the RTD is incorrectly connected, the screen displays RTD
Incorrect use 25°C. The system can still make measurements
without the RTD connected, but uses 25°C as the compensation
temperature.
Ensure that the RTD is connected before power is applied
so that the system can detect the resistance value of the RTD
(Pt100 or Pt1000) and the configuration (2-wire, 3-wire, or
4-wire).
12970-012
COMMON VARIATIONS
Figure 12. Photo of EVAL-CN0359-EB1Z Board Showing Home Screen in
Measurement Mode
12970-013
5.1kΩ
+2.9V
The system shown in the CN-0359 uses the ADuCM360
precision analog microcontroller for a highly integrated
conductivity measurement.
If the user desires a discrete ADC, the AD7794 24-bit, Σ-Δ
ADC is a good choice.
Rev. A | Page 8 of 10
Circuit Note
CN-0359
CIRCUIT EVALUATION AND TEST
3.
This circuit uses the EVAL-CN0359-EB1Z circuit board, an
external power supply, conductivity cell, and an RTD.
4.
Equipment Needed
The following equipment is needed:
The EVAL-CN0359-EB1Z circuit board
A 6 V power supply or wall wart (EVAL-CFTL-6V-PWRZ)
A conductivity cell
Pt100 or Pt1000 2-wire, 3-wire, or 4-wire RTD (if the RTD
is not connected, the conductivity measurement is referenced
to 25°C)
Setup
Connectivity for Prototype Development
Take the following steps to set up the circuit for evaluation:
1.
2.
6.
Connect the conductivity cell as follows:
a. 4-wire cell: connect an outside current electrode to J5
Pin 1 and the closest inner voltage electrode to J5 Pin 2.
Connect the second outside current electrode to J5
Pin 4 and the closest inner voltage electrode to Pin 3.
b. 2-wire cell: connect one electrode to J5 Pin 1 and Pin 2
and connect the second electrode to J5 Pin 3 and Pin 4.
c. If the conductivity cell has a shield, connect it to J5 Pin 5.
Connect the RTD as follows (if used):
a. 4-wire RTD (see Figure 5): connect the positive
current excitation wire to J3 Pin 1 and the positive
voltage sense wire to J3 Pin 2. Connect the negative
current excitation wire to J3 Pin 4 and the negative
voltage sense wire to J3 Pin 3.
b. 3-wire RTD (see Figure 6): connect the positive
current excitation wire to J3 Pin 1. Connect the
negative current excitation wire to J3 Pin 4. Connect
the negative voltage sense wire to J3 Pin 3.
c. 2-wire RTD (see Figure 7): connect one RTD wire to
J3 Pin 1 and the other wire to J3 Pin 4.
d. If the RTD wires are shielded, connect the shield to J5
Pin 5.
The EVAL-CN0359-EB1Z is designed to be powered with the
EVAL-CFTL-6V-PWRZ 6 V power supply. The EVAL-CN0359EB1Z requires only the power supply and the external
conductivity cell and RTD for operation.
The EVAL-CN0359-EB1Z also has an RS-485 connector, J2,
that allows an external PC to interface with the board.
Connector J4 is a JTAG interface for programming and
debugging the ADuCM360.
Figure 14 is a typical PC connection diagram showing an
RS-485 to RS-232 adapter.
Rev. A | Page 9 of 10
6V
SUPPLY
PC
J1
CONDUCTIVITY
CELL
J5
J2
RS-485 TO
RS-232
ADAPTER
EVAL-CN0359-EB1Z
RTD
J3
J4
JTAG
Figure 14. Test Setup Functional Diagram
12970-014
•
•
•
•
5.
Connect the 6 V power supply (EVAL-CFTL-6V-PWRZ)
to J1 of the EVAL-CN0359-EB1Z circuit board.
Turn on the power by connecting the EVAL-CFTL-6VPWRZ, and then push the button on the EVAL-CN0359EB1Z circuit board.
Follow the procedure previously described in the Software
Operation and User Interface section and enter the
following parameters: EXC Voltage, EXC Frequency,
TEMP Coefficient, and Cell Constant.
Return to the main screen and wait for the ADuCM360 to
flush the buffers and display the conductivity and the
temperature. If the screen shows an error and the buzzer
beeps more than 20 times, check the sensor connections.
CN-0359
Circuit Note
LEARN MORE
Data Sheets and Evaluation Boards
CN-0359 Design Support Package:
www.analog.com/CN0359-DesignSupport
AD8253 Data Sheet
ADIsimPower Design Tool. Analog Devices
ADA4000-1 Data Sheet
Linear Circuit Design Handbook. Analog Devices/Elsevier.
ADA4638-1 Data Sheet
Op Amp Applications Handbook. Analog Devices/Elsevier.
ADA4528-2 Data Sheet
The Data Conversion Handbook. Analog Devices/Elsevier.
ADA4077-2 Data Sheet
A Designer's Guide To Instrumentation Amplifiers, 3rd Edition.
Analog Devices.
AD8592 Data Sheet
MT-031 Tutorial. Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”. Analog Devices.
ADuCM360 Data Sheet
ADA4627-1 Data Sheet
MT-101 Tutorial. Decoupling Techniques. Analog Devices.
"Section 7: Temperature Sensors" in Sensor Signal Conditioning.
Analog Devices.
AD8542 Data Sheet
ADP2300 Data Sheet
ADP1613 Data Sheet
ADG1211 Data Sheet
ADG1419 Data Sheet
ADM3483 Data Sheet
REVISION HISTORY
11/15—Rev. 0 to Rev. A
Change to Setup Section ...................................................................9
1/15—Revision 0: Initial Version
(Continued from first page) Circuits from the Lab reference designs are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its
licensors. While you may use the Circuits from the Lab reference designs in the design of your product, no other license is granted by implication or otherwise under any patents or
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notice but is under no obligation to do so.
©2015 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN12970-0-11/15(A)
Rev. A | Page 10 of 10
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