CN-0411

Circuit Note

Rev. 0 | Page 4 of 12

Node B and Node C are constantly switched to impose a bipolar

square wave across RCOND. A multiplexer switches between

different gain resistors in Node A.

The applied excitation voltage at Node A to the voltage divider

is generated using AD5683R, a 16-bit SPI voltage digital-to-analog

converter (DAC). This allows the magnitude of the square wave

signal applied to the voltage divider to be user-configurable.

Choose the excitation voltage to maximize the signal while not

exceeding the probe’s ratings. By default, the software applies

0.4 V excitation. The AD5683R is also by default the source of

the system’s 2.5 V reference voltage, but can also be configured

to accept an external reference voltage.

Figure 4. Multiplexing Gain Resistors

Figure 4 shows the gain setting resistors and switch, where R1 =

20 Ω, R2 = 200 Ω, R3 = 2 kΩ, R4 = 20 kΩ, R5 = 200 kΩ, R6 =

2 MΩ, R7 = 20 MΩ and P1, P2, and P3 are GPIO outputs from

the AD7124-8.

The circuit has a usable conductance range of 1 µS to 1 S. The

voltage divider of the conductivity cell scales through these ranges

by switching between seven gain resistors using ADG1608 as

shown in Figure 4. The ADG1608 is an 8-channel multiplexer

with a typical on-resistance of 12.5 Ω when operating at a single

5 V supply. This on-resistance is significant when the conductivity

measurement is in the 20 to 200 Ω range. Pin S2 and Pin S3 of

the ADG1608, which connect to the 20 Ω and 200 Ω gain resistors,

respectively, are also connected to two input channels of the

analog-to-digital converter (ADC). The system can also be

configured to perform an initial calibration for measurement

errors in the 20 and 200 Ω range. Shown in Figure 5 is a 3-option

(6-pin) jumper selection header (P5), which connects to 20 Ω

and 200 Ω precision resistors. Shorting Pin 1 and Pin 2 configures

the system to measure the signal across the conductivity cell while

shorting Pin 3 and Pin 4 or Pin 5 and Pin 6 configures the

system to measure the signal across the precision resistors.

Figure 5. Conductivity Cell Connection

The conductivity cell is switched to impose a bipolar signal

across it using an ADG884 as shown in Figure 6. The ADG884

has 0.5 Ω typical on-resistance and is operating at a single 3.3 V

supply. The switching is controlled by a PWM signal from the

microcontroller board. The frequency of this signal is user-

configurable to 94 Hz for low conductivity measurements and

2.4 kHz for high conductivity measurements.

Figure 6. Conductivity Cell Switcher to Simulate Bipolar Pulses Across the

Conductivity Cell Using ADG884

The signal across the conductivity cell is amplified by a gain of 10

using the AD8220 low input bias current instrumentation

amplifier operating at a single 5 V supply with input signals up

to 0.25 V as shown in Figure 7. There is also a user-configurable

jumper selector P6, which provides for a system zero-scale

calibration.

Figure 7. AD8220 Instrumentation Amplifier

The output of the instrumentation amplifier passes through two

parallel sample-and-hold circuits. As shown in Figure 8, the

sampling of the AD8220 output is controlled by the ADG836, a

dual SPDT switch, which has low charge-injection and is operating

at a single 3.3 V supply with input signals of up to 2.5 V.

Figure 8. Sample-and-Hold Design Using ADG836 and AD8628

A0

A1

A2

P1

P2

P3

S1

S2

S3

S4

S5

S6

S7

S8

D

B

TO CONDUCTIVITY CELL SWITCHER

AND INSTRUMENTATION AMPLIFIER

VDD

VSS/GND

VEXC

3.3VA

EN

U5

ADG1608

GAIN

RESISTORS

CONDUCTIVITY PROBE

BNC CONNECTOR

2

4

6

1

PRB+

P5

PRB–

200Ω

20Ω

3

5

D1

D2

S1A

B

TO GAIN MUTIPLEXER

AND

INSTRUMENTATION AMPLIFIER

C

S1B

S2A

S2B

IN1

IN2

VDD GND

3.3VA

U5

ADG884

PRB+

PRB–

PWM1

GAIN = 10

–

+

REF

5V

A1

AD8220

B

SAMPLE-AND-HOLD

INPUT SIGNAL

C

–

+

D1

S1A

S1B

S2A

S2B

D2

IN1

IN2

–

+

VDD

GND

3.3VA

3.3VA

3.3VA

PWM2

INSTRUMENTATION

AMPLIFIER

OUTPUT

SIGNAL

PWM3

A2

AD8628

A3

AD8628

U7

ADG836

1kΩ

VOUT, A2 OR

POSITIVE

CONDUCTIVITY

SIGNAL

VOUT, A3 OR

NEGATIVE

CONDUCTIVITY

SIGNAL

1nF

1nF

1kΩ

47µF

47µF

150Ω

150Ω

150Ω