1

2

3

4

5

7

8

9

10

11

12

13

14

600

500

400

300

200

100

0

-100

-200

-300

-400

-500

-600

pH

mV

10°C (74.04 mV/pH)

25°C (59.15 mV/pH)

0°C (54.20 mV/pH)

ACID

BASE

0

2

4

7

10

12

14

+414 mV

-414 mV

-177 mV

0 mV

+177 mV

pH

LMP7701, LMP7702, LMP7704

SNOSAI9H – SEPTEMBER 2005 – REVISED MARCH 2013

www.ti.com

Figure 54. Output Voltage of a pH Electrode

The temperature dependence of a typical pH electrode is shown in Figure 55. As is evident, the output voltage

changes with changes in temperature.

Figure 55. Temperature Dependence of a pH Electrode

The schematic shown in Figure 56 is a typical circuit which can be used for pH measurement. The LM35 is a

precision integrated circuit temperature sensor. This sensor is differentiated from similar products because it has

an output voltage linearly proportional to Celcius measurement, without the need to convert the temperature to

Kelvin. The LM35 is used to measure the temperature of the solution and feeds this reading to the Analog to

Digital Converter, ADC. This information is used by the ADC to calculate the temperature effects on the pH

The output of pH electrodes is usually large enough that it does not require much amplification; however, due to

the very high impedance, the output of a pH electrode needs to be buffered before it can go to an ADC. Since

most ADCs are operated on single supply, the output of the pH electrode also needs to be level shifted. Amplifier

The LM4140A is a precision, low noise, voltage reference used to provide the level shift needed. The ADC used

in this application is the ADC12032 which is a 12-bit, 2 channel converter with multiplexers on the inputs and a

serial output. The 12-bit ADC enables users to measure pH with an accuracy of 0.003 of a pH unit. Adequate

power supply bypassing and grounding is extremely important for ADCs. Recommended bypass capacitors are

shown in Figure 56. It is common to share power supplies between different components in a circuit. To minimize

the effects of power supply ripples caused by other components, the op amps need to have bypass capacitors

on the supply pins. Using the same value capacitors as those used with the ADC are ideal. The combination of

these three values of capacitors ensures that AC noise present on the power supply line is grounded and does

not interfere with the amplifiers' signal.

22

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