SM72501 Datasheet, PDF (18/20 Pages) National Semiconductor (TI) – SolarMagic Precision, CMOS Input, RRIO, Wide Supply Range Amplifier

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FIGURE 11. Temperature Dependence of a pH Electrode
The schematic shown in Figure 12 is a typical circuit which
can be used for pH measurement. The LM35 is a precision
integrated circuit temperature sensor. This sensor is differen-
tiated 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 infor-
mation is used by the ADC to calculate the temperature
effects on the pH readings. The LM35 needs to have a resis-
tor, R_T in Figure 12, to –V⁺ in order to be able to read
temperatures below 0°C. R_T is not needed if temperatures are
not expected to go below zero.
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 A1 buffers the output of
the pH electrode with a moderate gain of +2, while A2 pro-
vides the level shifting. V_OUT at the output of A2 is given by:
V_OUT = −2V_pH + 1.024V.
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 con-
verter 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 12. It is common to
share power supplies between different components in a cir-
cuit. To minimize the effects of power supply ripples caused
by other components, the op amps need to have bypass ca-
pacitors 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.
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FIGURE 12. pH Measurement Circuit
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