LMP7701_06 datasheet(19/22 Pages) NSC

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Application Information (Continued)

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 differ-

entiated from similar products because it has an output

voltage linearly proportional to Celcius measurement, with-

out 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 readings. The LM35 needs to have a

resistor, 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 are usually large enough that

they don’t 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

provides the level shifting. V

OUT at the output of A2 is given

by: V

OUT = −2VpH + 1.024V.

LM4140A is a precision, low noise, voltage reference used to

provide the level shift needed. The ADC used in this appli-

cation 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

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 com-

bination 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.

20127361

FIGURE 11. Temperature Dependence of a pH

Electrode

20127362

FIGURE 12. pH Measurement Circuit

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