LMP7701MASLASHNOPB datasheet(29/44 Pages) TI1

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600

500

400

300

200

100

-100

-200

-300

-400

-500

-600

10°C (74.04 mV/pH)

25°C (59.15 mV/pH)

0°C (54.20 mV/pH)

LMP7701, LMP7702, LMP7704

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SNOSAI9I – SEPTEMBER 2005 – REVISED NOVEMBER 2015

Typical Application (continued)

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. Because

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. VOUT at

the output of A2 is given by: VOUT = −2VpH + 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 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 49. 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 must 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.

9.2.3 Application Curves

Figure 52. Temperature Dependence of a pH Electrode

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LMP7701MA/NOPB Datasheet(PDF) 29 Page - Texas Instruments