10

100

1k

10k

0

10

20

30

40

50

60

70

80

90

FREQUENCY (Hz)

+INPGA=100mV

TIME (1s/DIV)

-50

-25

0

25

50

75

100 125

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

TEMPERATURE (°C)

PGA Gain = 5V/V

16

LMP91200

SNAS571E – JANUARY 2012 – REVISED FEBRUARY 2016

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Copyright © 2012–2016, Texas Instruments Incorporated

Typical Application (continued)

8.2.1 Design Requirements

8.2.1.1 pH Measurement

The output of a pH electrode ranges from 415 mV to

−415 mV as the pH changes from 0 to 14 at 25°C. The

output impedance of a pH electrode is extremely high, ranging from 10 M

Ω to 1000 MΩ. The low input bias

current of the LMP91200 allows the voltage error produced by the input bias current and electrode resistance to

be minimal. For example, if the output impedance of the pH electrode used is 10 M

Ω and an operational amplifier

with 3 nA of Ibias is used, the error caused due to the input bias current of the amplifier and the source

resistance of the pH electrode is 30 mV! This error can be greatly reduced to 1.25 µV by using the LMP91200.

The pH measurement with the LMP91200 is straightforward. The pH electrode must be connected between the

VCM pin and the INP pin. The voltage at the VCM pin represents the internal zero of the system so the potential

of the electrode (voltage at INP pin) will be referred to the VCM voltage.

8.2.2 Detailed Design Procedure

The LMP91200 is configured to execute a pH measurement as described in the pH Measurement section.

8.2.3 Application Curves

Figure 35. PGA Gain Error vs Temp

Figure 36. PGA Time Domain Voltage Noise

Figure 37. PGA PSRR vs Frequency