+

-

LMP7721

V

+

V

-

+

-

Error

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

100°C (74.04 mV/pH)

25°C (59.16 mV/pH)

0°C (54.20 mV/pH)

LMP7721

www.ti.com

SNOSAW6E – JANUARY 2008 – REVISED DECEMBER 2014

Typical Application (continued)

Figure 51. pH Electrode Transfer Function

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

Ω to 1000 MΩ. The ultra low input

bias current of the LMP7721 allows the voltage error produced by the input bias current and electrode resistance

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

Ω, if an op amp with 3 nA of

electrode is 30 mV! This error can be greatly reduced to 30 nV by using the LMP7721.

Figure 52. Error Caused by Amplifier’s Input Bias Current and Sensor Source Impedance

8.2.2 Detailed Design Procedure

The output voltage of the pH electrode will range from 54.2 mV/pH at 0°C, to 74.04 mV/pH at 100°C. The

maximum input voltage will then be ±74.04 mV * 7 = ±518.3 mV. Allowing for output swing and offset headroom,

the maximum output swing should be limited to ±2.4V. The amplifier gain would then be 2.4 V / 0.5183 V = 4.6

V/V.

The output voltage from the pH electrode is fed to the signal conductor of the triax and then sent to the non-

inverting input of the LMP7721. In this application, the inverting input is a low impedance node and hence is used

to drive the LMP7715 which acts as a guard driver. The output of the guard driver is connected to the guard of

the triax through a 100-

Ω isolation resistor.

Figure 50 is an example of the LMP7721 used as a pH sensor amplifier.

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