Datasheet

IN-

IN+

N/C

OUT

N/C

pH ELECTRODE

out

100:

0.1 µF

TRIAX

LMP7721

LMP7715

LMP7721

www.ti.com

SNOSAW6D –JANUARY 2008–REVISED MARCH 2013

Figure 51. LMP7721 as pH Electrode Amplifier

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 via a 100Ω isolation resistor. Ideally, the inverting and the non-inverting inputs of the amplifier are kept

at the same potential through the operation of the amplifier. By connecting the signal conductor to the non-

inverting input and letting the inverting input drive the guard, the signal conductor and the guard are kept at the

same potential which prevents leakage from the signal source.

LAYOUT AND ASSEMBLY CONSIDERATIONS

In order to capitalize on the LMP7721’s ultra low input bias current, careful circuit layout and assembly are

required. Guarding techniques are highly recommended to reduce parasitic leakage current by isolating the

LMP7721’s input from large voltage gradients across the PC board. A guard is a low impedance conductor that

surrounds an input line and its potential is raised to the input line’s voltage. The input pins should be fully

guarded as shown in Figure 52. The guard traces should completely encircle the input connections. In addition,

they should be located on both sides of the PCB and be connected together.

Figure 52. Circuit Board Guard Layout

Solder mask should not cover the input and the guard area including guard traces on either side of the PCB.

Sockets are not recommended as they can be a significant leakage source. After assembly, a thorough cleaning

using commercial solvent is necessary.

Product Folder Links: LMP7721