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LMP91200

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SNAS571E – JANUARY 2012 – REVISED FEBRUARY 2016

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Copyright © 2012–2016, Texas Instruments Incorporated

9 Power Supply Recommendations

VDD should be bypassed with 10-µF, 1-µF and 0.1-µF capacitors, placed as close as possible to the LMP91200

VDD pin (pin 1). An LDO is recommended for the supply rail, but a DC-DC switcher may be used if sufficient

filtering is used to attenuate the switching frequency components.

10 Layout

10.1 Layout Guidelines

Due to the high impedance of the ph Electrode in the pH measurement, careful circuit layout and assembly are

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

of the LMP91200 from large voltage gradients across the PCB. A guard is a low impedance conductor that

surrounds an input line and its potential is raised to the voltage of the input line. The input pin should be fully

guarded as shown in Figure 38. 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. The LMP91200 makes the guard

ring easy to be implemented without any other external operational amplifier. The ring needs to be connected to

the guard pins (GUARD1 and GUARD2), which are at the same potential as that of the INP pin. 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.

Figure 38 shows a typical guard ring circuit when the LMP912000 is interfaced to a pH probe through a triaxial

cable/connector (usually referred to as triax). The signal conductor and the guard of the triax should be kept at

the same potential. Therefore, the leakage current between them is practically zero. Because the triax has an

extra layer of insulation and a second conducting sheath, it offers greater rejection of interference than coaxial

cable or connector.

10.2 Layout Example

Figure 38. Circuit Board Guard Layout