Datasheet

LMP91200

www.ti.com

SNAS571C –JANUARY 2012–REVISED MARCH 2013

The 3-step temperature measurement can reach a precision as high as ±0.1°C (with R

REF

@ 0.01% of tolerance)

when the analog signal is acquired by at least 16 bit ADC. With lower number of bit ADC this method gives the

same result of the 2-step measurement due to the low voltage offset of the signal path. As rule of thumb, the 3-

step temperature measurement gives good result if he the LSB of the ADC is less than the input offset of the

PGA.

Diagnostic Feature

The diagnostic function allows detecting the presence of the sensor and checking the connection of the sensor.

A further analysis of the answer of the pH probe to the diagnostic stimulus allows estimating the aging of the pH

probe. With the diagnostic function is possible to change slightly (+/- 5% VREF) the Common mode voltage. If

the sensor is present it reacts, this reaction gives some information on the status of the connection, the presence

of the sensor and its aging. In fact a typical symptom of the aging of a pH probe is the slowness in the answer. It

means that a pH probe answers with a smoother step to the diagnostic stimulus as its age increases.

The procedure is enabled and disabled by SPI (refer to ). Until bit D6 is at low logic level, VCM stays at the

programmed voltage independently by the SDO_DIAG pin status. When bit D6 is tied at high logic level, on the

first rising edge of SDO_DIAG, a positive pulse is generate. At the second positive rising edge of SDO_DIAG pin,

the positive pulse ends. At the third positive rising edge of SDO_DIAG a negative pulse is generated. At the forth

positive rising edge of the SDO_DIAG the negative pulse ends and the routine is stopped and cannot restart until

bit D6 is set again at 1.

Layout Consideration

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

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

LMP91200’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 pin should be fully guarded

as shown in Figure 56. 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 op amp. The ring needs to be connected to the guard pins

(GUARD1 and GUARD2) which are at the same potential 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.

In Figure 56 is showed a typical guard ring circuit when the LMP912000 is interfaced to a pH probe trough a

triaxial cable/connector, usually known 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. Since triax has an extra

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

cable/connector.

Figure 56. Circuit Board Guard Layout

Product Folder Links: LMP91200