LMP7721: LMP7721

Saish Kalaskar

Part Number: LMP7721

I am using LMP7721 as buffer for signal conditioning of pH probe . I am using lmp7721 as buffer as shown in fig.

My problem is that when it is used as buffer it gives faithful output for some time only.I mean when I used it yesterday it shows correct output but when I used it today it shows false output and also output was so fluctuating which vary from 300mV to 50 mV. even some time it does not show any output.

what should I do?

please tell me reasons for above problems and solution for it,

thank you!

over 7 years ago

0 kai klaas69 over 7 years ago

Guru 140200 points

Hi Saish,

have you connected the minus pole of pH probe to 0V of circuit?

Kai

0 Saish Kalaskar over 7 years ago in reply to kai klaas69

Prodigy 100 points

yes, I did

0 kai klaas69 over 7 years ago in reply to Saish Kalaskar

Guru 140200 points

Hi Saish,

have you scope? Can you check whether the OPAmp output is oscillating?

The LMP7721 is a fast OPAmp. So, you should shield the input signal from the ouptut. The best way to do this is to use a short shielded cable to the pH probe. Using a gain >1 can also stabilize the LMP7721. And you should insert a 100R resistor in the output line of LMP7721, directly at the output pin. This resistor isolates the output from capacitive loads and also stabilizes the OPAmp.

Section 8.2 of datasheet of LMP7721 contains useful information about connecting a pH probe.

Kai

0 Marek Lis over 7 years ago in reply to kai klaas69

TI__Guru 68461 points

In addition to making sure the op amp does not oscillate, please make sure to drive the shield of Triax guard with another op amp in order to minimize its leakage current which get coverted to voltage by the vary high impedance of the pH probe - see below.

0 Dennis Goeke over 7 years ago in reply to Marek Lis

TI__Genius 11135 points

Saish

We haven't heard back from you so we assume this answered your question. If not, post another reply below, or create a new thread if this one has timed out.

Thanks
Dennis

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LMP7721: LMP7721