In What is the purpose of this op amp? you see my interested circuit.

from lmp7721 user guide


The diagram above shows a single pH measurement "channel".
U1 sets a reference voltage of Vout_LM4140A/2, and U2 measures the voltage outut of the pH probe relative to the reference voltage.

I wish to add a second independent pH probe.
I could add two new amplifiers say U1 as reference and U2B for measurement.

However, is there any reason why I should not add a U2B but share the existing reference source from U1 for both probes?

Original question:

  • I measure output of u2 in single ended mode. There is no problem with one electrode (for example pH).
    But i need to measure different electrodes using same common reference in same time (in here reference is taken by u1). If i connect another electrode which have same characteristics with pH electrode and works with same reference to new lmp7721 (as u3) like u2, is it correct?

I understand you to be intending to add a new pH electrode in its own glass tube, to be monitored by "U3", and you wish to share the reference voltage produced by U1.

This is potentially acceptable BUT pH measurements are so very very very very susceptible to outside disturbance that you need to weigh up the cost gains compared with the potential for cross coupling between the op amps and probes in some way.

As long as the actions associated with one electrode do not affect the other in any way at all it will probably be OK. Can you guarantee this?

This LMP7721 application note is liable to be of use It relates to an evaluation board but covers much practical material.

In a pH sensor the output voltage is the voltage developed BY the glass due to hydrogen Ion concentration differences. Sensor impedances are in the order of semi-infinite - say 50 to 500 megohms range, so ie 100's of megohms load impedance is really lower than desirable.

The op amp cost $US10/1 at Digikey. See data sheet for why it costs so much :-).

Wow!!! What a superb op-amp - in some areas at least LMP7721 datasheet
3 FemtoAmp bias current :-) !!!


pH electrode performance

Wikipedia Glass electrode good

Combination electrode

Single and double junction electrodes

ISFET solid state pH sensors - - The future of pH sensors, once they get them properly sorted.

  • \$\begingroup\$ The LMC6041 has also 2 fA input bias current, but is only 1.64 dollar. GBW is only 75 kHz, but for this application that's not a problem. \$\endgroup\$ – stevenvh Jul 31 '12 at 12:52
  • \$\begingroup\$ First, using of one Ag/Cl reference for different ion selective electrode sensors is correct. There is no problem. I preferred lmp7721 because the circuit is easy to make. I don't know what i can do with lmc6041. Im open to all advices. \$\endgroup\$ – İsmailA Jul 31 '12 at 13:05
  • \$\begingroup\$ @IsmailA - IMO the LMC6041 can be used here instead of the 6 x more expensive LMP7721. The latter offers better specs regarding speed, but you would pay a lot for a feature you don't need: a GBW product of 17 MHz looks ridiculous on a pH-meter. \$\endgroup\$ – stevenvh Jul 31 '12 at 14:34
  • \$\begingroup\$ can i use lmc6041 in this circuit instead of lmp7721? \$\endgroup\$ – İsmailA Aug 1 '12 at 9:30

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