PH differential instrumentation amplifier

Question

I have a computer engineering background with minimal experience with real EE work. I want to interface to a ph probe and was wondering what the best way to do this is.

It seems to me there are 2 basic options for doing this. The first is fully differential, where the ph probe connects to Vin in the diagram below. The output can then be read from the output of the first 2 stages via differential ADC. Is this correct?? Are there any flaws with connecting a ph probe directly to this circuit??

Second stage can be excluded and is just if you want a single ended signal ended with additional gain

This second design is simply a single ended gain+offset configuration. First stage does some gain, second stage offsets and has a bit more gain.

Which one is ideal for minimizing signal loss/noise? My thought would be that the first circuit would be better at noise rejection since it is differential, and feeding this into a differential ADC, some common mode noise would be rejected.

Are there better configurations for doing this?? I am interested in reading range of 3-10ph only. In this case, is one better than the other?

Some notes on ph probe:

• Extremely high impedence opamps are required(eg femptaamp input bias current).
• -420mv is 0ph, 420mv is 14 ph(may have that switched around).

Answer 1

You cannot just connect the probe across inputs to the inamp and expect good results.

Operational amplifiers need a path to ground from each input. So the second circuit is better.

the 741 is totally unsuitable, but the LMC6041 in the notes should work well for this circuit. (2 fA is typical rating though, use the guaranteed value of 4 pA for design unless you want your circuit to only mostly work... 4pA through 300Mohm sensor is 1.6mV offset.)

LMC6041 will be happy with a single supply, so if the -5V supply is inconvenient have a look at this circuit: What is the purpose of this op amp?

I suggest you stop simulating and actually build the circuit. LMC6041 is $2 from a distributor or ask National, they'll give free samples for school projects. You don't need to hook up the ADC yet, just get the probe and signal conditioning amplifier working. Read the output with a voltmeter. Just like software, breaking the problem into small pieces is the way to make progress.

Answer 2

I like a hybrid of the two. Firstly, the first is an instrumentation amplifier build. You can't use off the shelf components to build an IA with anywhere near the CMRR of a commercial IA -- at least without a whole bunch of hard work. Spend the $6 on a real IA. If you'd like, use a high resistance to ground to deal with bias currents on each input. A modest gain IA stage to deal with CMRR, followed by modest gain opamps with appropriate filtering is often a good approach.