Hello,
I have been working on a schematic which supplies a steady current. which then feeds into a differential op amp to determine resistance as it allows me to divide the voltage by the current to get the resistance.

I have attached a schematic which was used to simulate. In reality the 1k Collector resistor is a varrying resistance between 1k and 100k which is the resistance of the skin. The final schematic will supply roughly 10-20 micro amps over a set of 2 electrodes to determine body resistance.

The question that i have is whether my 2nd schematic would be a proper setup for measuring the body resistance with 2 probes or if i could improve upon something else?

PS don't mind the values these were used for simulating only and are still subject to change.

Thanks in advance.

 

The 1M resistor in parallel with the 2K resistors doesn’t make sense.

It’s possible that 5V won’t be enough to drive 20 microamps. Depends how far apart the electrodes are and the skin conditions where the electrodes are placed. You’re not measuring the body so much as the electrode-to-body interface.

The body responds and the readings will change with time. It may be hard to get a reproducible result.

 

I don't see any schematics. If you cannot post images you can post a link to an external schematic or send them by private message for me and I will add them to your post.

 

These are the schematics i thought i added them.

@wayneh the 1M resistor was to get an output in my simulation it is true that it's a bit weird(it will be removed). As for the electrodes. The point of them is to measure body resistance which would change depending on the amount of sweat. So results will vary i expect.
Do you have a suggestion to make it more reproduceable?

Thanks.

 

One thing to watch out for is making sure that the input signals do not exceed the common mode input range of the opamps. That might mean using positive power supply for the opamp that is greater than the voltage (+5V at the moment) used tp supply bias. An opamp, the common mode input range of which includes the positive power supply rail would also satisfy this need.

 

"Normally" measuring body R involves a very high common mode environment
problem, solved by an IA.

Page 6-21..... https://www.analog.com/media/en/tra.../designers-guide-instrument-amps-complete.pdf


Regards, Dana.

 

"Normally" measuring body R involves a very high common mode environment
problem, solved by an IA.

Page 6-21..... https://www.analog.com/media/en/tra.../designers-guide-instrument-amps-complete.pdf


Regards, Dana.

So do you mean that i should try and use an instrumentation amplifier instead of the differential amplifier? or would an op amp with a high common mode rejection suffice?

 

One thing to watch out for is making sure that the input signals do not exceed the common mode input range of the opamps. That might mean using positive power supply for the opamp that is greater than the voltage (+5V at the moment) used tp supply bias. An opamp, the common mode input range of which includes the positive power supply rail would also satisfy this need.

It might be my misundestanding of the op amp working but would this be a problem considering there is a 5v stable supply into the op-amp?
Since the common mode input voltage is (Vin + Vin-)/2.

 

I depends on where you have that pot (connected to +5V at one end.) set and the highest voltage that can appear on the collector of the transistor.

 

An IA is, generally speaking, laser trimmed at factory to maximize CMR.

Making a diff amp out of ratioed Rs, unless you use thin film matching R network
part, will not achieve high CMR.

Here is an example I ran in Mathcad just how difficult this is -

Regards, Dana.

 

ASn IA is, generally speaking,m laser trimmed at factory to maximize CMR.

Making a diff amp out of ratioed Rs, unless you use thin film matching R network
part, will not achieve high CMR.

Here is an example I ran in Mathcad just how difficult this is -

Regards, Dana.

Thank you dana.
I appreciate your input and time.
I've been looking into instrumentation amps and found this schematic

I'm not sure what u meant by ASn but if i understood correctly then using this schematic should give me a high Common mode rejection ratio.
However im not entirely sure what use that would give me. I read up on common mode rejection ratio and it seems to be important for differential amps so as not to interfere with the signal. However how exactly this works i'm not sure about yet.

Edit: By laser trimmed IA do you mean like an IC or just some parts?

Regards, Christiaan

 

That is essentially the schematic of an IA. But problem is having hi
accuracy R's, ratios of them, to get CMR performance. Those are
laser trimmed in manufacturing the IA.

https://en.wikipedia.org/wiki/Common-mode_rejection_ratio

https://www.analog.com/media/en/training-seminars/tutorials/MT-042.pdf

The AC line stray signals a body is subjected to is a CM signal that you
do not want in your measurement. It will induce itsef on your electrodes.
Even if you have shielded cables, although that will help.


Regards, Dana.

 

Alright i will look further into all these suggestions.

Thank you Danadak and DickCappels for the tips.