voltage follower with an op amp

Ron H

Joined Apr 14, 2005
7,063
Post a schematic of your voltage divider. A couple of people here have suggested that your meter is loading it. I suspect that also.
 

Thread Starter

arry

Joined Apr 2, 2010
26
Below is my circuit. R3 is my meter .

When the voltage divider is 100k then:
+input 2.03V gets 2.08V on output. LTSpice shows about 6mv voltage drop when R3 is connected.

When the voltage divider is 50K then:
+input 2.03V gets 2.05V on output. LTSpice shows about 3mv voltage drop when R3 is connected.

So the difference should be 6mv with 100k divider and 3mv with 50k divider, but not 50mv and 20mv !?

Still with the smaller divider difference is much smaller - 20mv ???

I am lost :confused:.

PS. Or maybe the impedance of my meter is not 10M ?
 

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Have you tried buffering the "+" input with a small capacitor?

A trick I have used to measure high impedance voltages is to buffer it with a capacitor.

This can apply to your measurements on the test circuit to buffer the effect of the multimeter probe, as well as to the actual original problem you're trying to solve.

You're trying to buffer a high-impedance signal to be read by an ADC, right?

Most ADCs sample for a very short time, with a very small capacitance, like 20 pF on the MSP430, for instance. If you buffer using a low-leakage capacitor, then you can often get pretty good readings on a high-impedance source... the impedance is effectively lowered in the time domain, because the sampling occurs on a very low duty cycle. When it's not sample, the input pin is in high-impedance state. As long as the leakage current is not too much, then this technique will work fine. I have used it on sources with impedances up to about 100K Ohms. According to the data sheet on the MSP430, the input pin leakage is max 50 nA, but in practice may be better than that. You can try and see.

Anyway, when you're trying and seeing, you can use a low-leakage cap to buffer the signal even to read it with your multimeter. In that case, with just a polystyrene cap on the input signal, then you can touch the multimeter and see the voltage. If the input impedance is very high, then you will see it slowly droop with the current flowing into your multimeter, and it will build up again when you remove the probe.

Hope this helps some.

Anyway, I don't know what's causing your issues with the op amp follower. I have done op amp followers with MCP6041 and with OPA277 that were rock-solid on high-impedance inputs, so I don't know what's going on with your big offset errors.
 

eblc1388

Joined Nov 28, 2008
1,542
Below is my circuit. R3 is my meter .
We are asking you for the values of the voltage divider that you have actually used when those previous voltage measurement in post#18 were taken, not a simulation of different values in LTSpice.

I don't see a reference to any particular Opamp library in your attached asc circuit.

If you want to see what the MCP6022 output does, you should use an opamp model specifically built for that opamp, perhaps from the chip manufacturer.

Simply changing the opamp name to MCP6022 for an universal opamp in LTSpice would not give you a close approximation of a MCP6022 behavior.
 

Thread Starter

arry

Joined Apr 2, 2010
26
What is the make and model number of your meter?
DT-838.

We are asking you for the values of the voltage divider that you have actually used when those previous voltage measurement in post#18 were taken...
The divider was 100k all.

Have you tried buffering the "+" input with a small capacitor?
I will try this. Thanks.
 
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SgtWookie

Joined Jul 17, 2007
22,230
Try measuring it as in the attached simulation.

Having the meter from the noninverting input to the output will minimize the loading effect of the meter. You should then only measure the offset voltage of the opamp.
 

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Thread Starter

arry

Joined Apr 2, 2010
26
Measuring as suggested by SgtWookie gives 100uV which is well within the specification.

Well, it seems the meter loaded down the +input voltage from the very beginning.

Problem solved.

Thank you all.

Finally I can sleep :)
 
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