# 2 op amp followers on 2 capacitor plates not measuing correct voltage

1. Jul 10, 2012

### dustinthewind

I am currently trying to measure an 80pF capacitor using 2 voltage follower op amps. I have them both powered by a set of 2 1.25V rechargable batteries. They are op amp type LMP7721 link: http://www.ti.com/product/lmp7721 .

Each op amp is configured such that the voltage out terminal [end of the triangle] is connected by wire directly to the negative in therminal [voltage follower configuration]. The positive in terminal on each LMP7721 is connected respectively to 1 plate of the capacitor.

A keithly 182 volt meter is conected across each output terminal on the op amp to measure the difference in voltage.

As a test to see if they measure voltage correctly I put a battery across the plates in which I have already measured the voltage to be 1.25V. However only -0.98 shows one way and 1.02 volts shows the other way.

any idea why I am not measuring the 1.25V difference in voltage between the outputs of the op amps? No resistors are involved.

[I have also noticed when I test each voltage follower using a variable voltage (adjustable resistor) they seem to follow the voltage fine. However they will only drop to a minimal voltage of 9mV and 16mV respectively.]

I am also wondering why the voltage followers won't drop to zero volts individually wen I test them to see if they follow voltage.

2. Jul 10, 2012

### Staff: Mentor

It sounds like the input voltage to each follower is not referenced to ground or any other voltage? If you just put a differential voltage across the floating capacitor, it is not referenced to the opamps' ground or power supplies.

If you put the battery between ground and the input to one of the followers, does the output show the correct voltage?

3. Jul 10, 2012

### Staff: Mentor

BTW, what are the power supply voltages that you are using for the opamps?

4. Jul 10, 2012

### yungman

That's the very question I want to ask. If he power by +/-1.25V, that's not enough. The data sheet called for +/-5V.

5. Jul 10, 2012

### dustinthewind

The documentation states it will run on 1.8 to 5.5V if you read the 2nd page of documentation both in "features" and "general description". The fact that each of them follows voltage indicates they are working working. The problem is they will not drop below 9 and 16mV amp 1 and 2.

If both voltage followers give a voltage then connecting each capacitor plate to a voltage follower means the 2 voltage followers should give the difference in voltage on their outputs. The positive in terminal on the voltage follower I think should be a high impedance connection and have very low bias current.

However maybe your on to something. I'll introduce a 3rd battery of 1.25V in serries with the other 2 batteries giving 2.5V to make the power supply 3.75V then measure the 1.25V battery across the capacitor plate and see if it gives me a different voltage.

6. Jul 10, 2012

### dustinthewind

On 2nd thought berkman I might understand what you are saying. I'll try what you are telling me and see if that works too.

7. Jul 10, 2012

### yungman

You voltage is ok for the opamp. I am still confuse about your original circuit. You have two batteries to power the opamp. So you get 2.5V across the opamp. The mid point between the two batteries is the ground reference.
How do you connect the cap? Is it across the same two batteries also? I don't understand why you said you expect 1.25V across the opamp output.

8. Jul 10, 2012

### dustinthewind

the capacitor is not grounded. So it is floating. Each voltage follower just follows the voltage on each individual plate. I guess the circuit works. I found some wire problems and fixing that seemed to fix the problem with not showing the correct battery voltage.

I think the problem with the op amp not gong to zero is based on an offset voltage. There is some error involved in each op amp. They give a plus and minus for the offset voltage in the specs. When I introduced a negative voltage it cycled all the way through negative and ended at the positive 9mV and the other one at the 16mV. I guess that solves my problem afterall. Thanks everyone.

For the ground voltage on the op amps power supply I have them connected to the negative terminal of the battery. Since my measurement is just the difference between the 2 op amps it seems to work. Thanks again o/ , toodles.

9. Jul 10, 2012

### yungman

You cannot let the cap float. You have to reference the cap to some where. If you use a third battery to put across the cap, use two extra resistor of equal value and wire in series, then connect the two resistor across the cap. Then you connect the junction between the two resistors and connect to the junction between the two batteries that power the opamp. Now you set up a 0V reference at the half voltage of the capacitor. With this, you have 0.625V on one side of the cap, and -0.625V on the other side of the cap. Now if you measure using a probe with the ground reference to the point between the two batteries powering the opamp ( 0V), you will get the correct voltage.

You cannot let the cap float. the whole cap might be drag beyond the input range of the opamp just by any static. You have to make sure everything is referenced to a set voltage.....where in this case, I have you reference to 0V.

10. Jul 10, 2012

### jim hardy

Indeed the cap can't float .

That op-amp is not rail-to-rail, datasheet says output can only go within a few tens of milllivolts to V-. See "Output Swing Low" under "2.5V Electrical Characteristics".