# OP amp configuration?

1. Jan 5, 2013

### riie

Hi all,

What is the configuration of the op amp in the attached file?
The resistors is confusing me.
Can someone enlighten me?

Thanks alot!

#### Attached Files:

• ###### Circuit.jpg
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Last edited: Jan 5, 2013
2. Jan 6, 2013

Anybody?

3. Jan 6, 2013

### jim hardy

You haven't said where's the input to the circuit.

An "operational" amplifier will attempt to hold its input pins equal.
It's up to designer to surround it with a feedback that lets it do that.
Can yours do that with the feedback circuit you've given it?

Here's the motherlode of introductory op-amp info:
http://www.ti.com/lit/an/snoa621b/snoa621b.pdf [Broken]

Last edited by a moderator: May 6, 2017
4. Jan 6, 2013

### Staff: Mentor

It's an inverting amplifier. The presence of only resistors (i.e., no capacitors or inductors) means the feedback (hence gain) is independent of frequency (provided we overlook the frequency response of the op-amp itself).

5. Jan 6, 2013

### jim hardy

Are they anything more than a voltage divider?

What's voltage on left side of Runnumbered ?

6. Jan 6, 2013

### AlephZero

I'm guessing "Kelvin Capacitor" means some sort of capacitance measurement probe where the capacitance changes with time.

But my brain hurts trying to figure out how that relates to the rest of the circuit.

If one end of KC is at virtual earth and the other end is at a constant bias voltage, then changing the value of KC is equivalent to injecting some charge / current into the unnumbered resistor ....

7. Jan 6, 2013

### jim hardy

Found some info on it..

http://www.kelvinprobe.com/MRS Seminar - Notes.pdf

An interesting instrument for a R&D lab to study surface electric effects.

i'd wager that's how it works. Oscillating position of one plate of the capacitor gives alternating current, because to keep constant voltage the Q/C ratio must be constant.
So charge moves out when plates move apart and vice versa.

Amp gives voltage in proportion. The numbered resistor voltage divider just gives more gain.

One of its uses looks to be measuring affinity of a substance for electrons, ie how many ev it takes to withdraw one ? I always wndered how they measure that.

That Lord Kelvin was mighty clever.
Sure would be fun to watch someone operate one and understand its use !

8. Jan 7, 2013

### riie

Jim hardy is right, I am doing something similar to what he has stated.

Have been reading about circuits that are used to measure the induced AC current that is produced by the vibrating kelvin probe which can be represented by the kelvin capacitor.

This prove to be challenging as not much info are given on those circuits or rather no detail was provided for the I/V converter that was being used and the induced AC current produced are in the magnitude of pA to nA range. In addition, noises often predominate in the measurment of such a small signal.

9. Jan 7, 2013

### jim hardy

ahhh... that National AN20 is a start.

You will want to read some application notes for extremely high Zin amplifiers.

Search on "Electrometer opamp"
the ones i used are obsolete now, and i was doing DC to extremely low frequency work.
Hopefully your post will spark Yungman's curiosity, he is way beyond my level at such matters.

Here's an ancient Analog Devices paper that at least mentions "vibrating capacitor electrometer"
http://www.analog.com/library/analogDialogue/bestof/pdf/05_2.pdf

old jim

10. Jan 8, 2013

### Enthalpy

This circuit is used to create an equivalent feedback resistor bigger than the values available.

Say, with 10Mohm feedback and a divider by 100, you have the equivalent of 1Gohm. Useful to improve the low-frequency response with your capacitive source.

Though, one should be aware of the effects offset has in this circuit. As well, a very slow response time is unnerving because it takes minutes to stabilize and yuo never know if it has finished. It's better to pass DC when you can, and zero all offsets digitally in a subsequent stage, as this takes no time.

11. Jan 28, 2013

### riie

I am still exploring for solutions.

Would it work if
1) A high precision resistor(0.02%) 10Mohm is used to convert I(nA to pA range) to V with a gain of 10^7
2) Then passed on to an instrumentation amplifier to remove any noise that is present and provide further gain.

Am I oversimplifiying it? Or it is better to use a Electrometer opamp which functions as a tranimpedance amp to convert I to V.

12. Jan 29, 2013

### Staff: Mentor

What are the units of that gain of 10^7 you propose?

13. Jan 29, 2013

### riie

The 1x10^7 gain is from the 10Mega ohm resistor

Not sure whether the induced AC current can be converted to voltage that way.

14. Jan 29, 2013

### Staff: Mentor

You are thinking of including a 10MΩ resistor at the input where there is presently no resistor?

15. Jan 29, 2013

### riie

The vibrating kelvin capacitor is made of a piezo-vibrated kelvin probe and sample plate. The AC current induced will be of same frequency with the frequency used to vibrate the probe.

The attached image is a quick sketch of the method I am talking about.

Other methods were using opamp or Electrometer opamp and configured as a transimpedance amplifier to convert the AC current to a voltage for further processing or to be read by a daq.

Thanks for ur help too!

#### Attached Files:

• ###### cct.png
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16. Jan 29, 2013

### Carl Pugh

Connnect DC Bias + to ground.
There should be resistors to set A1 gain.
Add 10 kilohm in series with 1A-.
Add 1.0 megohm from 1A- to 1A Output

17. Jan 29, 2013

### riie

IA is a instrumentation amplifier.
So the last two steps are not required?
The gain of the instrumentation amplifier can be set by an external resistor, apologizes for not showing.

I am refering to the INA116 instrumentation amplifier as it's input bias current is 3fA(typ) so as not to affect the AC induced current which is already very small?

18. Jan 29, 2013

### Staff: Mentor

Gain is set by a single resistor?

19. Jan 29, 2013

### riie

This is the datasheet of the instrumentation amplifier, INA116.