OP Amp Configuration: Understand Resistors - Get Answers Now!

[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 a lot!

 

[riie]

Anybody?

 

[jim hardy]

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

Use your op-amp fundamentals:
An "operational" amplifier will attempt to hold its input pins equal.
It's up to designer to surround it with a feedback that let's 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

 

[NascentOxygen]

Hi all,

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

Thanks a lot!

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).

 

[jim hardy]

The resistors [STRIKE]is[/STRIKE] ARE confusing me.

Are they anything more than a voltage divider?

What's voltage on left side of R_unnumbered ?

 

[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 ...

 

[jim hardy]

Found some info on it..

http://www.kelvinprobe.com/MRS%20Seminar%20-%20Notes.pdf

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

then changing the value of KC is equivalent to injecting some charge / current into the unnumbered resistor ...

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 !

 

[riie]

Sorry for not providing more info.
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.

 

[jim hardy]

This prove to be challenging as not much info are given on those circuits

ahhh... that National AN20 is a start.

and the induced AC current produced are in the magnitude of pA to nA range.

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

Search on "Electrometer opamp"
the ones i used are obsolete now, and i was doing DC to extremely low frequency work.
http://www.ti.com/lit/ds/symlink/lmc6001.pdf
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

 

[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.

 

[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.

 

[NascentOxygen]

10Mohm is used to convert I(nA to pA range) to V with a gain of 10^7

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

 

[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.

 

[NascentOxygen]

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

 

[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!

 

[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

 

[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 referring 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?

 

[NascentOxygen]

The gain of the instrumentation amplifier can be set by an external resistor

Gain is set by a single resistor?

 

[riie]

Gain is set by a single resistor?

This is the datasheet of the instrumentation amplifier, INA116.
www.ti.com/lit/ds/symlink/ina116.pdf
You can see that the gain is set by the value of Rg.I am still learning by the day, so I'm not really sure will this method work.
Or configuring a transimpedance amplifer (using electrometer opamp) and 2nd stage amplification by another opamp.
Really appreciate your support and suggestions!