Measuring 0.0001V Using 741 OP-AMP

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Discussion Overview

The discussion revolves around methods to measure a very low voltage of 0.0001 volts using a 741 operational amplifier (OP-AMP). Participants explore various circuit configurations and considerations related to low voltage measurement, including the use of sample and hold circuits, inverting amplifiers, and potential issues with input errors and current flow.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant suggests using a sample and hold circuit to measure the low voltage.
  • Another participant proposes an inverting amplifier configuration with a high feedback resistor to amplify the signal.
  • Concerns are raised about the effects of current on very low voltage measurements, with a professor emphasizing that current should not pass through the circuit.
  • A participant mentions using a capacitor instead of a resistor in an integrator circuit to avoid current flow.
  • Another participant suggests building a chopper amplifier to mitigate current effects by alternating the direction of current through the source.
  • Some participants express confusion about the professor's intentions and the relevance of Control Systems to the question posed.
  • Several participants indicate a lack of familiarity with chopper amplifiers and seek clarification on low voltage measurement techniques.
  • One participant shares a resource found online regarding low voltage measurement techniques, questioning its simplicity and relevance.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best method to measure the low voltage. Multiple competing views and approaches are presented, and uncertainty remains regarding the professor's expectations and the appropriateness of the suggested methods.

Contextual Notes

Participants note limitations in their understanding of certain techniques, such as chopper amplifiers, and express uncertainty about the implications of the professor's comments on current flow and measurement methods. There is also mention of the complexity of the conversation with the professor, which may have influenced the responses given.

Who May Find This Useful

This discussion may be of interest to students and practitioners in electronics and electrical engineering, particularly those focused on low voltage measurement techniques and operational amplifier applications.

darkxponent
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Homework Statement



How will you measure a voltage of range 0.0001 volts using the 741 OP-AMP?

Homework Equations





The Attempt at a Solution



This a viva question and i couldn't answer. I don't know how to begin. Should i use Sample and Hold circuit or something else?
 
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darkxponent said:

Homework Statement



How will you measure a voltage of range 0.0001 volts using the 741 OP-AMP?

Homework Equations





The Attempt at a Solution



This a viva question and i couldn't answer. I don't know how to begin. Should i use Sample and Hold circuit or something else?

There are a number of things to consider What is a viva question? What kind of things are they looking for you to know and illustrate in your answer?

What can you say about the LM741 opamp? What are its input terminal errors (voltage, currents, etc)? And how would those affect amplifying a 1mV signal? What things do you need to know about the signal and signal source to help you answer this question?

Talk to us about how you would approach this question...
 
We were two students for the VIVA voice which is a part of practical exam. The converation went this way.

Prof: How will you measure a voltage range of 0.00001V using 741. Think and then answer. And show me the circuit.

Me: I will amplify it using inverting amplifier with Rf = 10^6 Ri. (the other guy gave similar answer)

Prof: Its a very low voltage. If some current passes, the voltage will drop. Current shouldn't pass.

Me: We can take higher value of Rf.

Prof: Even a small value of current will cause the voltage to drop.It is very low Voltage. Current shouldn't pass.

Me: I will use a capacitor instead of Rf. make it an integrator circuit. For DC no current through capacitor. Vo(t) = - (1/RC)*0.00001*t.

Prof: No. Remove one zero. Measure Voltage range of 0.0001V.

Other Guy: Sir we can use sample and hold circuit.

Prof: Show me how you will you use it.

He showed his work. He drew sample and hold circuit with digital sensor at output. The prof didn't say whether it was correct or incorrect.

Prof: Okay i remove one more zero. Measure Voltage range of 0.001V. Tell me

..silence

Prof: You have studied Control Systems.

We: yes.

Prof: Start paying attention to studies.

And the conversation ended.

I couldn't understand what he was trying to ask. I think he was more interested in amplification of Voltage.
 
I don't know any other approach except for those mentioned above. I also couldn't understand why he asked about Control Systems.
 
I'm with DXP on this. None of that exchange makes sense. Can you escalate it to the professor's supervisor? That exchange would earn your professor an infraction if he/she posted it here on the PF.
 
He was an external professor otherwise i would have asked him after the exam. And he asked this question to every group(at least 40 students). I don't know what were the exchanges with other guys. Maybe we were too dumb so our conversation turned out this way. He is a respected Professor.

I more interested in finding the answer to how can we measure low voltage using 741 rather than the meaning of this conversation.
 
It's been 30 years since I did this so I'm very rusty but I'm thinking perhaps build a chopper amplifier? You add a square wave to the input voltage, amplify it, then demodulate it...

http://electronicdesign.com/analog/chopper-stabilized-op-amps

Perhaps this is what he was hinting at when he said

"Even a small value of current will cause the voltage to drop.It is very low Voltage. Current shouldn't pass."

Perhaps by adding a square wave you can alternate the direction of the current through the source so on average it's zero?

Like I said I'm very very rusty!
 
Google found a block diagram...although it's not quite what I expected...

http://www.programmableplanet.com/author.asp?section_id=3034&doc_id=266134

160042_208399.gif
 
darkxponent said:
I more interested in finding the answer to how can we measure low voltage using 741 rather than the meaning of this conversation.
Sure, but a clue to what he had in mind probably lies in being able to interpret the gist of that conversation. :wink:

Maybe you could use the open loop gain, applying your small signal to one of the inputs. An adjustment for offset trim would be necessary. (Possibly taking the form of a feedback network passing frequencies << those contained in the signal to be measured.)
 
  • #10
Thank you guys for the reply.

berkeman said:
I'm with DXP on this. None of that exchange makes sense. Can you escalate it to the professor's supervisor? That exchange would earn your professor an infraction if he/she posted it here on the PF.

Will you please elaborate?

CWatters said:
Google found a block diagram...although it's not quite what I expected...

http://www.programmableplanet.com/author.asp?section_id=3034&doc_id=266134

160042_208399.gif

I tried understanding this Chopper thing but i just couldn't. I haven't studied this chopper amplifiers in my course work.

NascentOxygen said:
Sure, but a clue to what he had in mind probably lies in being able to interpret the gist of that conversation. :wink:

Maybe you could use the open loop gain, applying your small signal to one of the inputs. An adjustment for offset trim would be necessary. (Possibly taking the form of a feedback network passing frequencies << those contained in the signal to be measured.)

I did gave this answer also but the prof said-'We never use the Op-Amp in open loop configuration as it will start acting as comparator.
 
  • #11
I googled 'low voltage measurement techniques' and look what i found. I didn't include the word 'op-amp' in the search. The circuit seems to agree with the prof. Can't imagine the answer to be this simple. Do you guys agree with this paper.

View attachment Low level measurement.pdf
 
  • #12
darkxponent said:
I googled 'low voltage measurement techniques' and look what i found. I didn't include the word 'op-amp' in the search. The circuit seems to agree with the prof. Can't imagine the answer to be this simple. Do you guys agree with this paper.
That's a book. Is there an appropriate circuit that you have in mind?
 
  • #13
NascentOxygen said:
That's a book. Is there an appropriate circuit that you have in mind?

The circuit is in the book on the page 24.
 

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