Measuring 0.0001V Using 741 OP-AMP

In summary: I think the paper is basically a simple 'micro' voltmeter circuit.I would guess this is the sort of thing he was thinking of, but it's a bit of a leap to go from his question to this without any hints or clues.
  • #1
darkxponent
387
8

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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  • #2
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...
 
  • #3
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.
 
  • #4
I don't know any other approach except for those mentioned above. I also couldn't understand why he asked about Control Systems.
 
  • #5
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.
 
  • #6
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.
 
  • #7
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!
 
  • #8
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
 
  • #9
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.
 

Related to Measuring 0.0001V Using 741 OP-AMP

1. How do you measure 0.0001V using a 741 OP-AMP?

The 741 OP-AMP is a commonly used operational amplifier in electronic circuits. To measure a voltage as low as 0.0001V, you will need to use a voltage divider circuit with precise resistors to reduce the voltage to a value that can be accurately measured by the OP-AMP. The OP-AMP can then amplify this voltage to a level that can be measured by a voltmeter or other measuring device.

2. What is the accuracy of measuring 0.0001V with a 741 OP-AMP?

The accuracy of measuring 0.0001V with a 741 OP-AMP depends on the accuracy of the resistors used in the voltage divider circuit and the gain of the OP-AMP. Generally, the 741 OP-AMP has a gain of around 100,000, so it can accurately amplify the reduced voltage from the voltage divider circuit. However, the overall accuracy will also depend on other factors such as noise and temperature.

3. Can the 741 OP-AMP measure negative voltages?

No, the 741 OP-AMP can only measure positive voltages. If you need to measure negative voltages, you will need to use a dual supply OP-AMP or add a DC offset to the input signal.

4. What are the potential sources of error when measuring 0.0001V using a 741 OP-AMP?

There are several potential sources of error when measuring 0.0001V using a 741 OP-AMP. These include inaccuracies in the resistors used in the voltage divider circuit, noise in the circuit, temperature variations, and limitations of the OP-AMP itself. It is important to use high-quality components and properly design the circuit to minimize these errors.

5. Are there any precautions to take when measuring 0.0001V with a 741 OP-AMP?

Yes, there are a few precautions to take when measuring 0.0001V with a 741 OP-AMP. It is important to use a stable power supply and minimize noise in the circuit. Grounding and shielding can also help reduce noise. Additionally, it is recommended to calibrate the circuit periodically to ensure accurate measurements.

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