0
\$\begingroup\$

I want to measure nano voltages (of the order of 10^-7) for a transient hot-wire setup for measuring the thermal conductivity of nanofluids.The setup I'm using is one with a platinum wire incorporated in a Wheatstone bridge setup and the resistance change over the time is measured for the required thermal conductivity measurement. I don't have a nanovoltmeter with me to measure the bridge voltage measurements over time but I do have Tektronix TDS220 Real-Time Digital Oscilloscope with me that can measure the millivolt region and can be used as a data logger. So I'm planning to construct a voltage amplifier using opamp and the amplified output needs to be at least 10K times higher than the input, also I'm planning on using the IC741 opamp.So is it possible to construct a voltage amplifier with such parameters?Also, I have Keithley 487 pico ammeter with me and can I use it in some way?I really appreciate any help from anyone.

\$\endgroup\$
2
  • \$\begingroup\$ What time-constant of measurement must you achieve? 1Hertz fluctuations, with 1% accuracy? or 100Hz fluctuations with 0.01% accuracy? What sorts of electrostatic shields (sheets of metal) can you place around the fluids? What measurement RMS error (1 sigma) can you tolerate? NEED ERROR BUDGET. Will AC-drive to your bridge cause changing electric fields that will perturb the nanoFluid properties? \$\endgroup\$ Sep 1, 2018 at 4:49
  • \$\begingroup\$ I don't work anymore on the above mentioned project and the objective to measure the thermal conductivity was dropped.Anyway thank you for your response towards this post.Have a nice day :) \$\endgroup\$
    – RkDAS
    Sep 5, 2018 at 6:51

2 Answers 2

2
\$\begingroup\$

A 741 is utterly incapable of doing anything useful at that level. Drift and noise are orders of magnitude too high. It was hot stuff in 1968, not so much now.

You can use a "zero-drift" chopper amplifier with ~50nV/K drift but noise will be relatively large, depending on bandwidth. I suspect 100nV RMS (not P-P) should be achievable over a few Hz bandwidth.

Construction techniques need to be very good to live up to the amplifier, including especially minimizing thermal gradients to reduce thermal EMFs.

If possible, find a way to get more signal.

Picoammeters and electrometer amplifiers in general tend to have poor DC performance as they are optimized for extremely low bias current. Since you have, I believe, a very low impedance source, you have the opposite problem.

\$\endgroup\$
6
  • 1
    \$\begingroup\$ The 741 should be banned from everything except history books. \$\endgroup\$ May 3, 2018 at 11:38
  • \$\begingroup\$ Thank you for your reply.I will try to channel my search in the so given direction.Also is there any scope for a voltage to current conversion here so that I can somehow use the pico ammeter?Earlier we used a Fluke 8846A multimeter for this but now we don't have access to this multimeter though. \$\endgroup\$
    – RkDAS
    May 3, 2018 at 11:46
  • \$\begingroup\$ @RkDAS No you can't use the picoammeter (directly) for reasons I discussed above. It would be silly to use it a voltmeter wouldn't it? \$\endgroup\$ May 3, 2018 at 12:09
  • \$\begingroup\$ @VladimirCravero Its 50th birthday is this year. It can get senior discounts in some places. \$\endgroup\$ May 3, 2018 at 12:10
  • \$\begingroup\$ @Andyaka has a recommendation for a rather nice chopper stabilised amplfier that moves the LF noise up to its chopper frequency (200kHz) leaving pretty decent noise at LF. Its name escapes me at the moment but it could be ideal for the job. found it ADA 4528, see electronics.stackexchange.com/questions/141162/… \$\endgroup\$ May 3, 2018 at 12:30
1
\$\begingroup\$

I addressed a similar problem some years back. Instead of DC, I used AC excitation for the bridge so that my gain block didn't have to deal with the effects of DC and high gain. The gain block fed a center tapped transformer followed by a synchronous rectifier to obtain the DC component; a differential output from the gain block should work while eliminating the transformer.

Don't know if this would be applicable to your system but might offer another option to consider.

Edited to answer a comment: Error due to self-heating in the sensor is present whether AC or DC excitation is used for the bridge. Under constant drive conditions and at a given sensor resistance, there will be a known power dissipated in the sensor. The resulting temperature rise can be viewed as an offset that can be calibrated out. As the temperature under investigation varies the sensor resistance, that power dissipation will change, creating another source of error. Maybe this can be neglected. The thermal EMF mentioned in another answer is no longer a problem when using the AC excitation.

The sensitivity of the amplifier chain, whether AC or DC coupled, will have to be the same. It's just that the AC coupled version will not have to deal with the error caused by DC offset multiplied by the large gain.

A synchronous rectifier can introduce an error due to phase differences between the drive and the input signal. The same drive is used as the AC excitation to the bridge. As long as the bridge has no reactive components, this will not cause any errors. Phase shift through the gain stages will cause an error, but if the shift is constant and the excitation is constant, then it will manifest itself as an offset in the DC out of the rectifier. This of course is at high level and therefore not amplified by the gain stages...easy to calibrate out.

If a transformer is used, any phase shift it introduces will also show up as a DC offset. It can be replaced by an op-amp with gain of -1 to create the opposite phase for the synchronous rectifier.

Whichever method you choose, I encourage you to determine an error budget and analyze each step of the signal chain with this in mind.

\$\endgroup\$
1
  • \$\begingroup\$ thank you for your suggestion.In my case, the possible temperature changes associated with the experiment is in ranges of 293K-318K so I have to be very careful with the sensitivity and the resolution of the whole setup so using the transformer and then rectification might add to extra errors right? \$\endgroup\$
    – RkDAS
    May 3, 2018 at 14:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.