Best way to measure small (<500mV) voltages accurately? - diyAudio
 Best way to measure small (<500mV) voltages accurately?
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 Equipment & Tools From test equipment to hand tools

 Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
 30th December 2011, 11:51 AM #1 diyAudio Member   Join Date: May 2009 Best way to measure small (<500mV) voltages accurately? Hi all I'm currently working on a chemistry project for school, one part of which involves measuring electrode potentials and relating then to concentrations. This will involve making accurate measurements of voltages which will be all less than 1V. I have calculated that a 10x change in concentration will cause a change in the voltage measured of just under 30mV, if you want an idea of why I need accuracy. The voltages will be DC and not changing significantly though time. I currently have an old fluke 87 DMM, which reads to 0.1mV on the smallest scale. However, it has not been calibrated in at least 15 years. Leaving it switched on in the mV DC scale with no leads gives a reading of 0.4mV. I was thinking that perhaps creating a simple op-amp circuit with a gain of about 10 and using a higher scale might give a better accuracy. However, this raises the question of how well I can determine the gain, and if that measurement would introduce more inaccuracies than it prevents. To determine the gain I would either measure the R1 and R2 values in the circuit (and use 1% resistors) and calculate the ratio, or put a known standard voltage through the op-amp and measure the output (google suggests using an LED Vf). I would need a specialised low DC offset op amp. Is a typical inverting/non inverting setup adequate, or would a more complex setup be needed. I don't really want to spend much money of my own on this, so I would rather not need to send off my meter for calibration. I have a good set of parts but no PCB making facilities so it would have to be a stripboard project. Thanks all in advance.
 30th December 2011, 12:16 PM #3 On Hiatus     Join Date: Oct 2002 Location: Chicagoland Blog Entries: 2 The key to getting accurate measures of electrode potentials is very high input impedances. You absolutely need opamp buffers, using opamps with extremely low bias currents (generally FET opamps) and low offset voltages. Speed (slew rate, gain-bandwidth) is totally unimportant, you need tight DC parameters. If you use 0.1% resistors to set the gain, error from the opamp will be the least of your worries. Circuit leakages will be far more likely to be error sources. Real electrometers use exotic techniques like guard rings and active shields. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 30th December 2011, 02:35 PM #4 diyAudio Member   Join Date: Nov 2008 Location: Oakmont PA Set the meter to the range you need and have it hold there. Then press the offset button. Now the meter will read just your differences! No calibration is required.
 30th December 2011, 05:05 PM #6 diyAudio Member     Join Date: Aug 2011 Location: Utah A quick check of my own Fluke 87 shows a 0.2 (200uV) offset at room ambient. So I'd give the alcohol a try. Doc __________________ Noli timere problematum affigit
 30th December 2011, 06:34 PM #7 diyAudio Member     Join Date: Aug 2011 Location: Utah Okay, after further checking I think the ICL7650S op amp powered by 4 lithium cells per rail and using Vishay CMF series 0.1% 25ppm/C resistors will do the task for you. I'd used no higher than 10K/100K gain resistors. Use mylar or polyproplyene bipass caps and keep everything short and close to op amp built on double sided board with traces cut or etched out of ground plane. For a cheap fairly accurate voltage reference, look at the LM-10 which is a buffer amp combined with a precision voltage referrence. Not as good as the old LM199-399, but they are mostly old stock now and getting very pricey. If you can find a copy, fluke used to put out an excellent book on precision measurment. I have no clue where my copy might be right now. But somebody may have scanned it. There are lots of options, such as finding an old lead compensator, but I think the best low dollar solution is the chopper stabilized op amp. Keep in mind things get very strange below a mV. Don't expect absolute stability. There is a reason that things that will inherently measure such things cost a lot. Best of luck Doc __________________ Noli timere problematum affigit
 1st January 2012, 12:18 AM #8 diyAudio Member   Join Date: Apr 2010 Location: Bath, UK Am going to make an alternative suggestion that you get hold of a Solartron 7075 DMM. I use up to 3 of these for various measurements including from thermocouples and they have microvolt measurement capability with good thermal stability after a warm-up time of about an hour. They are available here in the UK for as little as £25 on Ebay, plus postage; which may well be the same cost as you building an external circuit for your Fluke. For you to have an idea of their resolution, have a look at the 7075specs.pdf available here: Friedrich Messtechnik - Manuals-Downloads They include the Guard facility for their inputs mentioned earlier and very high input impedance. hope this assists Mik
diyAudio Member

Join Date: Aug 2011
Location: Utah
Quote:
 Originally Posted by nickbob00 I don't really want to spend much money of my own on this, so I would rather not need to send off my meter for calibration.
Rather covers the Solartron or other high end DMM.

The chopper stabilized amp solution in front of his fluke might run \$12 US
Doc
__________________
Noli timere problematum affigit

 4th January 2012, 01:03 AM #10 diyAudio Member   Join Date: Apr 2010 Location: UK A tough one. What you really need is what is known as an 'electrometer', basically it is a voltmeter with both a very high input impedance, specialized circuitry to deal with enviromental noise and it's been designed to work at low potentials (mV/uV). A standard DMM will give you a false reading as it has a relatively low impedance (20Mohm perhaps) and is not really designed to be accurate at the uV scale. This doesn't solve your problem though! You've got a DMM and you still need to measure your cell. I would probably build an 'electrometer' front end with some gain in it (the gain will put you in a more linear part of the typical DMM's range, perhaps only x10 needed), if you look-up the AD846 datsheet (Analog Devices | Semiconductors and Signal Processing ICs) it'll give you an idea, or trawl through their applications page: http://instrumentation.analog.com/en/chemical-analysis/segment/im.html

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post rick57 Everything Else 0 4th May 2007 04:03 PM mikee55 Subwoofers 4 28th March 2007 01:41 AM lumanauw Solid State 21 12th January 2007 11:44 PM jteef Subwoofers 2 11th December 2005 10:55 PM maoumaou Multi-Way 5 8th December 2004 04:42 PM

 New To Site? Need Help?

All times are GMT. The time now is 04:50 PM.