Measuring NanoVolts - Page 1
Forum section: electronics / testgear

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: December 18, 2013, 04:30:30 pm
content: Does anyone know of a cost effective way to measure NanVolts accurately without having to buy like a 3K dollar Keithly meter?

author: robrenz
author_rank: Super Contributor
timestamp: December 18, 2013, 04:41:22 pm
content: What are you measuring, and what do you mean by accurately?

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: December 18, 2013, 05:34:36 pm
content: I guess it does not need to be extremely accurate, I'm just looking for something that is decent. The application is for BioMedical and BioFeedback prototypes, also extremely low level natural voltages which are inherently DC. I can get into the scale of uV but anything less than that gets unstable.

author: robrenz
author_rank: Super Contributor
timestamp: December 18, 2013, 06:07:27 pm
content: Aren't those signals fairly high source resistance? The near theoretical limits band says you need less than 1 Ohm source resistance to get to single NV measurements.

author: Unknown
author_rank: Unknown
timestamp: December 18, 2013, 06:13:49 pm
content: Look through the Keithley low level measurement handbook (available for free from their website). Cabling issues like thermal EMF (often in the order of uV/K temperature differential) can cause major issues, as can source impedance as robrenz mentions. I would probably look for a used Keithley nV meter or preamplifier and matching cables. There may also be purpose-made preamps designed for biomed applications, but I'm not sure if these would be any cheaper.

author: c4757p
author_rank: Super Contributor
timestamp: December 18, 2013, 06:17:35 pm
content: This looks interesting.Biopotential Amplifiers - Biomedical Engineering Handbook - J Nagel, ed. Bronzino.Of course, it doesn't recommend any parts. Still interesting. (And I'm still looking.)

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: December 18, 2013, 06:25:35 pm
content: Great information so far, keep it coming. This is new territory for me, so I am learning as I go. I can say without spilling too much of the project that I will have the need to measure in some degree the electrical properties such as voltage and current of micro organisms in a replicating state. In this state they produce energy which can be measured, but at an ultra small scale. Once I get the measurement, I can convert and process accordingly.

author: c4757p
author_rank: Super Contributor
timestamp: December 18, 2013, 06:27:31 pm
content: Microorganisms? Maybe this is stupid, since I don't really know what you're doing - but is there a way for you to use a larger number of microorganisms to somehow increase the amplitude of the signals? I know it would be rather hard to stack bacteria in series, though...

author: zapta
author_rank: Super Contributor
timestamp: December 18, 2013, 06:27:36 pm
content: Quote from: Terabyte2007 on December 18, 2013, 04:30:30 pmThat's another idea for a Dave's kick start project.Sent from my Nexus 5 using Tapatalk

author: jimmc
author_rank: Frequent Contributor
timestamp: December 18, 2013, 06:55:26 pm
content: Keithley do a good (free) handbook. Google 'keithley low level measurements handbook' and the first link should be a full copy.(Going via the Keithley site requires registration)Jim

author: robrenz
author_rank: Super Contributor
timestamp: December 18, 2013, 06:56:57 pm
content: Do you need dynamic measurements like low Hz scope trace or just discreet levels? to get noise down you will need to be using at least 100NPLC or higher which will be about 0.6Hz reading rate.

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: December 18, 2013, 07:15:26 pm
content: Quote from: robrenz on December 18, 2013, 06:56:57 pmMost likely discreet. Thanks for the advice on the 100NPLC, I was concerned about noise at that level.

author: robrenz
author_rank: Super Contributor
timestamp: December 18, 2013, 07:37:07 pm
content: This is my 8846A at 100NPLC and zeroed out on a 100mV signal. The entire vertical scale is about 188nV over 22 minutes. I doubt this would be good enough for you.I will post a shorted input short term plot as soon as it warms up.

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: December 18, 2013, 07:44:01 pm
content: robrenz, Is that just an average?

author: robrenz
author_rank: Super Contributor
timestamp: December 18, 2013, 08:19:55 pm
content: Each vertical bar is the min and max of that time slice. That occurs as the data is decimated to fit memory but not loose the min and max of any time period. Trend plot is not just a plotting function, it is designed to capture data for indefinite long periods.Here are two screen shots of single point data ofshorted inputs(befor any decimation occurs) over roughly 90 second and then 180 seconds.The first pics data covers the left half of the screen in the second screen. That just keeps repeating and the min and max of two adjacent time slots will be merged into one over and over again.The total vertical scale on both screens is 141nV

author: JDubU
author_rank: Frequent Contributor
timestamp: December 18, 2013, 09:07:04 pm
content: This app note and video by Jim Williams may be of interest to you:http://cds.linear.com/docs/en/application-note/an124f.pdf

author: robrenz
author_rank: Super Contributor
timestamp: December 18, 2013, 09:45:10 pm
content: That is an excellent app note but it is ac coupled for .1 to 10Hz and has a 160nV noise floor. Makes my 8846A noise floor look pretty goodI thought of this app note right away, that is why I asked if the OP needed low Hz dynamic signal or discrete levels.

author: eV1Te
author_rank: Regular Contributor
timestamp: December 18, 2013, 11:51:50 pm
content: Does anyone know how Keithley achieves nanovolt resolution in their meter by the way?Maybe you can try a TLC2652A, it is an op-amp with typical 3 nV/K drift, or maybe you will have better luck by measuring the current or charge produced by these organisms. Measuring small currents is much easier I would say even down to fA is possible (by using single op amp such as LMP7721 or LMC6001) that is about 6000 electrons per second, so with a scope you can almost see each electron!Do you know how the signals will look like, is it like a voltage/charge step or is it a slow drift over time (that would be hart to tell apart from drift in electronics)?

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: December 19, 2013, 12:19:40 am
content: eV1Te,The measurement would be a slow drift over time. I don't think it should be too hard to differentiate equipment noise from samples.

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: December 19, 2013, 12:21:43 am
content: The app note is great! Thanks!