10fA to 500mA measuerment, is it possible? - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 09, 2021, 03:49:02 pm
content: Hi,I came across this open source design, I want to know if it's possible? any Issues? or ideas?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 09, 2021, 04:29:13 pm
content: I don't see how this design could cope with 500 mA at the input. This upper limit is more like 10 mA and even this is with rather poor performance because of the switch resistance. The switch would also limit the usefulness to low currents, as there would be some leakage current, that is temperature dependent. So the practical lower limit is likely more like 100 fA-10pA.With the relatively large capacitance of the switches I am not so sure the TIA circuit would not oscillate. This can at least happen with some input capacitance.I see no real protection for the input.It is a bit odd to use a 3 V ref. with output buffer and than divide it down to 1.5 V for the ADC. There are ready made 2 V and likely some 1.5 V references with buffered output that would be used and give better performence.Anyway the TIA does not really need a high perfromance ADC anyway - it works for a large range, but with rather limited noise and stability.

author: penfold
author_rank: Frequent Contributor
timestamp: April 09, 2021, 05:09:01 pm
content: As mentioned, for the lower currents at 10fA, the temperature dependence is a huge factor, but so long as the circuit layout is focused on eliminating leakage, you can typically calibrate out the device leakages and compensate for temperature. I couldn't say immediately whether that particular circuit would work, but that kind of arrangement is frequently used in gas-ionisation based nuclear-radiation detectors to cover pretty massive dynamic ranges - all an absolute pain for calibrating and compensating... but do manage a consistent low end a little below 10fA.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: April 09, 2021, 06:22:30 pm
content: This is designed for something quite specific (analytical electrochemistry), not just measuring current. Probably everything you need to know about it is described here:https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0140349

author: ogden
author_rank: Super Contributor
timestamp: April 09, 2021, 07:18:40 pm
content: One can draw nice circuit using hi-end components, but unless there is proof - test results, that nice circuit isn't worth anything.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: April 09, 2021, 07:47:22 pm
content: Quote from: ogden on April 09, 2021, 07:18:40 pmHave you looked at the article about this exact circuit I linked to? It includes additional links to:Quote

author: ogden
author_rank: Super Contributor
timestamp: April 09, 2021, 08:57:08 pm
content: Quote from: JohnnyMalaria on April 09, 2021, 07:47:22 pmCould you point where exactly test results of whole 10fA to 500mA range are published in those papers?

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: April 09, 2021, 09:47:35 pm
content: Quote from: ogden on April 09, 2021, 08:57:08 pmI think the more relevant question is who is claiming that current range? It doesn't claim such on the schematic OP provided.Dryden and Wheeler make no such claims in any of the cited articles or athttps://microfluidics.utoronto.ca/gitlab/dstat/dstat-documentation/-/wikis/home

author: ogden
author_rank: Super Contributor
timestamp: April 09, 2021, 10:17:42 pm
content: Quote from: JohnnyMalaria on April 09, 2021, 09:47:35 pmRead carefully subject of this thread!

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: April 09, 2021, 10:26:38 pm
content: Quote from: ogden on April 09, 2021, 10:17:42 pmWell, did OP just pluck that out of a hat or read it somewhere? Or maybe misunderstand?It's intended for electrochemistry and so is commonly concerned with currents of the order of tens of microamps.

author: ogden
author_rank: Super Contributor
timestamp: April 09, 2021, 10:41:47 pm
content: Quote from: JohnnyMalaria on April 09, 2021, 10:26:38 pmThis is up-to OP to explain. My point still stands - unless you have hard proof, it may be not what you think it is. Kleinstein already said it all, I can only agree. Particular circuit can't come close to stated 500mA because author specify max otput current as 22mA which too optimistic anyway for 30mA short circuit current opamp. I won't even comment low current range - out of my scope anyway.QuoteRight. This is what author and papers say.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 10, 2021, 08:35:47 am
content: QuoteMaybe I miss read it, But say 50mA, the higher range is not critical in here,Suppose 10mA max, what about lower ranges? does the op-amp input bias currents way more higher than 10fA,so is it doable? How?

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 10, 2021, 09:19:30 am
content: These Chinese op-amps have 1fA input bias currents and only cost 0.1715$https://lcsc.com/product-detail/Low-Power-OpAmps_3PEAK-TP2121-TR_C248708.htmlso the questions is How it's even possible when the switch leakage currents are around 1nA and the op-amp input bias currents are around 20fA, how they claim 10fA range!How the design can be improved?

author: fcb
author_rank: Super Contributor
timestamp: April 10, 2021, 09:30:34 am
content: I'd have a problem with this design:1. Though shalt not design in MAXIM (lazy EE truism, but I have personal experience being screwed by them), certainly not 5 different MAXIM SKU's - guaranteed way never to get in production!2. The input switch MAX4737 (why?) is not very low leakage, also it's shared with some other signals - it would be like 'nailing jelly' chasing leakage paths.3. The MAX4617 is also not special (leakage), you'd end up with an awkward layout trying to put guard traces or slots around the legs.4. No idea where you get 500mA from the LMP7721 (a superb little opamp) has a source/sink short circuit current of +46mA /-15mA, and with the lowest resistor being 100R (R15) and a Vdd on the opamp of 3V3, you can't source >33mA even if your floating ground gets pinned to a hard rail.If you're trying to make an ammeter that goes from 10fA to 500mA (why???) then you're barking up the wrong tree with this design. This design is obviously for some other use case.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 10, 2021, 10:03:36 am
content: The LMP7721 is not just low bias, but also low noise. This may be a factor for the high current ranges, not so much for the very low current part.The Chinese example shows there are other interesting OPs too, though tis one is rather noisy and slow, which can be an issue with a TIA.Noise is an issue with the higher currents - not so much with a 100 M resistor. Even than the 100 M resistor is relatively small to measure 10 fA.One can still measure currents smaller than the bias currents. One can correct the zero point to some degree, though this is limited as the bias currents are usually quite temperature dependent.In the shown circuit the switch at the input is a big issue, it has way too much leakage. The MUX chip is actually not that bad from the leakage.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: April 10, 2021, 01:26:47 pm
content: Quote from: ali_asadzadeh on April 10, 2021, 09:19:30 amThere's nothing to improve. It's a potentiostat controller for cyclic voltammetry and, as the ample literature indicates, is fit for purpose. It does exactly what it needs to do for the scientist to do the experiment. Read the articles I found for you and you'll quickly realize it's more than just something to measure current.If you want to use this for something it isn't design for, you'll probably be disappointed.You have now entered the realm of science and left the world of engineering. The electronics are merely a tool to get the data. As long as you know the limitations of the tool, you accept it as suitable. If you don't understand the scientific application, you will find faults with the design and claim it needs improving. It doesn't. It does the job.The authors make it very clear that high input capacitance is an issue and they demonstrate that oscillation occurs - BUT IT DOES NOT MATTER for their application for the reasons they explain. It would matter if you were trying to use it for a more general purpose application.When I was a PhD student developing a lab instrument, if I had listened to the electronics engineers employed by the university, I would never have built it. They said things I was trying to do were impossible and they refused to help. Well, my impossible idea is now installed in thousands of labs around the world and a number of instrumentation companies sell their versions of it. In spite of their millions of dollars and professional engineers, my 34-year design still runs circles around theirs. But it is fine-tuned to do one thing and do it well, not be a general purpose tool.

author: Marco
author_rank: Super Contributor
timestamp: April 10, 2021, 04:00:58 pm
content: Ignoring the existing discussion a bit.I just noticed OPAX392 will offer another order of magnitude improvement on LMP7721 on most important properties, quite impressive.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 10, 2021, 04:36:26 pm
content: The OPA392 is not that different from the LME7721. For the typical bias the LME part looks better and also the low frequency (e.g. 10 Hz) noise the LME is about equal if not slightly better (probably depends on individual units). The current noise is also worse on the OPA . This is not such a surprise as the rail to rail input adds another pair of FETs that are incative most of the time. For a TIA one usually does not need a RR input.So in most aspects I would prefer the LME7721.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 10, 2021, 06:05:32 pm
content: QuoteI want to build exactly the potentiostat,So as an engineer I ask myself why they claim these specs? it's not making sense with my logicBut as I said, I want to build it, so I just want to make it logical or doable as the specs they claimed!

author: Kleinstein
author_rank: Super Contributor
timestamp: April 10, 2021, 06:38:41 pm
content: For the higher currents there may be some extra part missing, like a shunt used to measure higher currents. For the bias it depends on the temperature stability and also humidity - in a very stable environment and with a lucky sample for the input switch It may work also to low currents. It also depends on what one uses the 10 fA number for. Is it noise (over which BW ?), stability, offset after cal. (how long). The switch at the input may be used to do a kind of zero calibration.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: April 10, 2021, 06:56:32 pm
content: @ali_asadzadeh Contact one of the authors and ask. I'm sure they'd be more than happy to answer your concerns.

author: fcb
author_rank: Super Contributor
timestamp: April 10, 2021, 07:01:09 pm
content: Quote from: ali_asadzadeh on April 10, 2021, 06:05:32 pmI'm not aware of a commercial potentiostat that goes from 10fA to 500mA. That would be a pretty big electrode to need 500mA.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 11, 2021, 10:31:27 am
content: Hi,Here is a picture from my HP4155B semiconductor parameter analyzer:It shows the Id versus Vgs for a small MOSFETThe Y scale is logarithmic, from 1f (1E15) to 100mA (1E-1), This is a 14 decade range.This is a similar range in currents that the OP is asking about.This is a DC measurement.Current measurement over a similar range is possible.The most sensitive measurement shown in the Dryden paper is:This would require 30nA full scale, and may be a resolution of .3nA.3nA isHUGE(30,000) times bigger that 10fA.Jay_Diddy_B