Newbie Instrument Amplifier Question - Page 1
Forum section: electronics / beginners

author: ed_reardon
author_rank: Regular Contributor
timestamp: November 17, 2015, 05:53:15 pm
content: Hi all,I've set myself a small project which is connected to my job, I'm very fresh to all of this, (been interested in this stuff about ten days!) and I've designed what I think is a simple circuit that'll do what I need it to do.The basic idea is that I work with 'very' (I guess, depends how you qualify that)sensitive ion-selective electrodes which output a voltage of maximum +/- 250mV and I'd like to use an Arduino volt meter to measure this electrode output.The accuracy required isn't astounding, as this is simply a small tool "for indication only"to pick up well out of spec' electrodes rather than fractions of a percent accuracy, ideally I'd like a tolerance of +/- 2-3%; that'd make me more than happy, even drifting to 5-7% wouldn't be a major headache as this device would be to simply test that an electrode is doing broadly what it's expected to do, and an approximate decade shift (60mV roughly on these electodes) between decade standards.Having watched the Holy Dave's Op-Amp videos I've come to the conclusion I'll need a mega-impedance op-amp for this application as these electrodes are very sensitive and should be, by my understanding, sourcing as tiny weeny bit of current as possible -- well, less than that even.Having pulled a few industrial machines apart, it seems the LM6001 op-amp seems to be pretty common in the industry, I've very fresh to this but looking around this seems to (generally) sit right on top of the incoming connector and run with no gain, I guess from having watched a few things this means it's acting as a 'buffer?' (Feel free to correct me.)The final idea would be to power this as a small PCB from a few coin cells, guessing that the op-amps require only a gnats-fart of power.Now the questions, at last.1) Is it strictly correct to feed the non-inverting side directly from the output or should this be put through a resistor? Remembering I need as amplifer to provide no gain.2) Would there be benefit in passing this output in to further op-amps as buffers, is there an additive effect of stringing amps together (most of the designs I've looked at feed the LM6001 output in to more convetional op amps)?3) Do I need to add a resistor between the incoming BNC header and the op-amp input, or even capactiance?4) What have I missed?Thanks in advance for any input!Cheers,Andy

author: Simon
author_rank: Unknown
timestamp: November 17, 2015, 06:02:20 pm
content: The opamp you describe is indeed working as a buffer, yes the output goes straight into the inverting input. No you don't want to "string" opamps together, all this does is add more errors and noise. It is common practice to have a unity gain input stage as a buffer to seperate the souce from the rest of the circuit and guarantee that it is not loading the device under test. You then have whatever signal processig is needed. Cascading opamps can be done if say you need high gain which you don't, two stages with moderate gain can be better than 1 with a huge gain but putting two many stages in will eventually cause problems.No if your probes are that sensitive do not connect anything but the non inverting input.

author: ed_reardon
author_rank: Regular Contributor
timestamp: November 17, 2015, 06:11:13 pm
content: That's great, thank you Simon.Turns out you're really not far away from me either, few miles down the A6 for me....

author: Simon
author_rank: Unknown
timestamp: November 17, 2015, 06:12:37 pm
content: Ah small world, I actually know two people on this forum.

author: ed_reardon
author_rank: Regular Contributor
timestamp: November 17, 2015, 06:24:06 pm
content: Next question, (I'm sorry, forgive me) is that I guess I've got two options;The probes to 'wander' a bit, especially when they're stabilising - say it might heading for 50mV but it could swing between 51Mv and 49mV quite quickly for a few minutes which is unhelpful for me.I'd ideally like to smooth this effect, is this best done in either a) Software - as I'm coupling to an Arduino voltage sense pin or b) In circuit.There is another option I'd like to consider which would be to use a DVM rather than an Arduino, but for now the Arduino is best suited.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 17, 2015, 06:27:51 pm
content: The LMC6001 is good for a low input bias, but consumes quite some power. If powered from coin cells I would look for something with lower power (e.g. LMC6041) - though noise might be higher then.It might help to have a protective resistor (e.g. 10 K-100 K range), caps would be good to keep out HF pickup, but many have extra leakage.Depending on the ADC inputs, some amplification might help. At the low frequenies there is no need to have a separate buffer first.Smoothing in the slow range is better done in software. It's best to use a time window that is a multiple of 20 ms for averaging, to suppress 50 Hz pickup.

author: Simon
author_rank: Unknown
timestamp: November 17, 2015, 06:40:26 pm
content: some low pass input filtering is probably a good idea if your probes can take a little load, you would risk picking up things like mains hum if it's high impedence.Do you need to filter the measurement ? you could have detection of the swinging to tell you the probes are not ready yet.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: November 18, 2015, 12:30:47 am
content: ISE meters I've used have a 1 gigaohm input impedance, so you would have to be very careful about even a filter capacitor. I guess that sort of rules out most DVMs too, but you might want to try one just to see if the probes will put up with that kind of load.I'm thinking the reference pH meter schematic you find in the LMC6001 datasheet would be a very good starting point, since it's pretty much the same thing.Are you planning on prototyping the layout or going to get a board made? If prototyping I would be thinking about completely isolating the input pin with a teflon post or wiring it in the air or something.