Will this opamp/instrument amplifier amplifier circuit work as I hope? - Page 1
Forum section: electronics / projects

author: AlessandroAU
author_rank: Regular Contributor
timestamp: May 01, 2014, 03:52:22 pm
content: Hi everyone, I'm currently an engineering student doing a thesis investigating non-linear propagation of acoustic waves through composites. My test set-up involves using piezo transducers to record these sound waves.A large problem I'm seeing (edit: with the current setup) is common mode 50hz noise and the fairly small signal levels output by the piezos. I've designed an instrumentation amplifier based on some components I've found through research and designed a circuit based on examples in datasheets. Basically I want to take my small differential signal, use a combination of AD8250 and AD8253 which have digitally controlled gains to amplify this in steps of 1,2,5,10,100,200 etc up to 10000, then convert the single ended output into a differential output to feed into an ADC (http://www.analog.com/static/imported-files/eval_boards/EVAL-AD7760_7762EDZ.pdf).Since I'm not strictly an electrical engineer I have some gaps in my knowledge, I want to have a bandwidth of up to 1mhz at Gain =10.The input bias current return resistors, R10 and R11, how do I optimise these values? Obviously too large and I will have an error Ib*R, do these effect the input impedance much?Do I need a Ib return resistor R15, or will the previous opamp source/sink this current?Are there better op-amps for the application? I've spent alot of time researching and the AD825x seem the best but of course there could be better?I appreciate any input!!EDIT:fixed schematic error

author: madshaman
author_rank: Frequent Contributor
timestamp: May 01, 2014, 04:13:10 pm
content: Does the 50hz noise disappear when you run it from a battery (it might not)?Sent from my iPad using Tapatalk

author: AlessandroAU
author_rank: Regular Contributor
timestamp: May 01, 2014, 04:18:33 pm
content: Quote from: madshaman on May 01, 2014, 04:13:10 pmWhoops reading back I didn't explain myself clearly, so currently I'm using a scope to measure the output of the piezos and there is alot of common mode noise that is being picked up. This circuit is still theoretical I haven't committed to a build yet, I just wanted to make sure it would work before I did. This is basically going to be a front end to a 24bit ADC (8 bits of scope depth is not enough).

author: madshaman
author_rank: Frequent Contributor
timestamp: May 01, 2014, 04:29:23 pm
content: Quote from: AlessandroAU on May 01, 2014, 04:18:33 pmUnderstood. How are you shielding your test setup?

author: AlessandroAU
author_rank: Regular Contributor
timestamp: May 01, 2014, 04:56:17 pm
content: Quote from: madshaman on May 01, 2014, 04:29:23 pmI presume you mean my current setup? very poorly if at all. The current set-up involves piezo transducers adhered to a fairly small and light beam, because of the sensitivity of the beam due to external factors I cannot run thick shielded cables from the piezo as the added damping and stiffness would interfere with results. I am just using alligator clip to BNC leads to interface to the scope.I hope that using a differential amplifier close to the piezo transducers I can reduce a lot of the noise. I need a pre-amplifier design regardless to increase the signal level for an ADC. I plan to house my eventual design in a shielded metal box running from battery power for maximum mains isolation.

author: madshaman
author_rank: Frequent Contributor
timestamp: May 01, 2014, 05:35:01 pm
content: Quote from: AlessandroAU on May 01, 2014, 04:56:17 pmI highly suggest you take the time to build yourself a proper enclosure with BNC feed-throughs for both power supply and instrumentation first, it might save you a lot of pain. The enclosure also needs to contain your entire setup.I'm not experienced with piezo-electric transducers, but my intuition tells me that a 24-bit ADC is overkill; I don't think you'll be able to limit noise to the point where it will be useful.I'm not saying you don't, but why do you feel you need so much resolution?Please take my advice with a grain of salt, hopefully a more experienced forum member can provide better insight.

author: madshaman
author_rank: Frequent Contributor
timestamp: May 01, 2014, 05:40:16 pm
content: Quote from: AlessandroAU on May 01, 2014, 04:56:17 pmP.S. I think this in itself explains a lot of what you're seeing.

author: AlessandroAU
author_rank: Regular Contributor
timestamp: May 01, 2014, 06:09:25 pm
content: Quote from: madshaman on May 01, 2014, 05:35:01 pmI agree, 24 bits is overkill, but why not go for that anyway, who knows when the hardware could become useful? I don't really expect more than 16 bits of meaningful data.My plan is the build the circuit into a shielded enclosure, battery operated with separate power/control PCB. I've pretty much finished the design of the power supply PCB.