Measuring many impedances simultanoeusly - Page 1
Forum section: electronics / projects

author: fabiodl
author_rank: Frequent Contributor
timestamp: August 29, 2020, 10:58:22 am
content: I need to measure a time-varying impedance, both module and phase, between a point and multiple other points (few dozens). I need to sample it with few hundred hertz. Is there any commercially available solution(possibly composing multiple devices) that can do that?Otherwise, how would you do that? I'd like to avoid spinning a board w/ multiple (or multiplexed) adc on a resistive load, and get something already available if possible.

author: Marco
author_rank: Super Contributor
timestamp: August 29, 2020, 11:28:45 am
content: I'd use a standard x-channel digitizer to measure the voltage across the impedances, make a whole lot of parallel voltage to current converters with opamps and feed those with a single channel programmable signal generator. The rest is software.This seems most comparable to a multichannel bio-impedance analyzer, I don't see any commercial systems for it ... but if there are I'm sure the price will be immense.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 29, 2020, 04:00:15 pm
content: Is hundreds of Hz the frequency you want to measure impedance at or the update/scan rate? The most basic answer would be a standard multi channel DAQ from NI or anyone else should handle the data acquisition. The question is how much signal conditioning you would need to do and if you needed to roll your own of if the ones NI sells would be good enough. Basically you would send the excitation to all channels in parallel each with their own sense resistor and measure the voltage across the resistor with the scanned ADC.Another option is a AC impedance bridge (LCR meter) plus an external switch matrix. This will probably be slower than the multi channel DAQ but might be easier to set up.

author: Bud
author_rank: Super Contributor
timestamp: August 29, 2020, 04:16:12 pm
content: Is a requirement to measure all of the points at the same frequency? What physical distances are we talking about?

author: fabiodl
author_rank: Frequent Contributor
timestamp: August 30, 2020, 09:39:10 am
content: Quote from: Marco on August 29, 2020, 11:28:45 amThanks for the hint to the bio-impedance analyzer. This is not a personal project and a total cost under 100K $ would be ok.Quote from: ejeffrey on August 29, 2020, 04:00:15 pmFew hundred hz is the sampling frequency. The LCR meter + external switch matrix is probably a good setup. Do you have experience w/ any in particular?Quote from: Bud on August 29, 2020, 04:16:12 pmYes, all points should be measured using the same input, possibly synchronously. The physical distance is in the order of few inches

author: dmendesf
author_rank: Frequent Contributor
timestamp: August 30, 2020, 12:41:55 pm
content: I make boards for coil impedance measurements that are used in NDT. Highest number of channels up to now was 16 with simultaneous measurements, but it can be increased. Highest frequency was 400kHz. Measurement sample rate (impedance) about 500Hz. Actual impedance range varies with amplifiers used. What's your application?