High-sensitivity bioimpedance measurement analog front end  - Page 1
Forum section: electronics / projects

author: jt
author_rank: Regular Contributor
timestamp: January 22, 2016, 02:14:36 am
content: I’ve been getting some good advice here lately so I thought I’d try to push my luck with a more application-specific question.As a side project, I’m working on an instrument that makes bio-impedance measurements. The goal is to resolve <5-mohm modulation (BW of interest ~30Hz) on a ~150-ohm static component. The measurement involves applying a 100kHz constant-current 5mA sinusoidal signal across the skin through ECG pads. A second pair of pads are used to measure the voltage across the skin in-between the current electrodes; basically a 4-wire resistance measurement. There isn’t anything novel about this configuration – here is a random internet image for illustration:http://www.rehab.research.va.gov/jour/2014/512/images/jrrd-2013-04-0085f01lb.jpgRunning through ohms law, I’d need to resolve 25uV amplitude modulation on a 750000uV carrier – so I need a really sensitive amplitude demodulation circuit. I should mention the hope is that the circuit can be small and low power. At first I thought synchronous demodulation (aka lock in amplification) is the way to go. There is precedence in using this for bioimpedance measurements and I thought the ADA2200 was going to give me a very sleek solution. Getting into the details, it appears the output noise of the ADA2200 is going to be unacceptable, even with an instrumentation pre-amp front end. See top right chart of page 9.http://www.analog.com/media/en/technical-documentation/data-sheets/ADA2200.pdfSo, first question: would you concur, or am I misunderstanding the measurement (I’m not very practiced in analog signal chain analysis)?While I’d love to try to design my own lock-in-amp, I don’t feel like I could come up with a compact, low power solution without a lot of trial and error. My fallback is to use a rather unimaginative series of active filters and amplification, choosing high-end, low-noise components:Inst.Amp -> 100kHz BP filter -> 30Hz LP filter -> 0.1Hz AC filter -> Amplification -> A/DSo, second question: Can you think of a better approach? I have yet to look into AC to RMS circuits or active rectification stages, looks like there is precedence for using those as well for bioimpedance measurements.One other technique I found is called a slicing-amp (see link below). My intuition is that this won’t work well in real-world applications because the static component of the resistance (150-ohms in my example, translating to 750mV carrier wave) isn't going to be completely stationary relative to the slicing amp thresholds as the baseline wonders.https://www.ee.iitb.ac.in/~spilab/papers/2009/paper_bbpatil_pilani2009.pdfThoughts?

author: Marco
author_rank: Super Contributor
timestamp: January 22, 2016, 04:23:51 am
content: Quote from: jt on January 22, 2016, 02:14:36 amWithout a detector between band pass and low pass filter that obviously won't work

author: jt
author_rank: Regular Contributor
timestamp: January 22, 2016, 06:59:43 am
content: QuoteAh, great - you are absolutely correct. Do you have a recommendation? RMS detection or a full/wave or half/wave active detection? Or something else?

author: Pjotr
author_rank: Frequent Contributor
timestamp: January 22, 2016, 12:05:21 pm
content: Although I am not familiar with this kind of therapy, 5mA appears to me to be a quite high level of current. At low frequencies it is for sure above fibrillation level in the heart area.IMHO a synchronous detector is the way to go. It has the best noise performance of all. That sliding detector is nothing more than sliding the sampling phase with regard to the carrier: A synchronous (or lock-in detector) is phase sensitive. You can circumvent the phase sensitivity by using two detectors shifted 90 degrees apart. Remove 2x F_carrier with a simple low-pass at say 50 Hz and throw the resulting output at two AD converters. Do the sin-cos math and further low-pass filtering in the digital domain. Accurate synchronous detectors at this frequencies can easily be build wit C-mos switches.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 22, 2016, 01:33:16 pm
content: Is there a reason for using 100kHz as the modulation frequency? It is a bit high for measuring the resistive part of biological tissue - you are more likely to be observing a complex impedance - may be this is what you are looking at?Approaches I have used:Correlation of measured signal with excitation signal. Actually, the correlation was against an assumed sinewave and cosinewave. This was done, in almost real time by a TMS320 DSP - power wasn't a concern at that time. The DSP drove both the excitation signal at 12 bits, and recorded a 2k 12 bit sample window - we were correlating to recover multiple frequencies from the one recording.A battery powered impedance probe (the research prototype for thishttps://zilico.co.uk/zedscan/although I expect the production unit uses completely re-designed electronics). A PIC 16F877 was used to drive a DDS chip to provide the excitation current. The same chip also read an external 14 bit A-D converter at 0, 90, 180 and 270 degree phase positions. I can't recall the exact details but I suspect that the A-D readings may not have been consecutive at the higher measurement frequencies. The key point being that the PIC controlled the synchronisation between the drive signal and the measured signal.

author: Karel
author_rank: Super Contributor
timestamp: January 22, 2016, 03:39:51 pm
content: Many years ago, I designed and built a bio-impedance meter. It measured the complex impedance using 4 electrodes like you described.It used a constant current source of 100uA at 400Hz.It had 3 ranges, 100, 1000 and 10,000 Ohm with digital display. The samplerate was 100Hz and could be plotted and/or stored on a pc.The accuracy was better than 1/10,000 of full-scale. It used 2 dac's to generate two sine-waves (90 degr. out of phase) for synchronous detection.It has been used in some trials but unfortunately, there was too much deviation/artefacts in impedance caused by muscle contraction, not electrical,it was purely change in impedance because of change of density/contraction.When the subject didn't move too much, you could clearly see the heartbeat in the waveform caused by bloodpressure changes that caused impedance changes.(The electrodes were attached to an upper leg)Depending on what you want to achieve, be prepared on very big artefacts (assuming that you want to do tests on living subjects).In other words, your "static" component will not be static at all...

author: jt
author_rank: Regular Contributor
timestamp: January 23, 2016, 06:14:48 am
content: From the thread responses so far, It sounds like synchronous demodulation is the way to go. Does anyone have any thoughts on the viability of the ADA2200 component in this application? Or any other chip-set ideas as alternatives? As previously stated, I'd rather not try to do a full-up custom synchronous demodulation solution if I can avoid it.QuoteYes, this is a big concern of mine - and I've voiced that to my peers in the project. Since I'm not making a monetary investment in this, and I'm willing to write off the time investment as a learning experience, I'm going to compartmentalize my contribution to the performance of the analog circuitry and leave the system level concerns to others.Also, thank you for the description of your system performance. I'm very interested in the capability of "prior art" so that I can better understand what is within reason.QuoteI've been told by credentialed peers in this endeavor that the physiological modulation of interest is best detected with a 80-100kHz carrier range - but the why is beyond my field of expertise. I've also been told that understanding the real vs. imaginary component of the impedance is a nice-to-have but not necessary. With this in mind, one of my concerns with synchronous demodulation is that the I and Q outputs will individually have less SNR than a instrument that is sensitive to both real and complex impedance changes within the same analog signal chain (such as rectification or AC to RMS conversion).QuoteThis was also a concern of mine; at first blush 5mA at 100kHz is a violation of the IEC 60601 standard (which is touched upon on page 5 of the link below). I've been lead to believe that there are intricacies of the standard that can allow higher currents.http://www.analog.com/library/analogdialogue/archives/48-12/bio_imp.pdf