Experimenting the performance of Polymer Capacitors in Instrumentation Amplifier for Medical
1. Introducion
Medical application like ECG use instrumentation amplifier to condition small signals in the presence of large common-mode voltages and DC potentials.
With the electric current generated from the contraction of the heart wall, electrodes in skin surface can detect potential from 0.5- to 1.5-mV AC signal with a bandwidth of 0.05 to 100 Hz and sometimes up to 1 kHz.
The unwanted electrode DC offset potential of ±500 mV and a large common-mode voltage of up to 1.5 V shall be removed.
Low-pass filtering shall be implemented to remove unwanted common-mode voltage , which mainly comprises two parts: 50- to 60-Hz interference and DC electrode offset potential.
Therefor, low noise, fix gain instrument amplifier AD8424 shall be used in this experiment.Works include scheme design, simulate, PCB design and sold compononts.
With low ESR, the performance of polymer capacitor shall be tested and verified.
2. Priciples and Diagram
2.1 Low noise, fix gain instrument amplifier AD8424 shall be used. According to the datasheet, there are two basic circuits need good quality capacitor.
- The stable DC power supply need one 0.1µF capacity for high frequency filter and one 10µF capacitor for lower frequency in both Vs+ and Vs-.
- Differential Input +IN and -IN need low pass filter to remove input distortion and noise. 1nF and 10nF capacitor shall be used for this purpose.
2.2 Polymer Capacitor
In this challenge Kit, Polymer Aluminum Capacitor, 10 µF, 6.3 V, SP-Cap, CD Series is available
I would try to procure rest capacitor from market, including
0.1µF x2
10nF x1
1nf x2
After some research on the market, the polymer capacitor lower than 1µF is hard to find. So I have to replace them with some MLCC.
2.3 Further test with larger size.
Super-capacitor of 1F is sure to have different character. I would try to show it.
3. Test Procedure
3.1 Design and Simulation
I design the scheme with CADstar 16.0 or online easyEDA and simulte with ADIsimPE.
Then, sold the parts in demo board and test the customized ECG draft design.
3.2 Reports
Even with careful design and calculation, it is highly likely that the expected result is still hard to get.
More parameter tunning and design modification is needed.
I would blog the fault report if the real signal mixed with noise and special report on polymer capacitor.
4. Further work
I would use ADC chip to transfer the analogue signal after the AD8428 into digital values for data logging and analysis after this polymer capacitor experiment.
Another work to do shall be making one ESR meter with Arduino board to show how the ESR meter runs.
