Audioguru said:Your Sallen and Key lowpass filter has a gain of 11 which is higher than 3.0 which will cause it to oscillate. It is a Butterworth filter if its gain is 1.6.
LvW said:Hi aredhel,
one comment to the highpass stage: What is the required cut-off frequency ?
In your text, you have mentioned 0.5 as well as 0.05 Hz. But in the referenced document it is 0.05 Hz.
However, your design is according to 0.5 Hz. Please check again.
In addition, be aware that it will be very problematic to design an active filter circuit at a cut-off of 50mHz (large component values, bad capacitors, finite opamp input resistance).
Highpass: As 0.5 Hz perhaps are OK for you - did you make already some measurements ? All cut-offs around 1 Hz (and below !) are problematic.aredhel said:Yes I wanted a cutoff at 0.05Hz, but as I built the circuit and used the frequency generator (I wanted to test whether the circuit is working as it should) could not supply such low frequency. In addition, it seems completely useless to have a HPF with a cutoff at 0.05Hz since my circuit is not even functioning properly, in fact its cutoff turned out to be 4Hz ... nearly killed off the ECG signal. I am giving 0.5Hz HPF a try and hope the cutoff is not too high that it kills my signal.
I have one question ... I tried out my Notch filter from the previous schematic (the one before the new one I just posted). I got an square-like waveform when I supplied a sinusoidal wave from the frequency generator. However for my LPF I ended up with a sinusoidal wave for an output. I tested these two circuits on separate breadboards. Anyone knows what may be the reason?
Highpass: As 0.5 Hz perhaps are OK for you - did you make already some measurements ? All cut-offs around 1 Hz (and below !) are problematic.
Notch: There are better alternatives than double-T topologies. Double-T is hard to tune. As you got a squarewave at the output (which frequency ?) this indicates bad suppression (notch not exactly tuned) and overdrive of the opamp with a gain of "9".
Audioguru said:The input common-mode rejection ratio of the AD620B instrumentation amp is a minimum of 100dB. So if both inputs are fed the same signal then the output is reduced by 100 thousand times or better.
The datasheet also shows an opamp feeding back the offset voltage to the ankle of the patient.
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