Rectifier for biomedical analog front end

Thread Starter

alto125

Joined Mar 15, 2013
12
Hi there!
I'm working on a project that requires reading biomedical signals from the body, I'm making an analog front end with differential amplifier, bandpass filter, full wave rectifier, integrator and variable amplifier.
I'm testing the system with a sine wave. Everything is fine until the output of the rectifier - it seems extremely unstable! The amplitude after the bandpass filter is a 1.5v p-p sine wave and after the rectifier it ranges from 2.15 to 3.08V p-p but the shape of the waveforms is not very uniform

Any ideas?

Thanks
 

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#12

Joined Nov 30, 2010
18,216
A copy of a googled precision rectifier only tells us that the rectifier is correct. How you built it must be wrong.
Is that scope showing the first 3 pulses at start up or a steady state condition?
 
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#12

Joined Nov 30, 2010
18,216
Which amplifier chip are you using? How much voltage did you give it to work with (at the supply terminals)? How much current does it have to provide to its load? Stuff like that.
 

GopherT

Joined Nov 23, 2012
8,012
Looks like you are trying to rectify the signal from your sensors. Normally, you would amplify the signal from the sensor before rectification.
 

Thread Starter

alto125

Joined Mar 15, 2013
12
I'm measuring EMG signals, first of I'm amplifying with a ina129 differential amplifier (gain of 735) and then bandpass filtering it between (10-500Hz). It's a dual power supply of 9V supplied to the circuit

Thanks!
 

#12

Joined Nov 30, 2010
18,216
OK. That chip should do nicely. You have an available input range of +/- 5 volts and the available output into 10k ohms is +/- 7 volts. I shall now try to imagine which diodes you used and how the precision rectifier is connected to the previous and following circuits.

No, actually, I will not. I can not examine a circuit which you refuse to reveal. I am done begging you to let me help you.
 
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Thread Starter

alto125

Joined Mar 15, 2013
12
Take it handy!

I've no problem putting up all of my circuit at all, and the diodes are clearly labelled as 1N4148 on the post I put up earlier so there is no need for mad imaginations skills....

;)
 

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