Are oscillations of any type possible on this frontend?
Are oscillations of any type possible on this frontend?
(OP)
Hi all, I am seeing some unwanted signals in the recording . I have done many stability analysis including Bode plot but the amplifier seems not to have any problem. See attched schematic and plots. Can anyone please tell me if they see any issue with this setup? Is gain of less than 2 on stage 1 an issue?





RE: Are oscillations of any type possible on this frontend?
RE: Are oscillations of any type possible on this frontend?
RE: Are oscillations of any type possible on this frontend?
RE: Are oscillations of any type possible on this frontend?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Are oscillations of any type possible on this frontend?
RE: Are oscillations of any type possible on this frontend?
RE: Are oscillations of any type possible on this frontend?
RE: Are oscillations of any type possible on this frontend?
The node at the bottom of R11 is an half-supply 'artificial ground' created by OP4. So R11 is more of a load than positive feedback; or (another way to look at it) part of a Pi attenuator along with R1 and R2.
Schematics don't show layout or lead-length issues. But assuming for a second that those issue don't exist, all the amps are well bypassed with negative feedback to stifle high frequency oscillation.
Even still, one of the first things to check is to twist the timebase knob on your scope to see if there is any high frequency oscillation. Even a subtle broadening on the trace at low frequencies, may turn out to be RF upon closer inspection.
RE: Are oscillations of any type possible on this frontend?
Without seeing examples of those signals it is difficult to speculate. Is that Bode plot from a simulation, or is it measured from the actual physical circuit?
If there is instability, the first place I would look would be your dc power supply splitter. The half supply output should really be effectively decoupled to ground with some serious capacitance, as should your main dc power supply rails.
High gain biomedical amplifiers can suffer from some unique problems at the patient connection. Muscle artifact and electrode problems, as well as relatively high and changing common mode voltages are not unusual.
Your best bet might be to start off with the classic three op amp instrumentation amplifier circuit, with a third patient "ground" electrode to solidly connect the patients anatomy to your mid dc supply voltage reference point.
The two differential patient electrodes should be identical high impedance input circuits. This is all a very well trodden path for ECG and EEG instruments.
RE: Are oscillations of any type possible on this frontend?
Dan - Owner

http://www.Hi-TecDesigns.com