Are oscillations of any type possible on this frontend?

[europus2002]

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?

 

[VEBill]

Your schematic appears to be missing.

 

[europus2002]

Here is the schematic jpg..

 

[europus2002]

Also, Should the capacitance at the input effect the stability? ....There are 2 10uF caps to simulate the patient contact capacitance....

 

[Skogsgurra]

R11 is giving you some positive feedback. Never good for stability. From where did you get this diagram?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[europus2002]

Hi Skogsgurra, This is developed inhouse. The reason R11 is included is for 2 reasons 1) Set lower cutoff frequency through the cap and 2) Prevent DC build up on capacitor plate which would drive next stage into saturation like we observed. Is this thought wrong? Since on simulations and real world, this shows no signs of distortions / oscillations even with square wave testings.....It is intermittent and fairly random...... Do those input caps count towards instability contributors....thanks in advance

 

[sreid]

OP1 is a differential amplifier. You should have a 4.7 nF capacitor across R10 (as you do R4) for setting the bandwidth (and possibly stability).

 

[europus2002]

I had that cap in original simulation (4.7nF across R10) but that sends phase beyond 220 degrees with 100% oscillatory condition . So i took it off in schematic attached.That's why you do not see it across R2 either since that becomes capacitive load amplifier..

 

[VEBill]

I'd have drawn the circuit more symmetrically and consistently to make its operation more intuitive.

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.

 

[Warpspeed]

Unwanted signals on the recording ?

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.

 

[MacGyverS2000]

I have an old book (80's?) that covers nothing but biomedical instrumentation... I'll see if I can find it on the shelf when I get home. I vaguely remember it having an all-black cover, silver or gold writing. It spent considerable time on op-amp circuits with a lean towards protection from everything else attached to patients.


Dan - Owner

http://www.Hi-TecDesigns.com