Re: Opamp noise

On Nov 27, 7:58 pm, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:
On Thu, 27 Nov 2008 16:49:04 -0800 (PST), the renowned





"comp.arch.embedded.piclist" <Jim_L_Willi...@xxxxxxxxxxx> wrote:
On Nov 27, 6:17 pm, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:
On Thu, 27 Nov 2008 11:48:31 -0800 (PST), the renowned

"comp.arch.embedded.piclist" <Jim_L_Willi...@xxxxxxxxxxx> wrote:
I'm not sure if this is the correct place to ask this, but here goes.
I have build an opamp ciruit to amplify the signal from a force
transducer.  This is working fine, but the problem that I am having is
that if someone walks around it different spots, my steady state level
will change by almost 50mV or sometimes more.  I have removed the
transducer and shorted the diff inputs of the opamp to ground through
a resister and I get the same effect.  I have grounded the gnd 0V rail
to earth along with the transducer as well.  I have used caps near the
opamp but still no luck.  I have a Tektronix TDS2014 for which I am
analyzing the signal and if I touch the probe wire, the output level
of the opamp will go up 20 or 30 millivolts.  These are brand new
scope probes and I have tried four of them, they all do the same
thing, so I am ruling them out.

Is there a way that I can elliminate this.  I need to get acturate
reading into my process, but whenever someone moves, this level moves
up and interferes with the signal.  I am using a TI INA126
Differiential op amp with a gain of 1000.

I have even tried placing the entire circuit inside a grounded wire
screen cage and I still get the same effect.  I also know that it is
not vibration being transmitted to the transducer because of the above
forementioned removal of transducer and replacing with resister to
ground.

Thanks,

James

One possibility is that the op-amp is oscillating and you are getting
"theremin" type effects. Micropower chips are more prone to that sort
of thing. Do you have your 'scope set to BW limit?

It's also possible that the front end of the amplifier is acting as a
detector for RF signals.

You need proper filtering and construction techniques to get useful
results with G=1000, a plugboard will not cut it. It's possible to
prototype with a ground plane and some PCB material but you have to be
a bit careful.

Best regards,
Spehro Pefhany
--
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Filtering on the front end will elliminate my DC gain.

Low pass filtering will not affect the DC gain.

I have thought
of filtering, but then I lost my DC steady state force and only get
the force changes.

Sounds like high pass filtering. Not what you need.

Is it possible to use a two or three stage amp to
elliminate the unwanted signal.

Not really.

Also, I don't see how this is a
bigger problem when I get a DC level increase.  I and going to post my
scope reading in a little bit.  Also, I do have the scope on bandwidth
limit of 20MHz.

Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
sp...@xxxxxxxxxxxx             Info for manufacturers:http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com- Hide quoted text -

- Show quoted text -

Ok, here is at link to scope readouts and my circuit. In my circuit,
I would need a low pass response of 15 Hz or less should be
sufficient, so anything above that should not get amplied even if it
is micro Volts signals. I am wanting 0-15Hz response coming from my
transducer. Also, then INA 126 that I am using is an instrumentation
amp, not an op amp so I can't easily put capacitor in the feedback to
attenuate the high frequencies, so I am at a loss here. I have not
dealt with instrustrument amps before. Only opamps.

I will take any suggestions at this point. Normally I am a digital
person, not analog. But I will certainly try something new, as I have
done.

Heres the link.

http://www.williams-eng.com/circuit/circuit.html

Thanks
.

Relevant Pages

  • Re: Opamp noise
    ... I have build an opamp ciruit to amplify the signal from a force ... transducer and shorted the diff inputs of the opamp to ground through ... detector for RF signals. ... Filtering on the front end will elliminate my DC gain. ...
    (comp.arch.embedded)
  • Re: Opamp noise
    ... I have build an opamp ciruit to amplify the signal from a force ... transducer and shorted the diff inputs of the opamp to ground through ... detector for RF signals. ... Filtering on the front end will elliminate my DC gain. ...
    (comp.arch.embedded)
  • Re: Opamp noise
    ... I have build an opamp ciruit to amplify the signal from a force ... transducer and shorted the diff inputs of the opamp to ground through ... detector for RF signals. ... Filtering on the front end will elliminate my DC gain. ...
    (comp.arch.embedded)
  • Re: Opamp noise
    ... I have build an opamp ciruit to amplify the signal from a force ... transducer and shorted the diff inputs of the opamp to ground through ... detector for RF signals. ... Filtering on the front end will elliminate my DC gain. ...
    (comp.arch.embedded)
  • Re: Opamp noise
    ... I have build an opamp ciruit to amplify the signal from a force ... transducer and shorted the diff inputs of the opamp to ground through ... Do you have your 'scope set to BW limit? ... detector for RF signals. ...
    (comp.arch.embedded)