Why is an unloaded buffer showing an offset ?

[anushaas]

One key assumption is that this particular part is
pristine. After checking that the supplies are what
you think they are, I'd swap out for another part
and see whether the issue persists, or changes.

A little handling ESD can really whack op amp input
attributes.

I tried using a new AD8041 opamp (as an unloaded buffer) and it also showed the same offset at it's output.However,a LF347(also as unloaded buffer) did not show any such offset.

I shall try using some other opamp with 20MHz GBW as suggested by FvM

 

[anushaas]

The combination of the first C & D (4th and 5th) waveforms makes no sense. Either the oscilloscope has an offset error, or the output offset is changing when you connect the oscilloscope probe. In the latter case, it's most likely a case of high-frequent self oscillations, see my previous post #8. Also the distorted output waveform suggests an oscillation or other serious hardware problem.

My suggestion is to try a user friendly 10 to 20 MHz GBW OP in your circuit.

I tried the circuit using OPA827 and INA163.The schematic is as shown below:


The input waveform VG1 is obtained as:


The output waveforms as obtained from two INA163 instrumentation amplifiers(VF1 and VF2 in schematic) are shown below:
VF1:


VF2:


The waveform VF2 appers to have jitters.While a 480mVpp sine was directly given as input to U4(INA163),the output obtained was:


Is there any way of correcting this?

 

[Audioguru]

You see electronic noise, not jitters.
R6 and R7 attenuate lower frequencies but the 100pF capacitor passes high frequency noise.

 

[KlausST]

Hi,

Is there any way of correcting this?

It seams your breadboard can not handle the circuit precision.

Maybe bat contacts, bad wires, or just no star point connect to on of ground.
A PCB with proper GND plane should give better results.

****
Even your expensive OPAMPs won't improve a bad wiring or bad breadboard.

You don't say what your circuit is for... I assume the circuit is too complicated for the purpose... I don't know.

Klaus

 

[FvM]

You see electronic noise, not jitters.

Exactly.

In contrast to AD8041, I won't expect stability problems with OPA827 and IN163 due to breadboard circuit problems. But surely other "real world" circuit phenomena like noise.

A detail analysis can reveal the dominant circuit noise sources.

 

[Audioguru]

The 100pF capacitor is causing the noise because the resistors reduce the levels of low frequencies but the capacitor passes high frequency noise 12 times higher.