1
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

Currently facing some issues with noise/stability. When a multimeter is connected across Vout and Vref, there is a effectively no noise/oscillation i.e Multimeter reads 0.0012 mV r.m.s. However when an oscilloscope is connected to it (simultaneously with the Multimeter) the noise level rises to 1.8mV AC. I understand this is negligible however this is part of a current measurement circuitry with a T.F of 40mV/A hence its important to keep this reading as low as possible. The reference voltage is used to provide reference to the other subcircuits within the whole system. This is what I have tried so far, a snubber ciruit; series RC from Node B to ground of 10nF and 100ohm resistor. ALso tried a snubber between nodes A and B. Tried playing about with the compensating resistor R1 but increasing this seemed to made it worse. L1 and L2 are Ferrites respectively. I have tried shorting these out and also playing about with the resistance still no help. Anyone have any other ideas.

UPDATE: I have just found that its in fact the ground connection of the scope is what's causing the issue. just attaching the ground clip on the vref point or Vout upsets it and raises noise to 1mV AC

Regards.

|improve this question
\$\endgroup\$
  • \$\begingroup\$ The op amp used is a TLC2272 op amp \$\endgroup\$ – Tyson Adeyemi Oct 27 '16 at 13:29
  • \$\begingroup\$ Use the edit function to update your question instead of commenting on it ! \$\endgroup\$ – Bimpelrekkie Oct 27 '16 at 13:45
  • \$\begingroup\$ You included the filter in the feedback loop which creates even more risk of instability. Instead of connecting the opamp's - input to C1, connect it to the opamps output without anything in between. Then the R, C and L will work as a filter only and not harm the opamp's feedback loop. \$\endgroup\$ – Bimpelrekkie Oct 27 '16 at 13:50
  • 1
    \$\begingroup\$ Are you aware that the earth scope probe will inject some noise. Why not measure it differentially. \$\endgroup\$ – Andy aka Oct 27 '16 at 13:51
1
\$\begingroup\$

Treat the problem at the source, not after the fact.

The problem is that the opamp is going unstable. That should not really be too surprising with C1 directly on its feedback input. I don't know what you think R1 and C1 are supposed to do, but it won't be anything good.

Get rid of (short) R1, and put C1 on the other side of L1. Also make sure that the opamp is guaranteed to be unity-gain stable.

If loosing R1 and moving C1 don't do it, come back and we can get into more complicated ways to deal with the instability.

|improve this answer
\$\endgroup\$
0
\$\begingroup\$

Scopes have input capacitance and driving capacitive loads changes the system dynamic. Also, you mis-applied your snubber. R1 should isolate C1 from the feedback loop.

Check out: http://www.analog.com/en/analog-dialogue/articles/techniques-to-avoid-instability-capacitive-loading.html

And also Tim Green's Opamp stability series: https://e2e.ti.com/support/amplifiers/precision_amplifiers/w/design_notes/2645.solving-op-amp-stability-issues

|improve this answer
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.