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Cannot get an OPA4228 to Operate in a simple basic configuration

Russell Lait
Prodigy 20 points
Other Parts Discussed in Thread: OPA4228, LM324, OPA228, OPA227

I have a phase circuit that I have designed that uses four Op Amps to take an AC Input in the range of 50-60hz and then produce copies of that signal in but each phase shifted by 120 degrees lagging the next.  IE. Output 1 is 0 degrees, Output 2 120 degrees lagging Output 1, and Output 3 is 120 degrees lagging Output 2.

The power rails are set to 

+12v on pin 4

-12v on pin 11

OpAmp 1 is connected as an inverting amp with a gain of ~2 (using a 10k and 20k set of resistors. )

Op Amp 2 and 3 are connected as an inverting unity gain of 1 using 10k set of resistors.

Op amp 4 is connected as a non-inverting amp with a gain of 2.

Using an OPA4228:

When I run a signal of say 1vrms into the configuration I get what I expect except that there is massive noise all over what started as a very clean 60hz signal from a lab signal generator.

If I remove hte OPA4228 and drop in a more generic quad op amp (supposedly more noisy) like an LM324 all of the outputs clean right up and the circuit produces the desired results.  

My question is why is the OPA 4228 not dropping in and simply working?

Reading through the specifications it should operate better than the LM324 or other garden variety quad op amp. (I am trying to achieve a fairly tight SNR, and although the LM324 works, I can see noise artifacts on the 2nd and 3rd outputs that I would like to reduce. Output one is clean to about -80-90dB with mostly the same artifacts as the signal generator.)

Any thoughts or suggestions.  Seem like I am missing something simple here.

Thank you.

Russell

  • 0
    TI__Guru 62085 points

    Hello,

    The OPA228 is a decompensated op amp intended for applications requiring higher loop-gain at lower frequencies while operating in higher gains.  In the case of the OPA228 the minimum gain is +5V/V.  This is described on the first page of the OPA227/228 product datasheet. Use the OPA227 and it should drop right into your circuit without issue.

    Decompensated op amps are not unity-gain stable and require a minimum gain for stable operation. This can be seen when comparing the OPA227 and OPA228 open-loop gain and phase plots which I've copied below.  Notice that the dominant pole in the OPA228 open-loop gain curve is extended almost a decade further than the OPA227 which results in higher amounts of loop gain at lower frequencies.  Also notice that while the OPA227 has >60 degrees of phase margin when the Aol curve = 0dB, the OPA228 has less than 20 degrees which is considered unstable. 

  • 0 in reply to Collin Wells
    Prodigy 20 points

    Thank you for helping me see the difference.  I should have noted the gain spec on the OPA228.  Funny that I initially selected the OPA227 and that is what is on the schematic, but at the last minute purchased the OPA4228 for testing. 

    Happy New Year!

    Thanks again,

    Russell