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OP284 - Unusual output behaviour

Smann
on Sep 30, 2020

Hi. Looking for some help and advice.

I have a circuit using an OP284 which is showing odd behaviour to a stepped input that transitions from 0V to -9V.

The trace below shows the behaviour.

Yellow is the output of OP284 CCT ref LG_U42 pin 7

Blue is is the output of OP284 CCT ref LG_U42 pin 1

The step is caused by changing channels on the MUX (ADG1408) that feeds the first Op-amp. The -9V is a voltage rail, as is the 0V. The rails into the mux do not show the noise that is present on the blue trace.

The implemented circuit is shown below.

  • 0
    •  Analog Employees 
    on Oct 5, 2020 7:09 AM 4 months ago

    Hi,

    Can you transfer your probe from output of the 2nd amplifier to the input of the 1st amplifier?

    The oscillation in the output of the 2nd amplifier is most probably due to the oscillation in the output of the 1st amplifier.
    Let us try to capture the problem on the 1st stage amplifier.

    Also, is your circuit is on PCB?
    Additional information about the frequency of the input of 1st stage amplifier might also help to resolve the problem.

    Thanks and regards.

  • 0 on Oct 6, 2020 10:57 AM 4 months ago

    Hi

    Thanks for responding.

    The circuit is on a multi layer PCB with good ground planes and partial power planes. 

    I don't think the 1stamp I/P showed signs of oscillation. This trace shows 1st Amp I/P in yellow and O/P in blue.

     

    On the traces, the input to the 1st Amp id from the 8 channel Mux. The inputs that we are switching between on these traces are just voltage rails, so are reasonably stable. 

    I have looked more closely at a trace where this effect occurs and I would agree it does look like oscillation on 1st Amp. The period of these oscillations is approx 660ns.Trace below O/P 1st Amp Yellow. OO/P 2nd Amp blue.

    I've re-read this question and answer:

    https://ez.analog.com/amplifiers/f/q-a/104337/op284-oscillationg

    I suspect, that we are probably seeing a similar issue? In that case it was resolved by substituting OP284 with AD4084-2 that has superior phase margin.

    Something else that I am considering is to put a simple RC low pass filter in front of the 1st Amp I/P.  1KR and 470pF... ƒc ≅ 339kHz

    Would there be any issue to do this with either the OP284 or ADA4084 as Op-Amp?

    Thanks

    Steve

  • 0 on Oct 7, 2020 8:28 AM 4 months ago in reply to Fdelaram

    Hi.

    Thanks for your reply. I had responded earlier, but for some reason my reply was tagged as spam and deleted.

  • 0
    •  Analog Employees 
    on Oct 7, 2020 8:44 AM 4 months ago in reply to Smann

    Hi,

    Can I contact you thru email to further check the issue that you have?

    Thanks and regards.

  • 0 on Oct 7, 2020 9:01 AM 4 months ago in reply to Fdelaram

    The circuit is on a multi  layer PCB, with ground planes as well as partial power planes.

     

    I think that the input to the 1st Amp is  ok.  I’ll try to attach a trace later.

     

    The signal driving the I/P of 1st Amp, is the MUX selecting between a 0V and -9V signal. So the frequency of the original trace is just the switching rate of the Mux. We see the issue with differing Mux rates.. So perhaps it is more the edge rate at the Mux transition?

     

    Also, I have now properly read a previous issue on this forum  titled “OP284 oscillationg”.

     

    I suspect that this is very similar to what I have been seeing. Looking closer at the point of oscillation, I see an oscillation period of approx. 660ns, approximately what had been reported. The adopted solution in “OP284 oscillationg”  was changing the OP-Amp to an ADA4084, which has a better phase margin.

     

    If Oscillation is caused by edge rate / frequency of I/P signal.. I assume putting a simple low pass RC filter at the 1st Amp I/P would also help prevent oscillations?

     

    Many Thanks