This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

OPA2836 oscillation issue

Other Parts Discussed in Thread: OPA2836, OPA836

Hi,

We are using OPA2836 ICs on our board to support P and N lines for a differential pair. On the board after there are two paths TX and RX and at end of TX path and starting of RX path this IC is used in unity gain configuration. it has output pin shorted to negative input pin. when we do a loop back i.e. connect the output from IC in TX directly to positive input pin of other IC in RX path the output of TX Op-amps start oscillating. and the oscillation is same in amplitude as well as frequency for both P and N op-amps. Is this because of some specific requirements of OP2836 that are not met?? if we don't connect the output to input of other IC then both the devices work fine. Please help.

Thanks and regards,

Nikhil

  • Hello Nikhil,

    Without seeing a schematic I can only suggest a couple of general things to try. The first thing is that since you are driving the output of TX amp directly into the input of the RX amp, the TX amp will see the input capacitance of the RX amp and any parasitics associated with the traces/layout. This can cause instability or oscillations. The recommended solution is to try adding some series resistance to isolate as shown by figure 41 of the OPA836 datasheet (below).

    The next thing that could be at work is if both amplifiers are on the same power supply, there could be some backfeeding effects from the multistage configuration that turn into oscillations. There is a good blog post that addresses this issue and some potential fixes: http://e2e.ti.com/blogs_/b/analogwire/archive/2013/02/01/xavier-ramus.aspx

    The other thing that you could try is putting a resistor in the feedback path instead of shorting it. That could potentially help if capacitive loading is an issue. You could also try adding a resistor across the input terminals to try and increase the noise gain, making the system more stable. Even a noise gain of 2 should help.

    These are pretty general recommendations though. If you have a schematic you can post that would be very helpful to narrow down on the solution.

    If you're not comfortable posting your schematic you can also send it directly to me on E2E through the messaging feature.

    Regards,
    Luke Lapointe
    High Speed Amplifiers