OPA2690

[sparkyyyyy]

Hello,

I was wondering if any of you have used an OPA2690 in the unity gain configuration. I followed the data sheets suggestions of using a 25 Ohms resistor in between the output and inverting input to isolate the input capacitance from the output. I also followed the suggestions of a 50 Ohm resistor in series with the non-inverting input along with the power supply decoupling capacitors of 6.8 uF and 0.1 uF. With the 0.1uF from the V+ and V- supplies being as close ground as possible. My circuit output is oscillating at about 200 MHz with no input.

I am thinking about adding ferrites from my V+ and V- supplies to the capacitors in order to for a low pass filter.

This is a prototype circuit that I am adding to a board that is already built. Once I can get this to work, I will include the OPA2690 circuitry on board. Is there anything that I need to add to the circuit board such as ferrites to ensure against oscillation?

Does anyone have any suggestions?

Thank you.

Thank you,


George

 

[geekEE]

When you say it is "oscillating at about 200 MHz with no input," do you mean that your input is tied to ground or floating or something else?

 

[Noway2]

A couple of things about your post jumped out at me as potential red flags.

First, you say that the decoupling capacitors are "as close to GROUND as possible". Exactly what do you mean by this. Normally you want the decoupling capacitor to be as close to the POWER pin as possible; to provide the surge currents through as low impedance path as possible.

Second, you indicated that this is a prototype "being added to a board that is already built". I did a quick scan through the data sheet and I noticed a bunch of comments indicating the importance of the parasitic effects of the type of external components used and how it is important to NOT socket the device, etc. What I am driving at here, is that this part looks like it is extremely sensitve to parasitic effects.

My suspicion is that your prototype arrangement is at the root of your troubles and that a parasitic capacitance, inductance or both is either causing an unintended pole which is causing the amplifier to go unstable.

 

[sparkyyyyy]

I would like to thank both geekEE and Noway2 for taking the time to respond to my post. I appreciate it.

I found several things and now my circuit is working well. I had to use thick bus wire to extend my ground close to the chips plus and minus supplies. Also even though I was only driving though 1.5 inches of wire wrap wire, I needed to put a resitor right at the amplifier output so the amplifier would not see the capacitive load of the wire. I know the capacitive loading is mentioned in the data sheet, but at first glance I did not see that as a problem.


Thanks again for your help.

Thank you,


George