Weird Amplifier Problem amplifier is still amplifying a 5MHz

[psyhunter2001]

Weird Amplifier Problem

I don't know if anyone has run into this type of issue before but I have designed a simple amplifier using the THS3001. The problem I am running into is that when I have the input signal OFF, the amplifier is still amplifying a 5MHz signal. I have tracked down this signal to be coming from the power pins of the op-amp. Originally I figured that the power supply was broken but it looks like that is not the case. I currently have a 0.1uF cap connected to each power pin. Should I add another cap like around 6.8uF. Or is the problem somewhere else. If anyone has run into this type of problem please let me know. This thing has been driving me nuts to fix.
Thanks.

 

[E-design]

Re: Weird Amplifier Problem

It may be unstable and oscillating. Board layout and decoupling becomes critical with such fast slew rate amps. Remove any groundplane in a layout under the chip near inputs to prevent parasitic capacitive coupling and place decoupling caps very close to supply pins. Use at least a 10-100nF together with a low value electrolytic. Observe any minimum gain requirements and other precautions as per datasheet.

 

[FvM]

Re: Weird Amplifier Problem

To my opinion, THS3001 is a rather stable and easy-care OP among the high speed types. If wired uncorrectly, I would rather expect oscillations at several 100 MHz than at 5 MHz and I really wonder how you managed to achieve it.

Something like a completely wrong feedback dimensioning or a very unusual supply decoupling may be an explanation. You should show your simple amplifier circuit for better understanding. I assume, that you understood the basic requirements of current mode OP feedback circuits?

The above quoted datasheet suggestions are a good starting point, but also with less decoupling, the part would be stable, normally. At least if not driving heavy loads.