OPA690: Simulation issues with unit gain.

Rodrigo Bernardo

Part Number: OPA690
Other Parts Discussed in Thread: TINA-TI, LMH6559, BUF602, BUF802

Hi, I am simulating this device as unit gain with TINA-TI version 9.3. The simulated circuit is shown below:

I have a Sine wave with amplitude of 2V in the voltage generator (VG1). The op-amp is powered with +/-5V and the disable input is connected to +VS.

I was expecting this circuit operating as a high impedance unit gain buffer from 1 to 200MHz, but from frequencies higher than 100MHz it is not stable as you can see in the images below:

Generator at 1MHz = Input (VM1) and output (VM2) with same amplitude and phase.

Generator at 50MHz = Input (VM1) and output (VM2) with same amplitude but not completely in phase.

Generator at 100MHz = Amplitude of VM2 (output) higher than VM1 (input) and not in phase.

Generator at 200MHz = Amplitude of VM2 (output) lower than VM1 (input) and not in phase.

I would like to ask you guys why I am seeing these unstable simulations results? Is this device adequate for this application?

Thank you,

Rodrigo

over 2 years ago

0 kai klaas69 over 2 years ago

Guru 140200 points

Hi Rodrigo,

what you observe is correct and in full agreement with figure 1 and 2 of datasheet. And that a phase shift between input signal and output signal occurs at the highest frequencies is also normal. See figure 24.

For optimum results you should provide some gain reserve. So, for a closed loop gain of 0dB you will have proper results up to about 20MHz with a 200MHz OPAmp. At higher frequencies than 20MHz the performance more and more degrades.

Kai

0 Rodrigo Bernardo over 2 years ago in reply to kai klaas69

Prodigy 190 points

Hi Kai, as described in the datasheet "Unity-Gain Stable: 500 MHz (G = 1)" I thougt that it would work as a buffer up to 200MHz without amplitude oscillation. The phase shift is not an issue for me but the amplitude is not wanted.

Do you know any TI device that I could implement a high impedance input buffer from 1 to 200MHz?

Thanks,

Rodrigo

0 Sam Burnett over 2 years ago in reply to Rodrigo Bernardo

TI__Expert 3620 points

Hi Rodrigo,

The LMH6559 is a very simple high speed buffer with 200 kOhms input impedance that is flat out to 200MHz when operated on +/-5V rails. What is the application? Do you have any more design performance considerations?

Best,

Sam

0 Rodrigo Bernardo over 2 years ago in reply to Sam Burnett

Prodigy 190 points

Hi Sam, I would like to use a high impedance input buffer to "isolate" the 50R load of the Pi attenuator from the rest of the circuit.

Thanks,

Rodrigo

0 Michael Steffes over 2 years ago

Guru 48395 points

Morning Rodrigo,

1st be careful with that unstable word, what you are seeing is either the BW rolloff or slew rate limits, look at the BUF602

0 kai klaas69 over 2 years ago in reply to Rodrigo Bernardo

Guru 140200 points

Hi Rodrigo,

it all depends on your needs. You could also look at datasheet parameters like "bandwidth for 0.1dB flatness" or "harmonic distortion" to find the right OPAmp.

Kai