Driving AD80066

A bit quick on the 'Send' button there.

When I mentioned 'both planes missing' I was referring to power and ground planes, which are currently continuous under the buffer amplifiers.  We don't want large gaps in both planes anywhere, because then light can shine through the PCB - we don't want stray light.

And I should have mentioned that we have the possibility to slow the whole circuit even more, by a factor of about 4 (so the AD80066's ADCCLK would be 1MHz, and the individual channels would be running at 250kHz).

Hi pixelPhile,

Could you attach the scope shots of the oscillations? A schematic would help as well. What is your intended application?

Regards.

I hope these are visible.

The application involves a CMOS linear sensor (left hand side of the schematic).  The AD80066 is used in SHA mode.

Thanks for your interest,

Phil

Hi Phil,

Apologies for the late response. What we could do is help you troubleshoot the circuitry.

First, disconnect the amplifier and the AD80066 by removing R1. Check for the amplifier output to see if the oscillation is caused by the AD80066 or the component on the left side. If the oscillation still persists, please install a Low Pass RC filter after the Video DAC. Make the amplifier for gain of 2 and add a voltage divider network at the output to bring it back to a gain of 1. A 100R combination would do it. Also, could you further expand the schematic? I want to see more of the left side of the circuit. Thank you!

Regards,

jino

Hi Jino, and thanks again.

These are very worthwhile suggestions and I'll carry them out as soon as I can (will be a few days at least).

I cropped the schematic because I didn't want to publish details of what we're doing.  Can I share this with you privately?

Phil

One more question: to change the gain, I'll need to solder 'flying' resistors around the op-amp.  Is that likely to materially affect the result? 

Hi Jino,

I tried disconnecting the AD80066; the oscillation persists.

I tried using a x2 gain (100R + 100R from output to ground; centre taken to -ve input of op-amp); the oscillation persists.

Finally I tried using 51R/1nF RC filter on the output (i.e. I replaced C1 by 1nf); the oscillation persists (in fact, it's probably worse).

I do hope you have some other ideas...  Best regards

Phil

Hi Phil,

Have you tried the RC low pass filter before the input of the amplifier? In place of R5.

Regards.

Hi

What do you recommend?  RCR or can I just put a cap across the input of

the op-amp?

Phil

Hi Phil,

You can do these.

1. Test the amplifier separately. Disconnect all of the other ICs from the input and output side, Video DAC and AD80066, respectively. Check if the amplifier itself oscillates. The best way is to check it with our evaluation board if you are willing to spend for a separate board for testing purposes. Universal Evaluation Board for Single, 8-Lead SOIC Operational Amplifiers, for example. Or you can just test it on-board (your current board) to save you much time. Instead of using the Video DAC as an input, replace it with a signal generator for testing purposes.

2. Implement a voltage divider on the input side, after R5 add a resistance (100R) to the ground to attenuate the input by 0.5 V/V. Then add a capacitor in parallel with that resistor. Choose a value designed for your cutoff. Then gain up the amplifier by 2V/V to bring the signal back to its original amplitude. See figure below. 

Let me know if this works.

Regards,

Jino