ADS7863A: Programmable REFout Oscillation

Hello Emile,

Thank you for your post.

I noticed that R1690 says "jumper." Is this always installed, such that the REF_OUT voltage is always connected to the load capacitor (C1627)? The output capacitance is needed to ensure that the internal reference buffer is stable.

Is anything else loading the REF_OUT pin?

Regards,

Ryan

Hello Ryan,

Thanks for your response.

Short answers to your questions:

- Yes, R1690 is installed,

- No, nothing else is loading the REF_OUT pin.

A bit more details:

We have the possibility to use either the internal reference provided by REF_OUT or an external 1.5V reference (REF2030) by placing the jumper R1690 or R1689.

In the present case, R1689 is not placed, R1690 is placed.

Regards,

Emile

Thanks, Emile. Is the ADC converting when these oscillations are present for VREF = 1.5 V? I wonder if we are just introducing this instability with the scope probe itself. If the data looks good, then perhaps the reference voltage is stable without the probe connected.

What happens when you install R1689 and uninstall R1690? Does the reference voltage look clean on the scope?

Also, please make sure to measure directly across C1627 so that the ground clip of the scope probe is as close as possible.

Regards,

Ryan

Hello Ryan,

Yes, the ADC is converting. I do not think that the probe itself introduces these instabilities since the frequency of the ripple is quite low (~40 kHz).

When installing R1689 and uninstalling R1690, the reference voltage looks clean, no instabilities in this case.

Regards,

Emile

Thanks, Emile.

I'll see if I can test a couple things on our EVM tomorrow and get back to you with other suggestions. Can you confirm whether this is occurring on multiple boards/devices?

Regards,

Ryan

Hello Ryan,

Yes, this problem is occurring on multiple devices.

Regards,

Emile