High noise level on ADC input

Hi,

This can be caused by the capacitive load of 180pF. If you look at Figure 37 and Figure 40 in the datasheet of the opamp (www.analog.com/.../ADA4500-2.pdf), you can see that it is rather sensitive to capacitive load. The periodically switched sampling capacitor of the ADC means transient load of the opamp output, so this can cause ringing due to the180pF capacitors. You can try to increase the value of the series resistors. You can find more about the possibilities in this article: www.analog.com/.../ask-the-applications-engineer-25.html.

Replacing the opamp can also be a solution (there are examples in the datasheet of the ADC, also for the isolation of the capacitive load).

Hi ZoltanDTI , and JimBrady,

Additionally, I would recommend to make sure that 180pF cap is of C0G or NP0 type to achieve the best distortion performance (once you resolve the minimal count problem) to maximize ENOB. I've seen X7R type capacitors seriously degrade distortion and ENOB.

This source also notes that some ceramic caps can be noisy due to piezo electric effects:

https://www.allaboutcircuits.com/technical-articles/x7r-x5r-c0g...-a-concise-guide-to-ceramic-capacitor-types/

Regards,

Hooman

Thanks for feedback.  I checked the cap type and they are NPO.

I agree that the scope photo looks like ringing and I am going to try increasing series resistors from 24.9 to 100 ohms and see how it things look.  I need to order some parts so it will be a few days.  I also came across an online tool Analog Devices "Precision ADC driver tool" that is telling me that the ADA4500-2 is not a good ADC driver for this.

Jim