I see on the oscilloscope oscillation (6 MHz, 0.5 volts peak-to-peak) on the analog input (the signal to be digitized is connected to this pin) to an ADS7809 16 bit ADC. For troubleshooting purposes, the input to the ADC was isolated from its driving circuitry by removing a series resistor between it and the driving circuitry and the ADC input was tied to ground. The oscillation was still there.
I believe that the oscillation is due to instability in the internal amplifier that buffers the external reference (please see either the ADS7809 or its cousing the ADS8509 data sheet). Specifically, there's a recommended 2.2 uF tantalum cap that i placed between the "CAP" pin and ground. The block diagram shows the buffer amplifier driving that cap directly without an isolation resistor. I believe that the ESR of the capacitor may be too low resulting in instability in the buffer amplifier. I replaced the capacitor and the oscillation went away.
Would anyone please comment on this? Why does the data sheet specifically recommend a tantalum capacitor? Is it because there needs to be a minimum value ESR that's necessary to keep the buffer amplifier stable (the alternative type of capacitor, the ceramic, has very low ESR).
Ken