I'm driving an LTC2440 24 bit sigma delta ADC with an AD8475 funnel amplifier. If I short the ADC inputs together I measure about 7 counts RMS noise, which is about 2uV RMS. This matches my expectation and the figures quoted on the ADC datasheet for a system with a 10Hz to 1kHz bandwidth. My problem is that when I drive the ADC through the AD8475 I measure 105 counts RMS of noise, which is over 25uV RMS.
Can anyone suggest a way of improving on this?
Perhaps you could try throttling down the AD8475's bandwidth with a cap directly across the ADC differential input (after the 47 ohm resistors). 0.1uF might be a good starting point. The AD8475 is built for 4 MS/s @ 18 bit operation, so it may allow through some high-frequency noise that your ADC can't handle. The LTC2440 has a neat resolution vs. speed tradeoff feature, but ADCs are not always 100% resistant to out-of-band noise.