I know the data sheet says that the REF pin on the ADS7809 needs to be bypassed, and I believe it having seen noise problems with a similar converter when the REF pin was not bypassed with a fairly large capacitor. Why is this?
In general, a large capacitor (>1uF) must be placed at the reference input of analog-to-digital converters, and it has two functions:1) to limit noise bandwidth of reference and/or buffer2) to keep the reference voltage very stable during conversion time.
Since the ADS7809 has an internal buffer and reference, two 2.2uF capacitors are needed to limit noise bandwidth on both of them. In the case of the reference noise bandwidth, the 2.2uF capacitor at the REF pins limits reference noise bandwidth to about 20Hz.
Precision Data ConvertersApplications Team
Are dynamic currents pulled from the reference circuit during each conversion due to the switched capacitor architecture of this converter?
Yes, the reference is switched in/out with each bit decision that the ADS7809 makes. Can you tell us anything about your application? The ADS7809 has been listed as NRND (not recommended for new designs). The ADS8509 is pin-compatible to the ADS7809 and has the same features at a lower price. If this is a new project for you, I would suggest that you look into using the ADS8509 instead.
We're using the ADS7809 in a space application (equipment is mounted on a spacecraft) and the ADS7809 has flight heritage, that's why we like it.
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