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Precision Data Converters
Precision Data Converters Forum
ADS7809 - REF pin needs 2.2 uF cap - Why?
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.
How does the ESR of the recommended 2.2 uF tantalum capacitor connected from the the "CAP" pin to ground effect the performance? I see curves of SINAD and THD vs cap ESR in the ADS8509 (the recommended replacement for the ADS7809). We have a problem wherein we see 6 MHz oscillation on the input pin to an ADS7809. We changed the capacitor connected from the "CAP" pin to ground and the oscillation went away. Does the ESR of the capacitor connected from "CAP" pin to ground affect the stability of the input?
Why is a tantalum capacitor recommended for the capacitor between "CAP" and ground? Tantalum's have more ESR than ceramics, and a ceramic is (nowadays anyway) available in 2.2 uF. Is there a minimum amount of ESR that is required to prevent the internal buffer amplifier shown below (from the ADS8509 data sheet) from oscillating? If so, what is that minimum ESR value?
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