AD7779 High Noise when more 3 than Channels are Enabled

Hi all,

well this was a hard nut. We use the AD7779 and were surprised by the high self-noise on all channels at around 100 rms, so 105 dB (see page 33, Tab 10; high-res mode, max decimation).

To our surprise when only a single channel is enabled we reach an rms of 5 counts (~130 dB) which is more what we need. After playing around with the registers it turns out that this holds when less than 4 channels are enabled. When more than 3 channels are enabled (no matter which) the performance degrades from 130 dB to 105 dB.

As a power supply we are using a LTC3260 (https://www.analog.com/media/en/technical-documentation/data-sheets/3260fa.pdf) providing 100 mA -> LDO to +- 1.5 V.
Is that by design or is our frontend/power supply the weak part here?

Also we notice that the channels (also those in parallel) show different noise characteristic. But this is a second issue I suppose.

Thanks and best regards,

Marius



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[edited by: Lluis at 10:34 AM (GMT -4) on 2 Oct 2020]
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