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TLC3578: Some analog inputs are off by fixed amount, others are very accurate.

Part Number: TLC3578
Other Parts Discussed in Thread: ADS8678

I am using a TI TLC3578IPW to measure power supply voltages on my board.  Some of the digital results are very accurate compared to the analog voltages on the ADC input pins.  Some of the digital results are off by a fixed amount.  I have taken many readings, and the digital results are very consistent.  In other words, the results do not vary more than 10mV with every read.

The worst-case reading is for a 1.0V supply from a TPS7A9001DSKR LDO.  It reads 0.936V where the analog voltage is 0.999V.  The digital results for the 5V and 3.3V supplies are within 20mV of the respective analog voltages.

CFR is programmed to 0xE04 (BTC, Short Sampling, Internal Conversion clock, Single-ended, one-shot, EOC)

SCLK is 5MHz frequency.  Vref is 4.0V with 20mV ripple p-p.

Why is the digital reading for the 1V supply much more off compared to the other supplies?  I can supply schematic to TI.