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ADS1299 bias measurement with lead-off current source enabled

Shea K Ako
Prodigy 10 points
Other Parts Discussed in Thread: ADS1299

Hello,

I have been experimenting with the ADS1299 lead off detection functions and there is something that puzzles me.
With the bias lead disconnected and bias lead-off current source enabled (CONFIG4[PD_LOFF_COMP]=1, CONFIG3[PD_BIAS]=0, CONFIG3[BIAS_LOFF_SENS]=1) I can see that the bias lead off comparator is detecting the open lead (CONFIG3[BIAS_STAT]==1).
This indicates that the bias lead-off detection current sink has pulled the bias lead sufficiently toward VSS to trigger the comparator, due to the fact that there is no current path to pull the lead toward AVDD. In fact, the bias lead potential should be at AVSS. That all makes sense.
If I configure an input channel to measure the potential on the bias lead (eg. CH1SET=0x02, CONFIG3[BIAS_MEASURE]=1), the ADC input should measure a voltage of about AVSS-((AVDD+AVSS)/2), or in my case -2.5V, since I'm using a singled ended 5V analog supply. What puzzles me is that the ADC input is actually measuring about +1mV.
In the BIAS LEAD-OFF section of the datasheet it does say, "Only the comparator can be used to sense the voltage at the output of the BIAS amplifier". Does this mean that  when the bias amplifier is powered down with the bias lead-off sensing functions enabled, the input multiplexer settings (CH1SET=0x02, CONFIG3[BIAS_MEASURE]=1) are overridden?
Here is a complete dump of all register settings, in fact you can see CONFIG3[BIAS_STAT]==1:
ID          0x3e
CONFIG1     0xd6
CONFIG2     0xc0
CONFIG3     0xfb
LOFF        0x00
CH1SET      0x02
CH2SET      0x61
CH3SET      0x61
CH4SET      0x61
CH5SET      0x61
CH6SET      0x61
CH7SET      0x61
CH8SET      0x61
BIAS_SENSP  0x00
BIAS_SENSN  0x00
LOFF_SENSP  0x00
LOFF_SENSN  0x00
LOFF_FLIP   0x00
LOFF_STATP  0x00
LOFF_STATN  0x00
GPIO        0x0f
MISC1       0x00
MISC2       0x00
CONFIG4     0x02
Thanks,
Shea