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 0x3eCONFIG1 0xd6CONFIG2 0xc0CONFIG3 0xfbLOFF 0x00CH1SET 0x02CH2SET 0x61CH3SET 0x61CH4SET 0x61CH5SET 0x61CH6SET 0x61CH7SET 0x61CH8SET 0x61BIAS_SENSP 0x00BIAS_SENSN 0x00LOFF_SENSP 0x00LOFF_SENSN 0x00LOFF_FLIP 0x00LOFF_STATP 0x00LOFF_STATN 0x00GPIO 0x0fMISC1 0x00MISC2 0x00CONFIG4 0x02
Thanks,
Shea