I am trying to synthesize the information in the Rev C datasheet along with the digital lead mode discussion in follow-up question #3 in this thread and am finding that there are several details which are ambiguous. Can you please answer the following questions to help clarify the situation (note that I'm using CH0 ... CH4 to indicate the physical PGA + ADC paths in the LA, LL, RA, V1, V2 order):
I think that answers to these questions should resolve all the ambiguities that I currently see. Thanks.
Sorry, one more question to add to this query:
7. 'Common Electrode B' mode defines WORD3 = V1 - CE, WORD4 = V2 - CE, WORD5 = V3 - CE. This breaks the pattern of WORD3 uses CH2, WORD4 uses CH3, and WORD5 uses CH4. Are we sure that it isn't supposed to be WORD3 = V3 - CE, WORD4 = V1 - CE, and WORD5 = V2 - CE?
If the word order in the datasheet is correct, which bits in FRMCTL correspond to these words? Does bit 21 control WORD3 or WORD5? Does bit 20 control WORD4 or WORD3? Does bit 19 control WORD5 or WORD4?
Apologies for the slow reply. See my responses below:
This is correct. However, if configuring V1 or V2 electrodes for an auxiliary voltage measurement for purposes other than ECG the VCM can be disconnected from the PGA and VCM_REF can be connected instead. See table 50 in datasheet.
This is correct.
Yes VCM_CM_IN. And it is also a High Z because it faces a High input Z amplifier.
Yes it does
VCM follows the voltage at pin CM_IN and is not mixed with the internal VCM_REF (1.3V) .
The output of the amplifier is the voltage of V1 and V2 with respect to VCM. To determine the true voltage of V1 the VCM value is subtracted in the digital domain.
Yes, this calculation will always work regardless of VCM calculation
No that is incorrect, when CEREFEN = 0 common electrode is disabled. That configuration configures the part for Digital lead mode with the following
To produce above you configuration DATAFMT = 0, CHCONFIG = 0, and CEREFEN = 1
Yes, this mode will produce an output that digitizes all inputs in reference to one common electrode. See figure 60. The digitized data is signed and post processing on the MCU side is required to determine the standard lead values. Note, CEREFEN must be set also.
There´s a typo in the table in Figure 62. The drawing is correct (CH2 reads V3-RA) but the table suggests it somehow comes out in WORD5 instead of WORD3.
Thank you MLambe. I would appreciate it if you can spare time for a few follow-up clarifications:
Here are answers to your follow up questions:
3d. Yes, VCM will be 1.3V in this case.