Tool/software:
Hi,
I have an application using the AFE4960 for biopotential monitoring.
I'm using IBIAS AC Lead Detection to monitor the state of our electrode connections.
I believe that I have the system set up as the data sheet recommends:
LEAD_DET_MODULE_CLK_EN = 1
EN_IBIAS = 1
SW_IBIAS_AC = 1
IBIAS_AMPL = 5 (50nA)
IBIAS_BIAS_AMPL = 9 (corresponds to 50nA)
EN_AC_LEAD_DET_CLK = 1
AC_LEAD_DEMOD_CFG = 0x33
ECG_SAMPLE_SIGN = 0xAAAA
AC_LEAD_DET_CLK_PHASE = 1
Additionally, I have LEAD_DET_WIDTH set to 10 for an ECG acquisition rate of 1kSa/sec.
But the system is not responding as I would expect. I can monitor with a spectrum analyser (actually, an active differential probe and sampling oscilloscope with FFT analyser) the 4kHz tone being created by the lead detection system across the electrodes, and plot that as a function of the known resistance between the electrodes:
This tells me that the 4kHz tone amplitude rises monotonically and linearly with the resistance across the inputs of the AFE.
However, the output of the AC Lead Detection system, as read from the AC_LEAD_AMPL register does not follow the same linear, monotonic progression.
There are some coupling capacitors and parasitics in the electrode system which could give a small phase shift which is dependent upon the source impedance. Thinking that this might be something to do with the issue, I tried the only other value mentioned in the data sheet, which is to set AC_LEAD_DEMOD_CFG = 0x66 (the power-up default value). This gives the following:
I could just be happy that the system "seems to work" with this value for AC_LEAD_DEMOD_CFG. But I have no idea why it "works" this way, and not the way that the data sheet recommends. And I'm concerned that it might point to the fact that I have another configuration issue somewhere else with the device.
Can anybody offer any advice?
Best regards,
Rob