I'm working on a project to detect a small impedance change in tissue for a different reason than respiration but the technique is identical. I notice in all the data sheets, application notes, etc. that these devices are always used with a patient simulator which is purely resistive. Yet in the data sheet they show the model for the connection to the patient as a 51K resistor in parallel with a 47nF capacitor. This is never included in any of the descriptions of patient simulators. We have tested our concept with equivalent resistive simulators and even in a glycerin/water mixture acting as a phantom. Works great. However, as soon as we attach the sensors to human tissue (cadaver tissue in our case) the capacitance is so high that the RESP_MODP and RESP_MODN square waves because low amplitude triangle waves and we get no modulation.
In the application report SBAA181.pdf states in section 4 that the effect of cable capacitance will adversely affect the signal but there's not mention of this huge capacitance from tissue itself. I'm curious if anyone has actually manage to get the respiration function of this series of parts to even work? If so, how much did you have to change the EVM design?
