Dear TI team,
I am implementing the respiration feature on an 18-lead ECG.
The ADS129xR datasheet (SBAS459K –JANUARY 2010–REVISED AUGUST 2015) recommends a value of 470pF for C1 and C5 (chapter §10.2.1) while the ADS1298R evaluation board recommends a value of 2200pF for the same capacitors called C99 and C100.
Also, there are two main differences between the datasheet circuit and ours:
- defibrillation protection is implemented using 10kR for each electrode path on the ECG cable, a TVS (MAX30034, Maxim) and 1kR on each electrode path on our PCB.
- the device is an ECG so the applied parts are CF-type and the maximum allowed total current inside the patient is 10uA (No Fault Condition): taking into account the impact of lead-detection, I would like a maximum respiration current of 8uA. It leads to R1=R2=140kR instead of 40.2kR (chapter §10.2.1 of SBAS459K).
We performed some tests on the ADS1298R evaluation board as well as in our circuit.
On ADS1298R performances with a respiration simulator (Prosim 8, Fluke), we obtain best performance at 32kHz, gain 1 and 112.5°.
Still, the respiration signal is not exploitable with a baseline of 2000R and a 0.1R-variation with any software configuration
With our circuit, same result.
Which value shall be chosen for C1, C5 ?
Can you confirm that R1 and R2 shall be set to 140kR to obtain a respiration current of 8uA ?
Do you still recommend at 32kHz a gain of 3 or 4 and a phase of 112.5° or 135° (page 91 of SBAS459K) ?
PS: You can find attached our circuit that respects all the guidelines from the datasheet.
