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unstable DC level of ADAS1000 output, chip dependent

Question asked by shengwuei on Dec 18, 2015
Latest reply on Dec 21, 2015 by Padraic

Hi Sir,

 

I had made my own PCB with ADAS1000 on board, schematic as below. I believe the schematic is exactly the same as the evaluation board of ADAS1000 (ADAS1000SDZ).

schematic.png

 

 

I had made total 6 ADAS1000 PCB boards, but found output DC level of 4 of the boards is higher then the two others, see below :

(left : LL/RA/RL untouched, middle : put fingers on LL/RA/RL, right : then remove fingers from LL/RA/RL)

 

  • the four boards with higher DC level : DC level is about 2.6x10^6 before/during/after putting fingers on LL/RA/RL.

未命名1.png

 

  • the two boards with lower DC level : DC level is about 190 before finger touching, about 2.6x10^6 during finger touching, and about 1.8x10^4 after finger touching.

未命名2.png

 

 

In the previous figures, SNR is good(the middle figures) so DC level drifting is not a problem to me, but when signal is weak and SNR is low, it will be difficult to distinguish whether fingers are placed on leads or not, because middle figure will be similar with the right figure. Or, I will not be able to confirm if fingers were touched well on leads or not during finger touching.

 

Any suggestions on this ? Below is my register setting for your reference, thanks.

 

FF 0002e8// 1000 samples
01 000001// apply SWRST
05 e07002// LA/LL/RA contribute to RLD common voltage, necessary for DC lead-off detection
01 f800ae// ECGCTL, in digital lead mode
0b 000000// 40Hz LPF
0a BFD600// enable lead II, RESPMEG and LOFF
03 000079//  EXT_RESP_LL, external respiration path,  amplitude/1, 56 kHz
02 000015// enable only DC lead off detection, 50nA current

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