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ADS1291: DC Lead-off Problem

Part Number: ADS1291


Tool/software:

Dears,

I am working with my ADS1291 (not EVK, 1CH ECG) and getting trouble in DC Lead-off problem.

My board uses DC-lead off with 10M-Ohm pull-up/pull-down resistors. (RLD not used, dc blocking capacitors not used)

The DC Lead-off gives good results under normal conditions.

However, sometimes both leads are detected  as attached in short times even when both leads are disconnected definitely.

I am struggling with this problem for several weeks but I cannot find the reason.

And the problem is not reproduced normally but about 10% sample board (out of 100 samples) give the phenomenon randomly.

I have been searching the cause from some electro-static effect, or ground problem and so on, but I cannot find the cause of this problem.

Please give me some advice or comments for this problem.

Here is my related schematic.