ADS1294 Lead OFF detection issue

Sergii Makovetskyi

Intellectual 495 points

Other Parts Discussed in Thread: ADS1294

Hi guys,

we have made own PCB with ADS1294, and using it for 6 LEAD ECG capturing.

We have an issue with sensitivity of lead-off detection.

Even if electrode is disconnected we sometimes see it as connected.

We use DC Lead-off detection method with current source and tried to change current (6/12/18/24nAmp) with not difference.

We think we are doing something wrong.

Can you please help?

over 9 years ago

0 Brian Pisani over 9 years ago

TI__Mastermind 38745 points

Hey Sergii,

Can you post your schematic?

Regards,
Brian Pisani

0 Sergii Makovetskyi over 9 years ago in reply to Brian Pisani

Intellectual 495 points

Hi Brian,

yes, sure. Also i have ESD diodes at the inputs - SP724AHTG

Regards,

Sergii

0 Sergii Makovetskyi over 9 years ago in reply to Sergii Makovetskyi

Intellectual 495 points

Chip configuration: (Register/Data)
00h 10010000
01h 10000101
02h 10000
03h 11001100
04h 11
05h 0
06h 0
07h 0
08h 0
09h 0
0ah 0
0bh 0
0ch 0
0dh 0
0eh 0
0fh 1111
10h 110
11h 0
12h 1111
13h 0
14h 1111
15h 1001
16h 0
17h 10
18h 0

0 Brian Pisani over 9 years ago in reply to Sergii Makovetskyi

TI__Mastermind 38745 points

Hey Sergii,

I recommend narrowing the lead-off comparator thresholds by changing bits [7:5] in the LOFF register. What I think may be happening is that the diodes are limiting the voltage range of the inputs to within the comparator's thresholds thus preventing the comparator from triggering.

Regards,
Brian Pisani

0 Sergii Makovetskyi over 9 years ago in reply to Brian Pisani

Intellectual 495 points

Hi Brian,

thank you for the answer,

i forgot to describe the issue - sometimes we have situation when leads are detected ,but they are not connected to the body.

We have 6 PCBs, on most of them we have this issue, but 2 of them works OK. Also i can see that PCB is too sensitive to the humidity, so when i am trying to breath to the PCB - i can see that all leads are detected.

Some of the leads are always detected (but there is no short circuit to the ground).

Also we have tried to configure threshold from 95.5 to 70% and nothing were changed.

So, i don't think that it is due to the diodes (otherwise it will not work on any PCBs).

Regards,

Sergii Makovetskyi

0 Brian Pisani over 9 years ago in reply to Sergii Makovetskyi

TI__Mastermind 38745 points

Hey Sergii,

Try probing the inputs on the failing units and see what the voltages are when the leads are off. If the voltages are not near the rails, then the detection circuit is not triggering.

Regards,
Brian Pisani

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ADS1294 Lead OFF detection issue