ADS1292R: ADS1292 Lead-off detect abnormality

Hello Zhirong,

Welcome to our forum, and thanks for your post!

Can you please provide a schematic of your ADS1292R circuit?

You might also try increasing the LOFF current magnitude or lowering the comparator thresholds to see if lead-off is detected more frequently.

Is there a reason you have flipped the current source direction on both channels?

Best Regards,

Hi Ryan Andrews:

    We have tried to reduce the comparator threshold(Comparator positive side  70% and Comparator negative side  30% in LOFF: Lead-Off Control Register ), and this approach is invalid when the LOFF current magnitude is fixed to 6nA.

    When the comparison threshold is constant (Comparator positive side  95% and Comparator negative side  5% in LOFF: Lead-Off Control Register), increase the  LOFF current magnitude to 6uA, the lead-off is effective；but the ECG signal is too poor to be recognized. When the lead line is not connected, the read value is 0x80000. When the lead line is connected, the read value is mostly noise, not the normal ECG signal.

BRs

Wu, Zhi-Rong

Hi Ryan,

The figure as below is the schematic of our ADS1292R circuit, Thank you.

Hi Rex,

Thanks for sharing the schematic. A couple things you can try to improve the design:

1. Split the series 51k on each input to 22.1k and 10k, each followed by a common-mode cap (i.e. 47 pF). This will give you a second pole in your anti-aliasing filter.
2. Consider adding a 470 pF differential cap between INxP and INxN. This will reduce the overall frequency response to ~3.3 kHz, which is more than enough for ECG, and may help remove more of the input noise. A differential cap is also better for CMRR.

What is the input signal source? I would recommend testing with a function generator or patient simulator as a reliable input signal for comparison. Alternatively, you can short the input connections together (ELA_PAD with ERA_PAD and ELA_LEAD with ERA_LEAD) and analyze the RMS noise for each channel. I don't expect the noise to increase significantly with the differential lead-off current magnitudes.

Best Regards,

Hi Ryan,

Thanks for your suggestion and we'll test it. I have another question Is that fine we change Rp(R36) to 1.1K improving the CMRR & noise? Is there any side effect? 

BR
Rex

Hi Ryan,
We have some questions as below.
1.Is that available if Rp(R36) is set 30K ohm? And how low of Rp of the resistance can be set?
2.In our schemetic, Cf and Rf are 1M and 1.5nF, could we change Cf and Rf to 2.2M and 680pF? Is there any side effect?

Becasue we want to improve our CMRR or you could give us some suggestion.

Thank you.

BR
Rex