On the AD8232 datasheet only about LA, RA, RL three electrode ECG circuit,
But we want to make limb lead ECG circuit by adding LL electrode,
So I would like to ask whether there are associated circuit can provide references?
If I understand right you want to design a 2 or 3 limb leads ECG system. The AD8232 was originally designed for single lead bio-potential system but nothing stops you from using 2 or 3 AD8232 in parallel. Each electrode will go to two different AD8232. The 1st AD8232 will see electrodes LA and RA (lead I = LA-RA), the 2nd AD8232 will sense electrodes LL and RA (Lead II= LL-RA) and the 3rd AD8232 will sense LL and LA electrodes (lead III= LL-LA)
Three notes of caution:
- if input pull-up resistors are used remember to use just one per electrode. Do not duplicate them because that would decrease the input impedance.
- I would suggest DC lead off as a starting lead-off detection (this forces you to use input pull-ups to +Vs, one per electrode, not one per input pin). If you want to get rid of the input pull-ups and maximize the input impedance you would need AC lead off detection. If that was the case we woudl need to talk further about how one parts´ ac-lead-off signal may interfere with another part´s.
- Driving the RLD electrode is a little bit trickier in a multi-AD8232 system. You may decide to average the three RLD outputs connecting them with 1 resistor each (all three with the same value) to the RLD electrode. Or you may just take one of the three RLD output pins and use it as the system´s RLD, or you could use a constant DC voltage (any of the three REFOUT pins) and use it as RLD (although the system CMRR´s won´t be as good as using a real dynamic RLD)
For a fully featured multi-lead ECG AFE with respiration measurement, and pacemaker detection and many other features, please have a look at the 3 ecg-channel parts ADAS1000-3 and ADAS1000-4
or the 5 channel version: ADAS1000, ADAS1000-1 and ADAS1000-2
I'm not aware of any reference designs or circuits using the AD8232 which discuss the application you are talking about. I've forwarded this question to a colleague to see if he knows any more information for you, and he'll get back to you on this forum.
In the mean time, we do have other designs in the ECG reference designs using other products. So you could take a look at those and see if you they help your application www.analog.com/cn0308
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