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Use ADS1292R with only 2 leads(without RLD out)

Vishal Algure
Prodigy 185 points
Other Parts Discussed in Thread: ADS1292R

Hi,

I have to measure the ECG and respiration using only 2 leads without RLD out lead.

I am modifying schematic page from page no.58 of datasheet of ADS1292R.

1. Can I connect RLD_out to R4 and R6 instead of AVDD and remove R3 and R5.

2. I am connecting LA and RA to RLD_out through 10 MOhm resistor to improve CMRR as I dont have RLD out lead.

3. Can I put 51K resistor between C1 and IN1P and same for negative channel for patient protection.

4. Can I use DC lead off detection? As respiration channel has capacitors so is it possible to use DC lead off detection?

Thanks and regards,

Vishal 

  • 0
    TI__Mastermind 38745 points

    Hi Vishal,

    1.      Yes that is a viable method of setting the DC bias point for those inputs.
    2.      If you’re going to do that, you need to AC couple those inputs so that there is very little current flowing through those resistors. You can accomplish that similarly to how you planned to do it with the channel 1 inputs.
    3.      Just putting patient protection resistors on the lead traces should be sufficient. No need for patient protection resistors on each of the inputs.
    4.      It is not possible to do DC lead-off detection with an AC coupled input. I would recommend looking into the AC lead-off detection functionality for this device.

    Regards,

    Brian Pisani

  • 0 in reply to Brian Pisani
    Prodigy 185 points

    Hi Brian,

    1. If I connect RLD_out to R4 and R6. then how to drive RLD_out i.e. from internal (AVDD+AVSS)/2 or derive from any channel

    2. Means I need to add AC coupling to channel 2 also(ECG channel)? if yes what is the value of capacitor for that?

    3. If I add large value patient protection resistor on lead then respiration modulator current reduces.(Ib=(VREFP-VREFN)/(R1+R2+500E))=Ib= 30uA

    Do you mean to change only location of R2 and R1 to lead traces?

    4. But EVM of ADS1292R also has ac coupled for respiration channel and still it uses DC lead off detection with 6 nA current? 

    Thanks and regards,

    Vishal

  • 0 in reply to Vishal Algure
    TI__Mastermind 38745 points

    Hey Vishal,

    1. You should just drive the RLD amp using the internal (AVDD+AVSS)/2 only.
    2. The answer to this depends on your signal bandwidth. The AC coupling cap and the bias setting resistor serve as a high-pass RC filter whose half-power frequency is 1/(2pi*RC). If you expect a lot of signal content to be close to DC, then I would make that RC constant as high as possible to drive the cutoff as close to DC as possible.
    3.  It does change the modulator current, but you will know the value of those resistors, so you can factor that into your body-impedance measurement calculation. Think of it as changing the level of the baseline impedance you’d measure on the body.
    4. The EVM software provides as much flexibility as possible for evaluating the device. For example if a user wanted to acquire ECG data on channel one and did not want to AC couple it, they could place zero ohm resistors on the capacitor pads and remove the biasing resistors. In that case, they could definitely use DC lead-off detection for that channel. Though it may be possible to enable lead-off detection for AC coupled channels using the EVM, those channels would never trigger detection.

    Regards,

    Brian Pisani

  • 0 in reply to Brian Pisani
    Prodigy 185 points
    Hi,

    1. Yes, I will connect internal (AVDD+AVSS)/2.

    2. Datasheet of ADS1292R don't suggest AC coupled input for channel 2(page no. 58). It suggest AC coupling to Channel 1 only.

    3. If I put large value patient resistor at input then my respiration current will reduce and same will affect my change in voltage at the time of respiration. As well as in one of the application note(SBAA181) of TI mentioned that if we increase the base resistance then it will affect the measured respiration impedance(delta R). So i don't think that we can keep large resistor at input lea traces of ADS1292R and I will use 51K resistor between C1 and IN1P(page no. 50 of datasheet) for patient protection from channel 1. Or I think alternative option is put 40.2K resistors at input traces of lead only for patient protection from all 3 channels.

    4. I will use ac lead off detection in my design, but in EVM respiration channel is ac coupled and i haven't changed the settings in code, still it detects lead off detection(GUI shows that DC lead off detection with 6 nA).
  • 0 in reply to Vishal Algure
    TI__Mastermind 38745 points

    Hi Vishal,

    2. That figure exists to demonstrate the way by which only two electrodes can be used to acquire both respiration rate and an ECG signal. It assumes that there was some other way to generate a valid common mode voltage on the body such as use of the RLD electrode. If you are not going to use an RLD electrode connected to the RLD amplifier, then you must set the common mode some other way. The reason why I do not believe the configuration you proposed will work is because the RLD amp has very limited drive strength. If a large resistor was placed from the RLD amp to the input and the electrode offset drifted too high, it would have trouble regulating body common mode voltage to mid-supply. In that case the RLD amp is doing little to improve common mode rejection. If the inputs are all AC coupled, the common mode is set and the RLD amplifier is not significantly loaded.

    3. You are partially right. The ΔR will be smaller with respect to the entire resistance in the path, but you can still recover it if you know the other impedances. The demodulated voltage you will see on channel 1 will be VREF(ΔR + RBASE + RELECTRODE1 + RELECTRODE2 + 2RPP)/(ΔR + RBASE + RELECTRODE1 + RELECTRODE2 + 2RPP + R1 + R2) where RPP is the patient protection resistance you place on each electrode. If the waveform becomes too small to recognize in the output, you can simply increase your PGA gain setting granted your offset is not so high that the PGA is railed.

    If you use a 51 kOhm resistor on the RESP_MOD pins but on no other pins, then there will be no patient protection on any of the other pins which defeats the purpose of patient protection. If you put the 40.2 kOhm resistors on the shared input lead, then the voltage divider that is used to measure the respiration impedance will be shorted out which prevents you from measuring it.

    4. You are able to detect lead-off on channel 1? Or can you detect it for those electrodes using channel 2 which is not AC coupled? If a channel is AC coupled, then the input pins will stay at the bias point set by the resistors and lead-off detection would never be triggered for that channel.

    Regards,

    Brian Pisani

  • 0 in reply to Brian Pisani
    Prodigy 185 points

    Hi Brian ,

    Please find the below my understanding after our discussion for 2 lead configuration;

    1. I will use internal (AVDD+AVSS)/2 for RLD_out reference. 

    2. I have to AC couple channel 2(ECG channel) also by placing series capacitor between LA to IN2P and same for RA and IN2N.

        Respiration channel is already AC coupled as shown in datasheet of ADS1292R and I will connect R6 and R4 to RLD_out.

        I will add pull up resistor of 10 Mohm between RA and LA to RLD out pin.

    3. Relectrode(1.2K) is small as compare to patient protection resistor(51K), I think it will affect the respiration measurement and so i am adding patine protection resistor(51K) for respiration channel between IN1P and C1 and same for C5 and IN1N. 

    4. Yes, I am able to detect lead off detection of channel 1 as well as channel 2. I will use internal AC lead off detection.

  • 0 in reply to Vishal Algure
    TI__Mastermind 38745 points
    Hi Vishal,
    After looking at the numbers, it seems that 51 kOhm resistors on each input would increase the offset voltage too much relative to the small change in thoracic impedance you would want to measure. Since the offset exists, you can’t gain the input high enough to resolve the change in impedance. Does the patient protection impedance have to be as large as 51 kOhm? If so, then you could add the resistors in the path you suggested, but you should also put them on the channel 2 inputs to provide patient protection on that path as well. Otherwise, your configuration sounds like it would be good. Let me know if you have any more questions.
    Regards,
    Brian Pisani