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ADS1291: precisely what is missing compared to the 1292?

Eamon Egan
Prodigy 50 points
Part Number: ADS1291
Other Parts Discussed in Thread: ADS1292

There is one datasheet for both parts, and it doesn't seem very clear on precisely which resources are removed on the 1291 compared to the 1292.

Obviously I don't expect the channel 2 ADC to be functional. What about the second PGA, brought out to the PGA2N/P?

What about the input mux? Is it functional and connected to the IN2P / IN2N pins, for example for routing RLDIN out onto those pins?

What about IN3N / P?

Eamon 

  • 0
    TI__Guru 66156 points
    Hello Eamon,

    Thanks for your interest in our ADS1291!

    In theory, all of the Channel 2 functionality related to the input MUX should still be usable. Only the PGA and ADC for that channel will be powered-down and disabled in the ADS1291. This would mean that you could still route the RLDOUT signal into the device through RLDIN and configure the MUX to connect it to IN2P or IN2N. The same principle would apply to the auxillary inputs, IN3P and IN3N. Routing IN3P/IN3N to Channel 1 would disconnect IN1P/IN1N from the PGA and allow you to measure another input signal.

    I don't believe anyone on our team as tried doing this before, but by design it looks like it might be possible. Let me look into this further and confirm whether or not you can do this reliably. Have you tried it yourself?

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 50 points

    Hi Ryan,

    Thanks for the answer.

    I'm working on a wearable design that may or may not be used with a dedicated RLD electrode, but will have both IN1 and IN2 ports brought out to the electrode connector. It's primarily a 1 channel device, with a second ECG channel "just in case we want another ECG channel". What it won't have is a dedicated RLD pin defined on the connector, so if RLD is used, it will be in place of one of the 2nd input channel pins.

    The hope is that a real RLD (a third electrode) won't be necessary. We know it works with bias resistors on the input "most of the time" but since we can keep the RLD option alive, I was planning to use the RLDIN routing to provide this just in case there is some corner case that needs RLD for one reason or another.

    Since we will be putting the 1292 on our first prototypes, it's not a huge big deal right now. However, eventually if we want to use the slightly cheaper 1291 but want to maintain the RLD option on the other pins, the question will arise. I don't think we have any 1291s right now to experiment with, so no, we haven't tried it. Our previous generation design used the 1292. So if you could look into it (low priority, but nice to know within a month or so) I'd appreciate it.

    One related question: do you happen to know whether a 1292 with as much as possible powered down on CH2 should be expected to consume more power than a 1291? If not, that leaves only the (rather small) cost difference for us to consider in going for the 1291.

    Best regards,

    Eamon

  • 0 in reply to Eamon Egan
    TI__Guru 66156 points

    Hello Eamon,

    Sorry for the delay, but I was finally able to do some verification on the bench with the ADS1292ECGFE board. I could not communicate with the CH2SET register through our GUI software when an ADS1291 device was installed. However, I did verify with an ADS1292 that either (or both) channels could be powered-down and you can still route the RLDIN to any of the 4 input pins (INxP or INxN). Looking at the design files with our designer, that is how we expect the device to behave. We do not see any digital logic that would prevent you from writing to that register, if the Device ID reads "ADS1291." I have confidence that this will work, but I simply could not verify it myself with our platform.

    In addition, you can still use the IN3P and IN3N as an auxiliary input pair and route it to Channel 1 on the ADS1291. That option I know is listed in the MUX settings for both channels under the CHxSET register.

    You should see the same power consumption on an ADS1292 with one channel powered-down as you see with an ADS1291 (all features, supplies, and clocks being the same). The power per channel looks like it reduces with the ADS1292, but that is because we're dividing down the total power by the number of active channels. This includes quiescent power that is not specific to the number of active channels.

    Best Regards,