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ADS1299 How to check BIAS is working as expected and other problems

twistx77
Intellectual 480 points
Other Parts Discussed in Thread: ADS1299, ADS1292

Hello,

we have a device which is using the ADS1299 and a ADS1292 and it is already working, we have checked it works and we can dectect biosignals but we are seeing a weird signal of about 20hz in our samples and we are wondering if the bias is working as it should.

We tested a prototype before making our custom hardware using a ADS1299 Eval KIT but we didn't pay too much attention to the quality of the signal since we just wanted too see if it worked and that was it.

Now in our custom hardware we are using 8 single ended inputs which have the negative input of the amplifiers connected to a reference electrode through SRB1.

What we have tried to test the BIAS is connecting the BIAS, REF and two inputs electrode to the patient and then we tried disconnecting BIAS and REF electrodes to see what happened with the signal.

If we don't disconnect anything, we get the EEG signal with the 20hz noise in the expected channels and we can see when the patient blinks. The signal in the channels with no electrodes are basically a constant line.

If we disconnect the REF electrode we just get noise in all channels which is logical.

If we disconnect just the BIAS electrode, the signal in the channels with electrodes connected to the patient doesn't seem to have any change but the channels with no electrodes connected we get noise.

Is that the expected behavior? Is there any way to make sure the BIAS is working ok? We have tried reading with one of the channels the BIAS signal configuring its input and we get something like a noise that one we disconnect the BIAS cable it dissapears and if we connect it again we don't get that "noise" back.

We are filtering the signals with software filter to get rid of the DC because there is a lot of offset. How much offset should we be typically expecting?

The inputs in the PCB are basically the same as in the Eval KIT with the same filter in the input. The BIAS drive loop has a 1Meg in parallel with 1.5nF.

Also the DC lead-off feature isn't working at all when using the same configuration we had with the eval KIT which was working perfectly. We only get zeros in the lead-off status as all the leads where always connected. We've tried changing the lead-off configuration with no luck but we are still investigating.

Please, any help will be greatly appreciated.

  • 0
    TI__Guru 63086 points

    Hi Alex, 

    That's a lot of information there. :) 

    The REF electrode can be connected to all INxN inputs through SRB1, as you have described. You will need to keep this signal connected in order to take a proper single-ended measurement on each channel. Without it, I would expect nothing but noise like you observed. If any of the channels are unused, you can short the inputs and power down that channel.

    The BIAS amplifier should output a near-DC signal close to the voltage selected for BIASREF. Typically, this is near mid-supply. What are the power supply and BIASREF voltages in your system?

    Best Regards,

  • 0 in reply to Ryan Andrews
    Intellectual 480 points

    Hi Ryan,

    Thank you for your feedback. Sorry for the huge post. 

    I only talked about the REF electrode to let you know it was working as expected. We will be checking the BIAS which seems to be working correctly but we want to make sure it is since we got yesterday a very weird signal when we sampled the BIAS signal using one of the inputs of the ADS1299. 

    Also we are seeing a lot of DC offset and that was another of the reasons we have doubts about the BIAS circuit working correctly. Could you please give us an idea of how much offset we should we expect when the BIAS is working correctly?

    Thank you.

  • 0 in reply to twistx77
    Intellectual 480 points

    Hello Ryan,

    We've tested the device and the BIAS seems to be working. When active the BIAS pin goes to mid supply +-3mV and when disconnected the sensing or the bias in general we get mid supply +- 200mV.

    Also, we are getting about 7.9mV of offset and in some case 93,75mV, are this figures ok?

    We are also having a problem with the Gains from 8 to 24 which are not amplifying as expected. I'm going to make another post with details.

    Thank you.

  • 0 in reply to Ryan Andrews
    Prodigy 40 points
    Hi Ryan


    I am developing a EMG acquisition system for upper limbs and I am using the channels in differential mode (not single-ended). The ADS1299 is powered in dual-supply, between +2.5 and -2.5 V. I would like to use the Bias Drive function but, unfortunately, I am having some problems with it.

    In the initialization part:
    I write 0xE4 into CONFIG3 register to have:
    Internal reference buffer enabled, Bias signal generator and Bias buffer enabled, Reference on GND applied externally (BIASREF pin shorted to GND).
    In the registers BIAS_SENSP and BIAS_SENSN I am enabling the bias sensing in all 8 channels, both P and N side.
    Furthermore, I am setting all channels in "normal input mode", it means MUXn[2:0] = 000.
    My BIASDRIVE electrode is put on the elbow and connected at BIASIN(=BIASOUT) pin exactly as "ADS1299 User Guide" schematic at page 49. I would not "sacrifice" a channel to use it as BIASDRIVE, that is why I am using an other electrode connected to BIASIN pin.

    What I found is that in the BIASDRIVE pin I have a voltage of, more or less, -2.5 V, when all is connected and +2.5 V when all electrodes are disconnected from body. These are really different from the expected GND voltage.

    Do you have any idea on what is happening?
    Am I doing something wrong in the procedure written above?


    Thanks for your support,
    BR
    Enzo