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ADS1299 bias amplifier open-loop gain

Graham Kelly77
Prodigy 20 points

Other Parts Discussed in Thread: ADS1299, ADS1298

Hello,

We are using the ADS1299 in an EEG design and suspect we are having stability problems with our bias feedback loop. We would like to be able to do some analysis. I know TI doesn't have any SPICE models for the ADS1299. Is the open-loop gain versus frequency of the bias amplifier used in the ADS1299 available? Alternatively, is this amplifier an IP block that has been used in other TI silicon--and, if so, is there a SPICE model for that part instead?

Thank you,

Graham Kelly
Hardware Engineer

  • 0
    TI__Mastermind 38745 points
    Hey Graham,

    Unfortunately we do not have an easy to use SPICE model for the bias amplifier on the ADS1299. However, take a look at this document discussing the ADS1298 right-leg drive circuit that is identical to the bias circuit on the ADS1299. In Section 4, the author discusses compensating for low frequency poles created by cable impedance. The strategy he proposes involves placing a zero in the output of the amplifier near the pole created by the cable load.

    Another strategy that I have heard people use is to drive the cable shield with a signal corresponding to the common-mode voltage of the inputs. This will reduce the shunt capacitance between the electrode cables and the shield pushing the pole further out in frequency.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 20 points

    Hi Brian,

    I've read that document but must have missed the part you mentioned. Reading it now--it looks like the author doesn't have any explicit guidance on how to select the compensation capacitor in parallel with the protection resistor, but I assume the idea is to have the time constants for the parasitic LPF and the compensation network be equal?

    Thanks,

    Graham

  • 0 in reply to Graham Kelly77
    TI__Mastermind 38745 points
    Graham,

    That is correct. The zero created by Cp and Rp will be located at fp = 1/(2*pi*Rp*Cp). Try to match this relatively closely with the pole frequency created by the cable. The affect of the closely paired pole-zero is a net cancellation and improved phase margin.

    Regards,
    Brian Pisani