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Implementing shield driver with RLD amplifier

Rashid Peralta
Prodigy 130 points

Other Parts Discussed in Thread: ADS1296, ADS1298

Hi,

I am trying to implement a shield driver to avoid current leakage on electrodes which are connected to analog channels on ADS1296 and I have realized this chip can use all the  signals on these channels to do this task but I have realized the phase is 180º, this would not be useful for my purposes so I am using an external low-noise op amp as an inverting amplifier wih gain = 1, this way could be possible to implement the shield driver but I do not understand the purpose of R and C components shown on figure 51 and 52 of datasheet. Could anyone helping me with this problem?

Regards,

Rashid 

  • 0
    TI__Mastermind 43915 points

    Hi Rashid,

    Are you talking about the feedback resistor and cap that are in the feedback loop of the internal RLD amplifier? They are used to help with CMR. Give this app note a read -

    http://www.ti.com/lit/an/sbaa188/sbaa188.pdf

    Regarding a RLD shield driver technique, in our early pre-ADS1298 discrete ECG design solution, we included a shield driver. We decided on not including it populated on the ADS1298ECG-FE solution but did route the circuitry on board for someone to test their own solution. You can look at the schematic included in the ADS1298 User's Guide document and see that U5 and passive components around were left uninstalled but are in place. This circuit has not been tested with the ADS1298 but is routed on the board.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 130 points

    Hi Tony,

    Exactly I am talking about those components. I saw in the schematic you pointed out that there is an optional connection where RLD_INV can be directly connected to the buffer amplifier without using those R and C components so this way is implemented the shield driver with phase of 0º. If I configure RLD_SENSP[1:6]=1, RLD_SENSN[1:6]=1 and each MUXn[2:0]=000 registers I would have an average of signals fed into channels IN[1:6]P and IN[1:6]N, is it correct?

    Regards,

    Rashid 

  • 0 in reply to Rashid Peralta
    TI__Mastermind 43915 points

    Rashid,

    You can try the shield driver circuit but keep in mind that it was not tested by us.

    Setting MUXn = 000 will set the converter for normal electrode so the inputs will pass through the internal mux. Setting RLD_SENSP = 1 and RLD_SENSN = 1 will average the inputs from channel 2 out the RLD bias circuit. Take a look Figure 61 and you can see that you should be able to probe the RLDINV pin to see the average of the selected channels.

    Regards,

    Tony