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ADS1298 right leg signal derivation

Peng Liu50174
Expert 1510 points
Other Parts Discussed in Thread: ADS1298

Hi,

I'm not sure if I submitted this question accidently twice.

I have a question about ADS1298 RLD driving signal derivation.

As shown in Figure 33 at page 34, signals output from PGAs are configured to connect to RLD_AMP inverted pin ( RLDINV).

If I connect LA to IN2P, LL to IN3P, RA to IN2N and IN3N. I expect to make RLD=Gain*(LA+LL+RA).

per figure 29, I calculate out following PGA output:

PGA2_P=LA*(1+R2/R1)-RA*(R2/R1),

PGA2_N=RA*(1+R2/R1)-LA*(R2/R1),

PGA3_P=LL*(1+R2/R1)-RA*(R2/R1),

PGA3_N=RA*(1+R2/R1)-LL*(R2/R1),

By any combination, I can't get a summation of (LA+LL+RA).

e.g., if I set boh RLD_SENSP and RLD_SENSN to 0b110, then the summatio is (LA+LL+2*RA).

So anyone please help check if I have calculation error here, and what is the correct setting of RLD_SENSP and RLD_SENSN?

Thanks,

  • 0
    TI__Mastermind 38745 points
    Hey Peng,

    Simply clear one of the RLD_SENSN bits corresponding to RA. This will reduce its impact on the output to a single term.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Expert 1510 points

    Hi, Brian,

    Are you meaning the RA is not input to RLD derivation.

    The RLD is just Gain*(LA+LL)? 

  • 0 in reply to Peng Liu50174
    TI__Mastermind 38745 points
    Hey Peng,

    Let me provide a specific example. Suppose you set the RLDSENSP register exactly like you had before as 0x06 and then configure the RLDSENSN register to 0x02. This way only one of the inputs corresponding to RA is used in the RLD signal derivation. Does this make sense?

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Expert 1510 points

    Hi, Brian,

    IF RLDSENSP=0x6, and RLDSENSN=0x2, then as the output of correponding PGAs are listed below.

    PGA2_P=LA*(1+R2/R1)-RA*(R2/R1),

    PGA2_N=RA*(1+R2/R1)-LA*(R2/R1),

    PGA3_P=LL*(1+R2/R1)-RA*(R2/R1),

    The input to the RLD AMP INV input is summation of  PGA2_P+PGA3_P+PGA2_N, this is not LA+LL+RA, instead the summation is LA+RA+LL*(1+R2/R1)-RA*(R2/R1).

    This is the actual question, you can check if I have some wrong calculation here.

    Peng,

  • 0 in reply to Peng Liu50174
    TI__Mastermind 38745 points
    Hey Peng,

    I have another way to think about that may be more intuitive. Begin by assuming all inputs are centered around (AVDD+AVSS)/2. This is a reasonable assumption since the goal of using the RLD_SENS functionality is to center the inputs at (AVDD+AVSS)/2.

    Then we can say that each "end" of the differential ended PGA provides half of the final PGA gain since the PGA gain is differential. That is to say if you had a PGA gain of 2 and a differential DC input of 1 V centered around 0 V (for the case of bipolar supplies), the output of PGAP would be 0.5 x 1 V x 2 = 1 V and PGAN would be -0.5 x 1 V x 2 = -1. As expected PGAP - PGAN = 2 V = 1V x 2. Similarly just think of each PGA output independent of polarity as just Output = 0.5 x PGA x Input. In your case, each electrode figures the same amount into the RLD amp's transfer function.

    Finally, recall that the goal of this functionality is to improve CMRR and to actively maintain the patient's bias within the rails of the converter. If it can accomplish this, then I think is operating successfully. For more information, take a look at this application note: www.ti.com/.../sbaa188.pdf

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Expert 1510 points

    Hi, Brian,

    But when both input of PGA have common mode noise, I don't think ---each PGA output independent of polarity as just Output = 0.5 x PGA x Input.,

    let's see PGA_In_P=PGA_In_N=VCM. According to Figure 29 of the datasheet,

    Gain=(R1+R2+R2)/R1;

    PGA_Out_P=PGA_In_P*(1+R2/R1)-PGA_In_N*(R2/R1)=PGA_In_P=VCM;

    PGA_Out_N=PGA_In_N*(1+R2/R1)-PGA_In_P*(R2/R1)=PGA_In_N=VCM;

    So for the common node input, there is no PGA Gain when connecting to the summing point at RLD_AMP INV input.

    Overall I think I get the answer by your hint.

    In my case, when RLD_SENSP=0x6, and RLD_SENSN=0x2, suppose common mode noise is applied on LA,LL and RA,

    The PGA outputs have no gain on them with common mode input. The PGA outputs amplitude are the same as the input common mode noise , so the summation is LA+LL+RA. Then only RLD AMP gain has the impact in the close loop, PGA gain has no impact.

    Thanks,

    Peng

  • 0 in reply to Peng Liu50174
    TI__Mastermind 38745 points
    Hey Peng,

    That is correct. You do not want the PGA to have an effect on common mode signals since it is a differential amplifier. The RLD amplifier, however, should provide strong negative feedback to common mode signals in order to settle them down. The closed look gain will be determined by the feedback resistor placed outside the chip and the bandwidth can be controlled by using a feedback capacitor.

    Regards,
    Brian Pisani