Right leg drive integrator

Hi

I have a question regarding the driven right leg part circuit depicted in your ECG-circuit of your LT1168 or LT1920 datasheets.

I understand this circuit so: C1 in the feedback path together with R3/R4 (15k)  forms an integrator that attenuates the input common mode signal

with increasing frequency.  R2 in the feedback is for a stable biasing by restoring feedback at DC (where the circuit behaves like a high-gain inv. ampl.)

What does "POLE AT 1kHz" mean? Is that the "cutoff" frequency of the integrator? How is it calculated? Is this correct?:

1/(2π*((R3*R4)/(R3+R4))*C1) = 1061 Hz ?

What does then R1 do next C1? What is its purpose?



grammar
[edited by: pauledd at 6:42 AM (GMT 0) on 24 May 2019]
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  • +1
    •  Analog Employees 
    on May 29, 2019 1:02 AM

    Hi pauledd,

    Sorry about the delay in getting a response back to you.

    Your questions and my responses below:

    Question 1 : What does "POLE AT 1kHz" mean? Is that the "cutoff" frequency of the integrator? How is it calculated?

    Response: "POLE at 1kHz" is due to the final stage R7, and C3. It's the overall circuit roll-off.

    LT1168 will have plenty of BW at a gain of 10V/V and the R7, C3 combination ensures that unused bandwidth beyond 1kHz is attenuated which also serves to reduce unwanted noise.

    Question 2: What does then R1 do next C1? What is its purpose?

    Response: R1 creates a zero in the transfer function at around 1.3kHz where the integration roll-off ends. I cannot immediately think why it was deemed necessary to stop the integration beyond this frequency!

    However, I've seen series R placed in series with capacitors in the op amp feedback loop used before. The reason I had been given was to isolate the capacitor beyond its usable frequency such as when the cap approaches its resonant frequency where it could misbehave / act inductive and cause instability. I cannot be 100% sure that's the reason for having R1 in series with C1 but it's the only thing that comes to my mind (unless other folks on this forum can think of other reasons?).

    Regards,

    Hooman

Reply
  • +1
    •  Analog Employees 
    on May 29, 2019 1:02 AM

    Hi pauledd,

    Sorry about the delay in getting a response back to you.

    Your questions and my responses below:

    Question 1 : What does "POLE AT 1kHz" mean? Is that the "cutoff" frequency of the integrator? How is it calculated?

    Response: "POLE at 1kHz" is due to the final stage R7, and C3. It's the overall circuit roll-off.

    LT1168 will have plenty of BW at a gain of 10V/V and the R7, C3 combination ensures that unused bandwidth beyond 1kHz is attenuated which also serves to reduce unwanted noise.

    Question 2: What does then R1 do next C1? What is its purpose?

    Response: R1 creates a zero in the transfer function at around 1.3kHz where the integration roll-off ends. I cannot immediately think why it was deemed necessary to stop the integration beyond this frequency!

    However, I've seen series R placed in series with capacitors in the op amp feedback loop used before. The reason I had been given was to isolate the capacitor beyond its usable frequency such as when the cap approaches its resonant frequency where it could misbehave / act inductive and cause instability. I cannot be 100% sure that's the reason for having R1 in series with C1 but it's the only thing that comes to my mind (unless other folks on this forum can think of other reasons?).

    Regards,

    Hooman

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