Post Go back to editing

[AD8232] Rigth Leg Drive amplifier

Category: Hardware
Product Number: AD8232

Hi team,

I have some questions for right leg drive amplifier.

#1

The data sheet says that it inverts the common mode signal at the input of the instrumentation amplifier.

Since it is an integrator, the voltage phase will be shifted by 90 degrees.

Will this phase shift affect CMRR?

#2

If the answer to #1 is Yes, I would like to insert a resistor of about 10 megΩ instead of a 1 nF capacitor between the RLD and RLDFB terminals to prevent phase lag.

Is there any problem with this usage?

Best regards,

T-yoshi

Thread Notes

Moved from Instrumentation Amplifiers to Operational Amplifiers on Tuesday, November 21, 2023 1:57:45 AM by LiliPod

Top Replies

    •  Analog Employees 
    •  Super User 
    Nov 21, 2023 +1 suggested

    Hi  ,

    Good day. The effect of the integrator configuration on AD8232 depends on how you use the RLD and RLDFB pins. If the RLD pin is not used to drive an electrode as seen in Fig. 62 of the datasheet…

  • I have moved this question about AD8232 to the OP-AMPS  communitySomeone here should be able to assist you. 

  • Hi  ,

    Good day. The effect of the integrator configuration on AD8232 depends on how you use the RLD and RLDFB pins. If the RLD pin is not used to drive an electrode as seen in Fig. 62 of the datasheet, the common-mode rejection will improve. If the RLD pin is used to drive an electrode as shown in Figure 64 of the datasheet, the system would be susceptible to common-mode interference.

    Changing the 1nF capacitor to 10MΩ would greatly affect the loop gain for the frequency range of 50 Hz to 60 Hz for common-mode line rejection which is not advisable. Hope this helps.

    Regards,
    GIlbeys

  • Hi Gllbeys,

    Thank you for your reply.

    I understand that changing the 1nF capacitor to 10MΩ  is not reccomended.

    We are going to use RLD as the third electrode as in fig.64.

    The datasheet says that the RLD improves CMRR by generating a voltage that is in antiphase with the input common mode voltage.

    Since the RLD forms an integrator circuit, I believe that the output will be out of phase by 90 degrees with respect to the input common mode voltage.

    Is this correct?

    Does this mean that CMRR will be improved even if the deviation is 90°?

    Best regards,

    T-yoshi

  • Hi T-yoshi,

    You don't have to worry about the phase shift, since the integrator is used in a feedback loop. Think about any voltage feedback opamp: they are integrators above a very low frequency. For example, the open loop gain frequency dependence of the OP07 tells you that above 1Hz this opamp is an integrator. See this figure:

    But when you apply negative feedback, the closed-loop corner frequency gets much higher, so the phase shift is eliminated. In a follower configuration (gain of 0dB) up to about 1MHz, so million times higher:

    The integrator is useful in the RLD loop, because the gain is high at low frequencies, so it improves the CMRR at the mains frequency. So as   says, do not use a resistor instead.

    I hope it helps.

    Zoltan

  • Hi  ,

    Good day. I agree with Zoltan. In addition, when the right leg drive output current is injected into the subject, it counteracts common-mode voltage variations, thus improving the common-mode rejection of the system. Since we apply the capacitor as negative feedback, the closed-loop corner frequency gets much higher, eliminating the phase shift.

    Regards,
    Gilbeys

  • Hi Gilbeys,

    I'm sorry for late reply.

    I understand your opinion and will maintain the recommended circuit configuration.

    However, there is a problem that common mode noise other than 50Hz to 60Hz cannot be removed completely.

    When measured with an actual device, the gains of +IN and -IN are slightly different, and as a result, it appears that common mode noise cannot be completely removed.

    Since I am referring to the circuit in Figure 62, and I believe that there is no difference in PCB impedance.

    Is the input impedance slightly different between +IN and -IN?

    I have an additional question.

    A 180kΩ resistor is recommended for the +IN and -IN terminals.

    I understand that this is to limit the electrical current to the human body.

    If it's just for current limiting, can I place it closer to the electrode instead of near the IC?

    Best regards,

    T-yoshi

  • Hi  ,

    Good day. Sorry for the late response. There should be no difference between the impedance of the +IN and -IN terminals of AD8232

    It is recommended that the 180 kΩ resistor be placed in between the electrode and the terminal. Please take note that these resistors are not a comprehensive patient protection system. The resistors may not protect against supply line transients or leakage currents from power and acquisition systems.


    Regards,
    Gilbeys

  • Hi Gilbeys,

    Thank you for your support.

    This means that there is no difference in the internal circuit between IN+ and IN-.

    I have confirmed that there is no impedance difference in the PCB pattern, so I don't know the cause.

    Is it possible for you to check the circuit?

    I have no plans to change 180kΩ.

    I want to see where it should be placed on the PCB.

    From the circuit diagram, I think it should be placed close to the IC, but would it be okay to place it near the connector to the electrode, that is, on the edge of the PCB?

    We want to place 180kΩ on the side closer to the electrode.

    Best regards,

    T-yoshi

  • Hi  ,

    Good day. There was no issue if you place the resistor closer to the electrode as long as it is between the IC and the electrode. However, please take note that the additional resistor can also helped with filtering as stated in the datasheet:

    Regards,
    Gilbeys

  • Hello,

    It appears that the above question has been answered already so I'm temporarily closing this thread. In case there are additional questions, feel free to reply to this thread. Thank you!

    Best regards,
    Paul