[AD8232] Rigth Leg Drive amplifier

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 GLADION  says, do not use a resistor instead.

I hope it helps.

Zoltan

Hi T_yoshi ,

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 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 T_yoshi ,

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 T_yoshi ,

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