PCM6240-Q1: How to calculate resistor values between MICBIAS pin and INxP/INxN pins

Hi Kento-san,

Can you confirm if the PCM6240-Q1 is used in AC-Coupled mode or DC-Coupled mode?

If it is used in AC-Coupled mode, the bias resistance can be calculated using this calculator:

SBAC263 Support software | TI.com

Thanks and Regards,

Lakshmi Narasimhan

Hello Lakshmi,

Thank you for the support. As shown in the figure below, they are measuring with AC-coupled mode. On difference is they inserts 100ohm between measurement point and INxxpin as shown below.

Based on that, could you answer the questions? Though I calculated effective resistance with the calculator, 20kohm resulted in 10kohm which is diffrent from customer's measurement (=16.7kohm)

1. How can they calculate resistance between MICBIAS pin and INxP/INxN pins when they populate R0?

2. When they populate 20kohm resistor, the 16.7kohm measurement result correct?

Thanks,

Kento

Hi Kento-san,

Thank you for the clarification. Let me get back to you with a response for these queries by tomorrow.

Thanks and Regards,

Lakshmi Narasimhan

Hello Lakshmi,

Thank you! Looking forward to hearing from you.

Thanks,

Kento

Hello Kakshmi,

I am looking forward to hearing from you on this topic soon.

THanks,

Kento

Hi Kento-san,

1) 

Kento Adachi said:

How can they calculate resistance between MICBIAS pin and INxP/INxN pins when they populate R0?

In order to calculate what the resistance needs to be, in the Excel sheet calculator, we can mention the max. rms swing expected from the input, and the programmed MICBIAS voltage (circled in green). Based on that, the calculator returns a max. resistance value allowed (circled in red)

2) 

Kento Adachi said:

When they populate 20kohm resistor, the 16.7kohm measurement result correct?

Regarding this measurement, the calculator mentions an effective look-in impedance of the inputs, and the corresponding common-mode voltage seen at the INxP/M pins, as indicated in the block diagram as well.

I believe in this scenario we are referring to measurement between the input pins and the MICBIAS pin, which is different from what is mentioned in the calculator, please correct me if I am wrong.

Thanks and Regards,

Lakshmi Narasimhan

Hello Lakshmi,

Thank you for your reply.

Lakshmi Narasimhan Badrinarayanan said:

I believe in this scenario we are referring to measurement between the input pins and the MICBIAS pin, which is different from what is mentioned in the calculator

Yes, they are measuring the resistance between input pins and MICBIAS pin. Is this different from the look-in impedance?

Could you please tell me how to calculate the resistance between input pins and MICBIAS pin based on Rext value?

Thanks,

Kento

Hi Kento-san,

Yes, the look-in impedance refers to the effective input impedance at the INxP/M terminals, hence is different to the impedance measurement between INxP/M and MICBIAS.

Tnaks and Regards,

Lakshmi Narasimhan

Hello Lakshmi,

Understood. Then, could you please tell me how to calculate the resistance between input pins and MICBIAS pin based on Rext value?

At least, when they populate 20kohm Rext resistor, the 16.7kohm impedance result between input pins and MICBIAS pin correct?

Thanks,

Kento

Hi Kento-san,

There is no internal resistance expected between the input pins and MICBIAS pin, so if a 20kOhm is connected between INP/M and MICBIAS, the resistance between INP/MICBIAS should be 20kOhm

If the measured value is 16.7kOhm, that seems to indicate a parallel 100kOhm impedance. Is there any other connection on the INxP pin or MICBIAS pin? Another possibility (although a lesser possibility) could be that the multimeter used has an input impedance of a value close to 100kOhm.

I will also try to replicate these connections on a EVM and confirm this by tomorrow.

Thanks and Regards,

Lakshmi Narasimhan

Hello Lakshmi,

Thank you for yoru reply. 

Lakshmi Narasimhan Badrinarayanan said:

Is there any other connection on the INxP pin or MICBIAS pin?

No, there is no other connection on the INxP pin or MICBIAs pin.

Lakshmi Narasimhan Badrinarayanan said:

I will also try to replicate these connections on a EVM and confirm this by tomorrow.

That is really helpful. Thank you for your coopeation.

Thanks,

Kento

Hi Kento-san,

I have checked on the EVM, the measured value between INxP/M and MICBIAS for an external 20k resistor is indeed 16.7kOhm.

It is also confirmed that this is expected due to the presence of internal input impedances impacting the effective resistance measurement.

Thanks and Regards,

Lakshmi Narasimhan

Hello Lakshmi,

Thank you for your confirmation at your side.

Customer had tried the 20kohm external resistor placement for 40 times, and the measured value between INxP/M and MICBIAS varies from 12k~17kohm. As described in the first post, they need to check that the resitor change is approprately done by measuring the resistance. Thus, the min/max value between INxP/M and MICBIAS is important.

1. Could you please let me know the min/max resistance value range between INxP/M and MICBIAS?

2. Does it come from the variation of internal components?

Thanks,

Kento

Hi Kento-san,

I am assuming that resistor placement for 40 times refers to 40 different PCBs with different units, please correct me if I am wrong.

Kento Adachi said:

1. Could you please let me know the min/max resistance value range between INxP/M and MICBIAS?

Let me look into the exact numbers and get back to you

Kento Adachi said:

2. Does it come from the variation of internal components?

While variation of internal resistances from unit to unit can play a role, tolerance of the external resistance can also play a role. Can you share the tolerance of the external 20k resistance used?

Thanks and Regards,

Lakshmi Narasimhan

Hello Lakshmi,

Lakshmi Narasimhan Badrinarayanan said:

I am assuming that resistor placement for 40 times refers to 40 different PCBs with different units, please correct me if I am wrong.

Yes, you are correct.

Lakshmi Narasimhan Badrinarayanan said:

While variation of internal resistances from unit to unit can play a role, tolerance of the external resistance can also play a role. Can you share the tolerance of the external 20k resistance used?

The external resistance variation is 5% max and does not affect so much.

I am glad if you check the variation of internal resistance and min/max resistance value range between INxP/M and MICBIAS when they place 20kohm external resistor.

Thanks,

Kento

Hello Lakshmi,

I would like to resolve this question soon. If it is difficult for you to answer the question, could you pleae loop a system engineer of this IC in an e-mail?

Thanks,

Kento

Hi Kento-san,

Per your suggestion I have started an email communication looping in our systems engineer.

Thanks and Regards,.

Lakshmi Narasimhan

Hello Lakshmi,

Thanks to your looping, we mostly cleared customer's question.

Could you please let me know one more point? 

Lakshmi Narasimhan Badrinarayanan said:

I have checked on the EVM, the measured value between INxP/M and MICBIAS for an external 20k resistor is indeed 16.7kOhm.

In your measurement, I believe you used PCM6xx0Q1EVM-PDK Evaluation Module, is it correct?

If yes, could you please let me know how you modify the circuit and the conditions of jumpers (Jxx)?

Thanks,

Kento

Hello Lakshmi,

Thank you for your strong support! You can close this thread.

Thanks,

Kento

Closing this thread as mentioned.