• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 2 answers
  • Subscribers 9 subscribers
  • Views 485 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMX8410L: VCM_IN pin - using it with two ADCs

Charles Luke
Intellectual 270 points
Part Number: LMX8410L

I have two separate ADCs, one for I and one for Q. They each have a common mode output. The LMX8410 only has a single common mode input VCM_IN pin 36. It would have been better if the part had a VCM pin for I and one for Q. Maybe I can combine these for a single input to the LMX8410. What is the impedance of the VCM_IN pin? What is your recommendation for dealing with two common mode inputs from the ADCs? I could go with the eval design and just use a resistive divider off of the 5V supply. I also see the common mode can be supplied externally or internally. How is the internal mode implemented?

  • 0
    TI__Mastermind 18070 points

    Hello Charles,

    For internal VCM, there are three options: 1.4V, 1.7V and 2V. For external VCM, anywhere between 1.2V and 2V would be fine, although different register configurations are required. Details can be found in datasheet section 7.3.1.6

    There is only one VCM value and one VCM pin because the common mode voltages for both I and Q differential outputs are the same. I didn't quite understand why the ADCs for I and Q channels need to be biased at different levels.

    Yes you can use voltage divider to obtain VCM from power supply directly. Just make sure that the voltage variation is under control. In general, external VCM is more accurate than internal VCM in terms of IF output common mode voltage level.

    We don't have the value of VCM pin impedance. And may I ask why this number is required? Because at DC, no impedance matching is needed.

    Regards,

    Hao

  • 0 in reply to Hao Z
    Intellectual 270 points

    With the ADCs, they provide an output common mode voltage to be fed to the ADC driver amp. In this case the driver amp is in the LMX8410. Because I am using two different ICs for the ADCs, there may be slight differences in their common mode voltage. With only one input into the LMX8410 for ext CM voltage I can't use the ADCs CM outputs. I'll have to use the external voltage divider or use one of the values for the internal CM.

    As to why I asked about the impedance of the ext CM, if I do an external resistive divider, I would like to know if this LMX8410 ext CM input is going to place a load on the voltage divider.

  • 0 in reply to Charles Luke
    TI__Mastermind 18070 points

    In that case VCM can be considered to have very high impedance, at least equivalent to open for 12k resistors.

    Have you considered using the average of two ADC CM outputs? Since they shouldn't differ too much