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TRF371135: Common mode voltage range

James Gilb
Prodigy 40 points
Part Number: TRF371135
Other Parts Discussed in Thread: ADS62P49, TRF371109, TRF371125, , TRF3710

The datasheet is mostly silent on the VCM pin, except when it is contradictory.  The pin definition is "Baseband common-mode input voltage", implying it is an input, but then in "ELECTRICAL CHARACTERISTIC" it has: Vcm | Output, common-mode | Measured at I- and Q-channel baseband outputs | 1.5 V

So, is it an input or an output?  If it is an input (as shown in the EVM documentation), what is the range of voltages over which it is compliant?

  • 0
    TI__Guru 67954 points

    Hi James,

    The VCM is an input from an ADC like the ADS62P49

    The ideas is that the VCM output of the ADS62P49 can slightly shift the VCM output of the baseband to ensure the VCM of the basenband output follows the AC swing center of the ADS62P49 input.

    -Kang

  • 0
    Prodigy 40 points

    Kang

    Thanks, but that only answers that it is an input and that the data sheet is poorly written.  If it is an input, then why is listed as

    What about the range of voltages over which it is compliant? 

    The TRF371109 data sheet does have information on VCM in the section titled "ADC Interface" that says it is 1.5 V to 2.8 V and has a helpful graph.  This is missing TRF371135 and TRF371125 data sheets (but is in the TRF3710).

    Can you confirm that the information in the TRF371109 is the same for the TRF371135?

    It seems like there was an error when the data sheets were duplicated for three new parts and this section got left out.  The TRF310 helpfully offers min, typical and max values for VCM, but these were deleted from the other three.

    Basically, the data sheets are badly written and need to be fixed.

  • 0 in reply to James Gilb
    TI__Guru 67954 points

    Hello James,

    Apologies for the confusion. Please allow me to look into this. I have to find the people who worked on this project to see if they can help on this inquiry.

    -Kang

  • 0 in reply to Kang Hsia
    TI__Mastermind 18095 points

    James:

    I think the confusion related to Vcm input/output is because the datasheet is using the same term to refer to two different areas.  The Vcm pin 24 is an input.  This input pin accepts the common mode voltage typically supplied from the ADC.  That voltage is in turn supplied as the common mode voltage to the BB output pins (27,28 and 33,34) so that it will perfectly match the common mode voltage of the ADC.  Alternatively, you can simply supply a nominal 1.5V to that pin.  The Vcm specification in the table is referring the typical common mode voltage of the BB output pins (27,28 and 33,34) of 1.5V given a nominal 1.5V input to pin 24.

    The TRF3711xx family all utilize the same IP for the BB sections; hence, it is valid to apply the information related to the Vcm in section "ADC Interface" of TRF3711-09 to the TRF3711-35.  Indeed, that section should be included in the all of the TRF3711-xx family datasheets.  That section outlines the useable range of Vcm.  The range of 1.5V to 2.8V corresponds to the range where there is no impact to P1dB performance; however, per figure 46, you can extend the operation range from 0.9V to 3.0V if needed with some degradation in output compression.

    --RJH

  • 0 in reply to RJ Hopper
    Prodigy 40 points

    Yes, it is very confusing to create an entry in "Electrical Specifications" for the output common mode voltage that is determined by VCM input and not the part itself.  Does that mean if VCM is open circuit, then it will float to 1.5 V?  Furthermore, there aren't min, typ and max number listed for VCM in the datasheet.  Every input should have them.  When will the data sheets be fixed?  If I design to a specific common mode voltage for a released design, how do I verify that future changes by TI won't affect the rated input range if it isn't in the data sheet?  Do I need to find a part from another vendor?  Like the ADL5380, LTC5586 or LTC5594?

  • 0 in reply to James Gilb
    TI__Mastermind 18095 points

    James:

    I see your point given the BB VCM output is set by Vcm pin 24 input.  The intent is to supply a Vcm input at pin 24 either from the ADC or through a simple resistor divider from the device supply.  The typical Vcm is specified at 1.5V since that was the predominate common mode of ADCs available at the time of the device release.  On the TRF3711EVM, the Vcm pin 24 input voltage is provided through a resistor divider from the 5V supply.  There are no characterized performance metrics to specify min/max other than the guidelines provided in section "ADC Interface".  No change is forthcoming on that issue.  If you are specifically concerned about Vcm shifting, there are no device modifications which will impact Vcm range of the device in the future.

    --RJH