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LMX8410L: Bandwidth of the capacitors

Charles Luke
Intellectual 270 points
Part Number: LMX8410L

The part has the capability to select different output capacitors to limit bandwidth. While this is mentioned in the data sheet, I don't see and numbers for what the bandwidth would be. Maybe because you have to use external matching resistors to match into your particular differential baseband circuitry? If I were to use appropriate series resistors for matching into a 100 ohm differential load, what would the different bandwidth be based on the programmable capacitor choices?

  • 0
    Intellectual 270 points

    Also it's not clear why this cap value should be applied to a particular gain value. Can you please elaborate?

  • 0 in reply to Charles Luke
    TI__Genius 12705 points

    Hello Charles,

    Typically the IF output bandwidth is kept wide as possible.

    Gain adjustments on IF I/Q can be used to tune IMRR.

    Can you provide some more information on what you are trying to accomplish with capacitive filtering?

    Regards,

  • 0 in reply to Liam Keese
    Intellectual 270 points

    We are using the demod chip in a receiver that has a wideband mode and a narrowband mode. When in narrowband mode the filtering is performed in an FPGA but the wideband noise still enters the ADC. At low SNR, this can affect the AGC setpoint and cause the ADC to have to be backed off more than what we would like. Being able to reduce the baseband bandwidth for the narrowband mode reduces the broadband noise into the ADC and keeps the ADC operating at an optimal signal level.

    If you didn't intend to have variable bandwidth, then what is the purpose of the different caps?

  • 0 in reply to Charles Luke
    TI__Genius 12705 points

    Hello,

    Could you reference which section of the datasheet your question refers to so I can understand better what can be improved?

    Thank you,

  • 0 in reply to Liam Keese
    Intellectual 270 points

    Referencing SNAS730A –MARCH 2018–REVISED NOVEMBER 2018 doesn't have the same information as the earlier data sheet SNAS730 - MARCH 2018

    R82 and R83 are listed in much more detail in the earlier document.

    R82

    bit 13-11   IFA_COMPCAP

    bit 10        IFA_BOOST

    bit 9-6       COMP_VCM_CONFIG

    R83

    bit 8-5      IFA_CAP_PROG

    bit 4-0      IFA_GSTATE

    Looks like you guys are now having fixed values for these registers. I assume they are still there and would like to use them if they prove useful.

    These are the registers I am interested in for more information.

  • 0 in reply to Charles Luke
    TI__Genius 12705 points

    Hello Charles,

    The bandwidth is largely set by the gain value.  You are correct in that the capacitors have only some limited effect and were set to fixed value for the production release. 

    I will look into this some more and see what other details I can provide you on R83.

    Regards,

  • 0 in reply to Liam Keese
    TI__Genius 12705 points

    Hello Charles,

    As discussed previously, the IF bandwidth of LMX8410 depends upon the feedback resistor and capacitor of the IF amplifier.

    We had fixed IFA_GSTATE to get the maximum gain from the chip and IFA_CAP_PROG was set lowest to maximize the bandwidth.

    Setting of the resistor can be changed with IFA_GSTATE with 0 being highest gain and ‘31’ being lowest gain.

    Setting of the capacitor can be changed with IFA_CAP_PROG with 0 being lowest cap and 15 being highest cap. Settings above 9 are not recommended.

    If you want to reduce the bandwidth without changing the gain then you will need to program IFA_CAP_PROG. Increasing the bit will reduce the bandwidth. It is not recommended to go below 400MHz in 3dB bandwidth. Each step from 0 to 9 will reduce bandwidth by roughly 100MHz.

     For these settings validation must be performed at the system level as this is outside of our standard production configuration and test.

    Hopefully this helps you in your evaluations.

    Regards,