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LMH5401: VCM input range from ADC

ttd
Mastermind 8770 points
Part Number: LMH5401
Other Parts Discussed in Thread: ADC32RF52, LMH6554, TRF1305, TRF1305B1

Tool/software:

Dear Technical Support Team,

I'd like to confrim VCM input range of LMH5401.

What the meaning of VCM voltage range low and VCM voltage range high?

If VS＋=5V and VS-=GND , the input range of VCM is from 1.4V to 3.6V?

My target ADC is ADC32RF52 and VOCM is 350mV typ. It seems to connect it directly.

LMH6554 seems to be OK the range of VCM is ±1.25V(typ).

Best Regards,

ttd

  • 0
    TI__Intellectual 1320 points

    Hi there,

    The VCM range high and low are the limits of the common mode at the input or the output. 

    Your example is correct. So you can operate the FDA with output common mode voltage anywhere in the range of 1.4V to 3.6V with a 5V single supply. For input common mode, the limits get a bit looser if you require any common mode shifting from input to output of the FDA.

    However, the ADC32RF52, the input common mode voltage is 350mV, so you require split supply on the LMH5401 to be able to match the common modes. I recommend -2.2V and +2.8V to center the supply around the ADC input common mode. We have a good example of this operation with the TRF1305-ADC32RFEVM, which shows good practice when matching the ADC input common mode. Schematic can be seen below:

    TRF VS+ and TRF VS- in the above come from the following combination of power components. 

    Full schematics are also attached, if you are interested. The implementation will basically be the same with TRF1305 and with LMH5401. AMPS227A.pdf

    For the LMH6554, you are correct, it will be +/-1.25V around the center supply. 

    Thanks, and let me know if anything else comes up.

    Gideon

  • 0 in reply to Gideon Townsend
    Mastermind 8770 points

    Hi Gideon,

    Thank you for your reply and reference schematic.

    I understand split supply is the best for LMH5401 and TRF1305. 

    In my case, I’d like to use Gain=1V/V.

    LMH5401 seems to be more than G=2V/V and TRF1305x1 is fixed gain such as 5/10/15db.

    LMH6554 is OK for G=1V/V and accept VOCM=350mV with 5V supply, because current circuit is a ADA4930-1. 

    By the way, How is the input voltage forvTRF1305-ADC32RFEVM?

    According to user guide,-20dBm=22.36mVpp.

    TRF1305B1 has 10db, so 70.6576mV？

    How is the max Vpp for input?

    Best Regards,

    ttd

  • 0 in reply to ttd
    TI__Intellectual 1320 points

    Hello, 

    One option for both LMH5401 and TRF1305 is to configure for lowest gain, then add a small attenuator to fine tune for your required gain. This attenuator will be required anyways from a matching perspective, as the ADC32RF5x is unbuffered ADC and presents a not great input impedance. 

    Keep in mind for LMH6554 will also require split supply if you want to configure for DC coupled operation, as well as require an attenuator at the input to the ADC for matching. So for an overall system gain of 1V/V, you would have to configure the device itself for a gain of 2V/V or 4V/V and then add a 3 dB to 6 dB attenuator to bring the impedance to 50 ohm for the antialiasing filter design. 

    The TRF1305-ADC32RFEVM has an input power requirement of around -2 dBm single ended to bring the ADC to full scale. This equates to ~500 mVpp on the 50 ohm input to the TRF1305. This is including a 3 dB pad, splitter, and antialiasing filter at the input of the ADC.

    Thanks,
    Gideon