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ADS4449: Additional Inquiry of VCM resister

Louis Han15
Genius 4680 points
Part Number: ADS4449

Hello

I received an answer about VCM resister as below E2E link.

https://e2e.ti.com/support/data-converters/f/73/p/821528/3039338

I have additional Inquiry.

1. After mounting the 25 Ω resistor, the output reduce by about 3 dB. so It is necessary to explain why the output reduce.

2. when there is no 25 Ω  resistance on one side, please let me know something additional problem.

Best Regards.

  • 0
    TI__Genius 12780 points

    Hi Louis,

    1. After mounting the 25 Ω resistor, the output reduce by about 3 dB. so It is necessary to explain why the output reduce.

    Looking at the previous E2E post that you shared, the schematic/diagram shows that the input sinewave signal has a 50 ohm impedance. Since both back-to-back transformers have a 1:1 impedance ratio, we need to terminate with resistance that matches the input resistance. We do this by using two 25 ohm resistors (connected differentially), so this acts as a voltage divider. This explains the 3 dB loss.

    2. when there is no 25 Ω  resistance on one side, please let me know something additional problem.

    If one of the 25 ohm resistors (highlighted in yellow on previous post) is missing, then that differential leg will not have the common mode voltage (DC) required for proper operation of ADC. Additionally, there is not proper termination for the input signal. The ADC will not provide datasheet results.

    Best Regards,

    Dan