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How to calculate the actual value of VIN of ADC12J4000EVM?

Tong Xu
Intellectual 515 points
Other Parts Discussed in Thread: ADC12J4000EVM, ADC12J4000, DAC38J84EVM

Hi everyone,

I'm using ADC12J4000EVM and I just got the 12-bit sampled data.

The configuration are as follows.

Single ended input. Bypass Mode; DDR.

Fs =4GSPS, Analog input: 500MHz, Input Power = -6dBm

Now I want to convert the 12-bit data to the actual value of VIN, but I cannot find the equation in the ADC datasheet. Can anyone tell me how to do the conversion?

Thank you.

Regards,

Tong

  • 0
    TI__Mastermind 44470 points

    Hi Tong

    A 12-bit ADC has output codes ranging from 0 to 4095. From the ADC12J4000 datasheet, the typical value for the Full-scale analog differential input voltage is 725 mVpp (peak to peak). Therefore, the relationship between converter code and input voltage is approximately:

    Vin = ((Code - 2047.5) / 4095) * 725 mV

    Here are the input voltages for some specific code values:

    0: -362.5mV

    2047: -0.0885mV

    2048: +0.0885mV

    4095: +362.5mV

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    Intellectual 515 points
    Hi Jim,

    Thank you very much.

    I noticed that there is a transformer (BD0430J50100AHF) on the DAC38J84EVM which converts single-ended input signal to differential signals. This transformer will cause insertion loss and return loss. I also noticed that these effects have a frequency dependency of input signal.

    I think the effects of this transformer are not included in the equation you told me, are they? If not, which kind of effects of the transformer should we consider?

    Regards,
    Tong
  • 0 in reply to Tong Xu
    TI__Mastermind 44470 points

    Hi Tong

    You are correct, the equation above is for the signal right at the inputs to the ADC. There will be some additional loss in the balun and in the signal traces routed across the board. The actual signal at the input to the EVM will be larger by the amount of the losses. These losses will be a function of frequency so aren't easy to quantify in a simple equation. If you have a narrow frequency range you can add a fixed compensating factor to the equation.

    You can estimate the signal loss versus frequency for the balun from the plot in the balun datasheet. The losses in the circuit board should be fairly small since the PCB dielectric is low-loss Rogers RO4350.

    Best regards,

    Jim B