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ADC1175: VRT and VRB values for conversion of video Signal

Jeff Paranich
Prodigy 50 points
Part Number: ADC1175
Other Parts Discussed in Thread: LMH6702,

Good Day, I am struggling with knowing what the optimum/proper values of VRT and VRB should be used if I am using the ADC1175 to digitize a video signal (15khz 240x320).  A typical video signal on either R,G or B lines is 0v to 0.7v, thus I was going to use the LMH6702 to amplify the signal (as recommended) and then set VRT and VRB accordingly so I have a band of 0.7v to digitize across, however the datasheet says on page 14 that VRT should be 1.0v to 2.8v more positive than VRB thus giving me a wider band than the video signal will have.  However the ADC1175 does specify on the cover sheet it may be used for video applications.  

Will the ADC1175 operate okay with a 0.7v difference across VRT and VRB?

Or will I still get good video conversion results just setting VRT 1v greater than VRB? 

Am I missing something in my understanding?

Thank you!

  • 0
    TI__Mastermind 44680 points

    Hi Jeff

    The minimum recommended level for VRT-VRB is 1.0V. The device will function with a smaller reference difference, but device performance may be degraded.

    I would recommend using 1.0V or large for the following reasons.

    1. This will give better ADC performance.
    2. If the analog video signal you are digitizing also includes the SYNC signal it may be closer to 1Vpp instead of 0.7Vpp. A 0.7V reference different may result in clipping of the converted waveform.

    If you want to set the VRT and VRB levels to give 0.7V or 1.0V difference I would recommend a circuit similar to that shown in Figure 21 of the ADC1175 datasheet.

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    Prodigy 50 points

    Thanks Jim, very helpful... lastly, page 6 indicates that typical voltage input (Vin) is 2.0v.  Would it be therefore be optimum to configure the external op amp for 1.65volts of gain, such that the midpoint of the original analog signal (0.35v) plus the gain (1.65v) matches the typical voltage input on the ADC1175?  Much appreciated.

  • 0 in reply to Jeff Paranich
    TI__Mastermind 44680 points

    Hi Jeff

    That is probably the most effective way to match the applied signal to the ADC full scale range, and eliminates the complications of driving the reference ladder externally.

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    Prodigy 50 points

    Hi Jim,

    Digging up my thread on the ADC1175 again, as you know a lot about high speed conversion.

    Likely a ridiculous question, but on page 14 of the datasheet it lays out resistor values for the LMH6702 to drive ADC1175

    1. What do the commas mean after the values 150 & 237 & 57.6? I'm guessing nothing and the values are in ohms?
    2. Vout/Vin= 1 + Rf/Rg = 1 + 237/150 = 2.58 ; which for a video signal b/w 0 and 0.7 v would give you amp of 0 to 1.8V.  Will this be sufficient or is there better resistor values to use for analog video signals?

    Thanks again I appreciate your help. 

    -Jeff

  • 0 in reply to Jeff Paranich
    TI__Mastermind 44680 points

    Hi Jeff

    1. I think those commas are separating the resistor value from the tolerance (for example "150, 1%"), but since the values are above and below each-other the commas are redundant.
    2. I think those values are an OK starting point. If your planned signal is truly 0V to 0.7V then it would make sense to enhance the circuit with some offset control to shift the output signal positive to be between the Top and Bottom reference voltages of the ADC. The Figure 19 circuit will have a lower ADC input voltage of approximately Vrb = 0.6V.

    Best regards,

    Jim B