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Original question:

UCD90320EVM-783: UCD90320 configuration

UCD90320EVM-783: ADC input resistance for the UCD90320

George Fourie
Prodigy 20 points
Part Number: UCD90320EVM-783
Other Parts Discussed in Thread: UCD90320

You previously suggested for the below circuit that: "The Vout Scale Mon has to be set to equal to scaling ratio in your schematics. For your case, the ratio is 2.49/12.49 = 0.1994. this is the numer you need to input."

How can this be true when the input resistance for the Amon inputs (ADC input) of the UCD90320 is specified as only 2.5KOhm max ?

Surely this low input resistance will affect the ratio. There is also no nominal value of input resistance specified.

Even with only the 200Ohm resistor on the Amon input as suggested in the datasheet this will already introduce larger than 7% error ?

  • 0
    TI__Guru 66780 points

    Hi

    The 200ohms resistor is recommend if voltage divider is not used.

    the 2.5Kohm is the equivalent resistance all the way up to the sampling node of the ADC and it is not going to impact your external divider circuit.

    Your circuit is good for 12V.

    Regards

    Yihe 

  • 0 in reply to yihe
    Prodigy 20 points

    Thanks for your reply. It then seems that the 2.5Kohm is not really an input impedance but rather a resistance between the ADC input pin and a high impedance ADC sampling node.

    That will definitely simplify interfacing requirements.