Electronic – ADC input circuitry and source impedance

adcimpedanceresistors

I'm using a maxim ADC in my application from this family: http://datasheets.maxim-ic.com/en/ds/MAX1304-MAX1314.pdf

The devices have relatively low input impedance. If you look at Figure 5 in the above datasheet, on page 19, you can see the equivalent input circuit. My ADC is 0-5V range. My input signal however is in 0-10V range, so i have to reduce it to match the ADC.

If I use a resistive divider at the ADC input, it seems to me that I can not get a proper reading from the ADC due to the interaction between my 2 resistors, and R1 and R2 in figure 5. For example:

|                      Vdiv
|   Vin------Rdivider--------Rdivider---------> GND
|                       |
|                       |
|                       |
|                       R1 (3.33k)
|                       |
|                       |             Csample
|                       |-------/ ------| |------- 
|                       |
|                       R2 (5K)
|                       |
|                       |
|                       |
|                       v
|                     0.9V

In the above figure, no matter what Rdivider values I chose, it seems to me that Vdiv will not be Vin/2.

Do I have to use an amplifier in the input for this case?

Best Answer

Yes, from the datasheet:

Due to the analog input resistive divider formed by R1 and R2 in Figure 5, any significant analog input source resistance (R SOURCE) results in gain error. Furthermore, R SOURCE causes distortion due to nonlinear analog input currents. Limit RSOURCE to a maximum of 100Ω.

So practically, this means you need to drive the input with an amplifier. Although, a resistor divider made with a 192 Ω and 200 Ω resistor will meet specs if your signal has a low output impedance.

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