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18 bit ADC thermal noise

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A simplified model of ADC noise refers the noise to a noisy input source resistance Rn while assuming the rest of the signal path to be noiseless. Attached Figure represents a particular 18-bit ADC that has a 10V input voltage range. The ADC has a bandwidth of 1 MHz. Calculate the maximum value of Rn if the resolution of the ADC is not to be adversely affected by thermal noise. Assume the ADC operates at 25 degree C. [N.b. The voltage resolution of an ADC is equal to its overall voltage measurement
range divided by the number of discrete values possible on its output.] Please see the attachment for a schematic.

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Solution Preview

To find Rn = ?

No. of bits N = 18
Voltage range V = 10
temperature T = 25+273 = 298 K
Band width f = 1 MHz = 10^6 Hz

ADC voltage resolution, Vr ...

Solution Summary

This problem estimates maximum resistance, to overcome thermal noise threshold so that ADC signal is resolved properly.