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Conditioning an ADC Input Signal

Thomas Foxcroft
Expert 2996 points


An analog-to-digital converter (ADC) takes a sample of an analog signal and then converts it to a binary number that represents the value for the sample. The basic parameters for the analog input signal are voltage swing and frequency.

If needed, the voltage can be conditioned with amplification and division in order to fit the input parameters for an ADC.

As for the input signal frequency, I would like to have the TI Community Member’s opinion on the following statement.

"If the analog input signal frequency is too high for the ADC to sample, the signal frequency should not be conditioned. Instead, a different ADC should be used that can adequately sample the higher frequency and properly convert it."

  • Guru 227245 points

    Thomas Foxcroft said:
    As for the input signal frequency, I would like to have the TI Community Member’s opinion on the following statement.


    Well, if something doesn't fit it doesn't fit.

    If the signal frequency is higher than the ADC conversion time, it is impossible to sample the signal in a way that gives any valuable/usable information about the signal. You need at least twice the signal frequency to get hold of the signal frequency. But then you cannot say anything about the voltage (unless you know that it is sinusodial and fixed frequency, then you can reconstruct the signal).
    If the frequency of the signal is higher than 1/2 teh sampling requency, all you get is nonsense.
    Input signal shaping usually means to put a filter on the input signal. To use a low-pass filter on a frequency higher than what you can sample with the ADC just results in - nothing. A flat line. 0V, 0Hz.

    Input shaping is only useful if the signal you are interested in is slow enough and you want to remove high-frequent distortions which will add some random noise to the sampled signal. (random because of the inability to reconstruct the HF signal).

    So the TI advice is 100% correct: if the frequency you want to sample is too high for the ADC, use a faster external one. IMHO, this advice is more or less superfluous as this should be obvious to everyone who wants to deal with signal sampling/processing.

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