Electronic – Understanding CT and ADC resolution

Since the jumpers have to be set manually, a simple solution might be to use a second jumper that indicates the selected gain to the microcontroller. So the user have to set two jumpers to identical positions. Or just use a rotary switch with two poles and three positions (e.g.: SS-10-23NPE).

A completely different approach would be to implement a autoranging function, where the microcontroller selects the gain factor automatically. Just use a small analog switch that will be controlled by the microcontroller. Now it is possible to auto gain the input signal: start with the lowest gain factor and work your way up just one step before saturation of the ADC input would occur. If neccessary protect the ADC input from overvoltage: just add a clamping zehner or TVS diode (important: low leakage current!) to the output of the amplifier.

A 3 bit ADC means you have 3 physical bits, and can output up to 8 different codes. It doesn't say anything about how accurately those voltages represent any particular coding, or whether any will be missing.

Two effective bits means that when using some specified measurement method, the ADC behaves as well as a perfect ADC with 2 bits would.

Usually, the method is to drive a nearly full scale sinewave into the ADC, spectrum analyse the result, and calculate the SNR, the signal to noise ratio. The effective number of bits (ENOB) is the size of theoretically perfect ADC (could be fractional) that produces the same SNR. Or the sum could be done for SINAD, the signal to noise and distortion ratio, which would produce a slightly different answer.

As you can see, this definition produces essentially no useful information about what the average code would be for any specific DC input, or the 'size' of the LSB when measured with a DC input.