selecting the right adc

Selecting the most suitable A/D converter (ADC) for your application is based on more than just the precision or bits. Different architectures are available, each exhibiting advantages and disadvantages in various data-acquisition systems. The required accuracy or precision of the system puts you in a category based on the number of bits required. It is important to always design your system to allow for more bits than initially required: if an application calls for 10 bits of accuracy, choose a 12-bit converter. The achievable accuracy of a converter will always be less than the total number of bits available.
Depending on the system requirements, your accuracy might be better expressed in micro-volts, decibels or LSBs (least significant bits). A FFT showing the frequency spectrum of a device can be useful in determining the noise performance of a given device. All Microchip stand-alone ADCs show typical performance data for AC specifications, such as THD, SINAD and SNR. The following table shows performance, in dB and V/V, for 8- through 24-bit converters.
Typically, an amplifier is required if the magnitude of the input signal is significantly lower than the full-scale input range of the ADC. However, by selecting an ADC with a higher resolution, the need for an amplifier can be eliminated.