Charge-sharing ADC input designs are not appropriate for all signal conditioning requirements. Care should be taken to select a high-speed op-amp based design when it is more appropriate, including:

• High Speed Sampling

  When sampling at high speed on a single channel, the tracking error in a charge-sharing design will become prohibitively large. In this case, a high-speed op-amp based ADC driver will be needed.

• Compatibility With Oversampling Methods

  Sampling the same signal multiple times back-to-back and then averaging the results can be a good method to reduce system noise or sampling noise. However, in the case of a charge-sharing ADC driver, these multiple samples will increase the effective sample rate on the channel, increasing the tracking error. In the case the extra samples are back-to-back, the channel sample rate becomes as fast as the ADC sample rate, which will likely create excessive tracking error.

• High Bandwidth Signals

  The large capacitor needed to meet the charge-sharing criterion will inherently provide some amount of low-pass filtering. For undersampling applications or other applications that require sampling of a high bandwidth signal, charge sharing may not be feasible.

• Extremely Low Distortion

  C0G and NP0 type capacitors provide very low capacitance vs. voltage nonlinearity. If the ADC signal chain uses these capacitors it is possible to achieve excellent distortion performance. These capacitors are less dense than higher distortion alternatives (for example X7R type). This can make component selection for the very large capacitances needed for charge-sharing designs infeasible, particularly at high resolution (for example 16-bit resolution). For more information, see Selecting capacitors to minimize distortion in audio applications.