Comparison of noise performance of capacitive sensing amplifiers

Single op-amp, continuous-time charge amplifier stages and voltage amplifier stages are analyzed in an effort to determine if there is any fundamental noise performance advantage associated with either topology for the case in which the amplifier stage is interfaced with a capacitive sensor of low nominal capacitance. Signal-to-noise ratio is the performance metric chosen for comparison. It is found that the signal-to-noise ratio of the charge amplifier is fundamentally the same as that of the voltage amplifier for a simple case in which the amplifier is assumed to be ideal (requiring no DC bias path to its inputs) and the capacitive sensor is assumed to be affected only by stray capacitances to ground.; The capacitive sensor model is then extended to include parasitic series and parallel resistances and this more realistic sensor model is incorporated into noise models of charge amplifier and voltage amplifier circuits in which resistors are employed to establish DC bias at the amplifier inputs. It is found that the large-valued resistors necessary to ensure proper frequency response contribute significantly to the total output noise of both circuits over the audio range. Additionally, it is found that the signal-to-noise ratios of the more realistic charge and voltage amplifiers are essentially identical for identical source impedances and frequency-dependent with a single-pole high-pass type response leveling off somewhere above the tens of kHz. AC bias is briefly discussed since it could be used to modulate away from the baseband, where the signal-to-noise ratio is poor, up into a frequency band of higher signal-to-noise ratio.