A new approach to DC removal in high gain, low noise voltage amplifiers

When performing voltage noise measurements on a biased device, the DC component due to the bias must be rejected in order to avoid saturation of the high gain low noise input stage of the voltage amplifier employed to raise the level of the signal prior to sampling for spectral analysis. This is normally done by interposing an RC high pass filter between the device and the DC coupled amplifier input. The problem with this approach is that in order to reduce the noise contribution coming from the resistance R in the filter, its corner frequency must be chosen much smaller than the minimum frequency of interest, thus resulting in extremely long transients upon device connection or bias change. In order to address this problem we propose to employ a low noise, programmable floating voltage source that, together with a proper feedback loop, behaves as a noiseless AC coupling filter, thus allowing to considerably reduce the duration of the transients.