Read-out photodiodes using sigma-delta oversampled analog-to-digital converters

Sensing apparatus comprising:

• a photosensor for responding to a respective element of a radiant-energy image for at least a prescribed period of time to generate a photocurrent;
• a preamplifier having an input port connected for receiving photocurrent from said photosensor and having an output port for supplying an analog output signal responsive to the photocurrent its input port receives, which output signal is undesirably accompanied by wideband noise originating in substantial part within said preamplifier;
• a sigma-delta modulator of order L, having an input port connected for receiving an analog input signal, and having an output port for supplying at an oversampling rate digital responses to said analog input signal, which responses are accompanied by quantizing noise generated within said sigma-delta modulator, L being an integer larger than zero;
• means for applying to the input port of said sigma-delta modulator as its said received input signal the analog output signal from the output port of said preamplifier and an aboveband component consisting of at least a portion of said accompanying wideband noise, whereby the digital response of said sigma-delta modulator is accompanied by undesirable response to said aboveband component as well as the quantizing noise generated within said sigma-delta modulator;
• a decimation filter having an input port connected from the output port of said said sigma-delta modulator and having an output port for supplying a decimation filter response at an output sample rate that is a submultiple of said oversampling rate, said oversampling rate being R' times said output sample rate, R' being an integer greater than one, in which decimation filter response a component corresponding to said aboveband component is attenuated respective to a component corresponding to the preamplifier analog output signal as well as the quantizing noise generated within said sigma-delta modulator being attenuated respective to the component corresponding to the preamplifier analog output signal;
• and
• means for adjustably determining R'.

Sensing apparatus comprising:

• a photosensor for responding to a respective element of a radiant-energy image for at least a prescribed period of time to generate a photocurrent;
• a preamplifier for generating an analog output signal responsive to the photocurrent of said photosensor which output signal is undesirably accompanied by wideband noise originating in substantial part within said preamplifier;
• an analog filter for responding to the output signal of said preamplifier with a response in which said wideband noise above a cut-off frequency is suppressed;
• a sigma-delta modulator of order L, having an input port connected for receiving an analog input signal, and having an output port for supplying at an oversampling rate digital responses to said analog input signal, which responses are accompanied by quantizing noise generated within said sigma-delta modulator, L being an integer larger than zero;
• a subsampler for sampling at an output sample rate that is a submultiple of said oversampling frequency and that is below said cut-off frequency, from which subsampler an output signal for said sensing apparatus is to be taken;
• and
• a finite-impulse-response low-pass digital filter for said sigma-delta modulator digital responses for supplying to said subsampler a response to said sigma-delta modulator digital responses substantially free of said quantizing noise and of said wideband noise above said output sample rate.