Why prefer CMOS over CCD? CMOS detector is radiation resistant Fast switching cycle Low power dissipation Light weight with high device density Issues: High read noise

Analog to Digital conversion Incoming photons are converted to current by the The analog current is digitized by Analog to Digital converter ‘Photodiode’ How to tackle noise issues??

Low Noise Modulation scheme Oversampling: Sampling signal at frequency much higher than twice the bandwidth( Nyquist rate) of the signal - PCM Analog to digital conversion -  Analog to digital conversion Benefits: - Anti aliasing - High resolution - Noise reduction/cancellation Fast Fourier Transform diagram of a multi-bit ADC with a sampling frequency F S Fast Fourier Transform diagram of a multi-bit ADC with a sampling frequency kF S Effect of the digital filter on the noise bandwidth

 Analog to digital conversion The input is fed to the quantizer via an integrator, and the quantized output feeds back to subtract from the input signal. This feedback forces the average value of the quantized signal to track the average input. Any persistent difference between them accumulates in the integrator and eventually corrects itself. The density of "ones" at the modulator output is proportional to the input signal. For an increasing input, the comparator generates a greater number of "ones," and vice versa for a decreasing input. By summing the error voltage, the integrator acts as a lowpass filter to the input signal and a highpass filter to the quantization noise. Thus, most of the quantization noise is pushed into higher frequencies Transfer function: x Q [n]=z -1.x[n]+(1-z -1 ).e[n]

Oversampling does not change the total noise power, but alters its distribution. If we apply a digital filter to the noise- shaped  modulator, it provides a 9dB or 1.5 bits improvement in SNR for every doubling of the sampling rate. The output of  network is a 1-bit data stream at the sampling rate, in the orders of megahertz. Digital and decimation filter extracts information from this data stream and reduces the data rate to a more useful value. Advantages: Averages the 1-bit data stream Improves the ADC resolution Removes quantization noise that is outside the band of interest It determines the signal bandwidth, settling time, and stopband rejection. Decimation and Digital filter  Analog to digital conversion

Second order  Analog to digital converter A second order  modulator structure is obtained by extending the first order  modulator with an additional integrator unit. Transfer function: x Q [n]=z -1.x[n]+(1-z -1 ) 2.e[n] Benefits: e[n] is filtered with second order high pass filter Quantization noise even more suppressed SNR increases by 15 dB or 2.5 bits for every frequency doubling