A 16 channel analogue sparse readout I.C. for INTEGRAL (International Gamma-Ray Astrophysics Laboratory)

A low-noise, low-power, 16-channel pre-amplifier/shaper IC designed at RAL in 1.5 μ CMOS is described. The IC is designed to read out photodiodes which detect energy deposits in CsI scintillation bars. Each channel amplifies the input charge from a photodiode and shapes it with a 5-μs time constant (a requirement of the detectors). The peak of the shaped signal is held by a peak hold amplifier and stored onto a holding capacitor for readout. Each channel includes its own comparator circuit, whose output state switches when the peak hold amplifier output crosses a threshold. A data ready flag is output when a comparator fires and the peak voltages along with the digital addresses of the corresponding “hit” channels, can be read out, controlled by a data accept signal. Two readout modes are available, one allows only those channels with “hit” comparators to be read out, while the other allows all 16 channels to be sampled simultaneously and read out, regardless of the comparators state. In each case analogue data is read out in series. The first method skips channels which have no useful data to be collected, reducing the data read-out time. A channel noise of 500e- ENC rms is required to achieve the lower energy threshold of the instrument, using only 5 mW of power per channel from a 5 V supply and with a 70 pF detector capacitance