A Front-End Readout for Micro Bolometers Having Sub Nano Volt Noise Floor

We present our circuit solution for the second stage of the front-end readout of an array of mu-bolometers, cryogenic detectors having Si or Ge thermistor as sensing element. The front-end consists in a cold unity gain buffer stage (with single sided input connection) followed by a room temperature second differential amplifying stage. The optimization of the input transistor of the set-up has been determined by comparing the performances obtained applying a low pass filter or the optimum filter at output of the front-end. To minimize EMI interference the output of the cold buffer stage and its ground reference signal, taken close to the buffer itself, are readout in differential way from the second stage. To minimize noise, in our circuit solution the differential amplifier has an input stage with a noise that is 1.6 times smaller (1.27 times smaller if we consider the Square Root noise, SRn) than in a standard differential solution. Since the series noise is inversely proportional to the Gate area, to further reduce the noise the input JFET of the second stage has a large Gate area, of about 1 nF, and a very small feedback resistance. The performances of the presented second differential stage amplifier are about 0.194 (nV)
²
/Hz (440pV/radicHz for the SRn), and a figure of less than 1.21 (nV)
²
/Hz (or 1100 pV/radicHz SRn) at 1 Hz. Parallel noise is completely negligible over the ranges of the expected output impedances of the cold stage. Dynamic input impedance is a few tens of pF. The voltage gain is 500 V/V, AC coupled down to 80 mHz