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 μ-bolometers, cryogenics detectors having thermistors as sensing elements. 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. 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 equivalent to about 1.2 transistors, saving a factor of about two in performances with respect to the standard solution. To make further negligible the second stage noise, its input JFET has a large gate area, of about 1 nF, and a very small feedback resistance. The performances of the presented second stage amplifier are about 440 pV/√Hz for the square root of white noise, and a figure of less than 1100 pV/√Hz at 1 Hz. Parallel noise effect is completely negligible across the ranges of the expected output impedances of the cold stage. Dynamic input impedance is a few pF. The voltage gain is 500 V/V, AC coupled down to 80 mHz.