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The signal processing electronics supporting the soft x-ray tomography of the reversed field experiment (RFX) is described. Current from Silicon diodes, of the order of several hundred picoamperes in the most severe cases, is amplified by the I-V stage and then, after further amplification and filtering, digitized at a frequency of 1 MHz. The digits are optically sent to VME boards in the acquisition room, which provide standard tomographic inversions. Performance of the circuits (maximum transimpedance 108, maximum bandwidth of about 200 kHz) and their noise behavior have been tested extensively during various experimental campaigns of RFX. A new prototype, with a higher transimpedance of 109, has already been developed, tested on the bench, and successfully operated on RFX. A remotely controlled calibration unit, designed to automatically determine amplifier gains and the bandwidths to within a few percents, is also presented together with the most significant results obtained in the calibration of the complete system.