The SIXS instrument is designed for detecting and measuring solar X-rays and solar energetic particles in a Mercury orbit onboard BepiColombo. The particle detector consists of a CsI(Tl) core detector and 5 GaAs surface detectors. The front-end electronics of the particle detector is implemented as an ASIC which requires a large dynamic range of 2,000e- to 1,500,000e- and an integral non-linearity (INL) < 3% for a temperature range of -30deg C to +55deg C. The SIXS ASIC test structure has been manufactured to determine performance of differential vs. single-ended designs for noise, power supply rejection and current consumption. The ASIC has 8 channels, each of which consists of a preamplifier, a CR-RC shaper, a peak hold, a comparator and a buffer, all designed using enclosed geometry transistors. There are 6 channels for GaAs detectors with 3 different preamplifier types consisting of a single-ended with pole zero compensation; single-ended without pole zero compensation and a differential configuration. There are 2 channels for Csl detectors with single- ended and differential preamplifier designs. This ASIC has been manufactured in a 0.35 mum CMOS process. The ASIC architecture and design flow are described including simulated and measured results.