Development of a CMOS amplifier/shaper/ discriminator for the ATLAS Muon detector is presented. The first phase of this work has resulted in a simplified 4-channel device (ASD-lite) fabricated in HP 0.5μm n-well CMOS operating at 3.3 volts. Highly accurate DC models resulted in performance which closely matches SPICE predictions. On-chip crosstalk, a parameter not easily simulated, is measured to be below 0.5%. Results of chip measurements and on-chamber tests will be presented. This device will be used for early testing of ATLAS MDT (Monitored Drift Tube) modules. Further work on the final octal MDTASD, which will be fully programmable and include Wilkinson leading edge charge measurement, will be presented. Front End Requirements The Muon spectrometer aims for a PT resolution of 10% for 1TeV muons. This translates into a single wire resolution requirement of <80μm. The average drift velocity is about 20μm/ns, which implies a systematic timing error for an individual tube of about 500ps. The planned gas gain is low, about 2⋅10, to avoid aging problems. The expected signal (collected charge) is roughly 1500 electrons (0.25fC) per primary electron, so good position resolution requires a low noise front-end. A specified preamp peaking time of 15ns is a good compromise in terms of resolution and stability . The channel to channel crosstalk is specified to be less than 1%. The high count rates of up to 400kHz/wire together with the long electron drift times require either a bipolar shaping scheme or active baseline restoration to avoid resolution deterioration due to baseline fluctuations. At the time of the TDR the baseline MDT gas was Ar/N2/CH4 91/4/5 (3 bars absolute) which is very linear and has a maximum drift time of 500ns. The choice for the ASD shaping scheme was unipolar shaping with active baseline restoration for the following two reasons. First, it allows the measurement of the signal trailing edge , which has a fixed latency with respect to the bunch crossing, with an accuracy of about 20ns. Second, it avoids multiple threshold crossings per muon track, which would increase the hit rate and therefore the readout occupancy. Aging problems with all MDT gases containing hydrocarbons caused a change of the baseline gas to Ar/CO2 93/7 (3 bars absolute) which has a maximum drift time of 800ns and is very nonlinear. The long drift time and the non-linearity degrade the trailing edge resolution to about 80ns and cause multiple threshold crossings even for a unipolar shaping scheme. We therefore have adopted a bipolar shaping scheme since it does not require an active BLR and also does not require programmable filter time constants. To avoid multiple hits from multiple threshold crossings for a single signal we introduce a fixed dead time equal to the maximum drift time. It was shown that the overall increase in dead time does not cause a degradation of the pattern recognition efficiency. An ADC will measure the signal charge in a 20ns gate following the threshold crossing time. The charge is then encoded into a pulse width in the usual Wilkinson technique. This information allows a resolution improvement by performing a time slewing correction. Additionally, it is useful for diagnostics and monitoring purposes and might also be used for dE/dx identification of slow moving heavy particles like heavy muon SUSY partners. Two modes of operation will be provided. In one mode the ASD output gives the time over threshold information, i.e. signal leading and trailing edge timing. The other mode measures leading edge time and charge and is considered the default operating mode. Readout System Packaging The ATLAS MDT system consists of about 350,000 pressurized drift tubes of 3cm diameter, with lengths from 1.5 to 6m. The MDTs are read out by an ASD at one end, and the other end is terminated in the characteristic impedance of the tube (380Ω ). The preamp input impedance is a relatively low ~100Ω to maximize collected charge. To minimize cost, the MDT signals are carried on two-layer "hedgehog boards" to a mezzanine board, which contains 24 readout channels: 3 Octal ASDs, a single 24-channel TDC, and associated control circuitry. A single MDT chamber may have as many as 432 drift tubes or 18 hedgehog/mezzanine board