The GASSIPLEX0.7-2 Integrated Front-End Analog Processor for the HMPID and Muon Tracker of ALICE

The most recent member of the Gasplex family has been designed in a 0.7 µm n-well CMOS process to meet specifications for the ALICE applications: 500 fC linear dynamic range and a peaking time of 1.2 µs. Its internal circuitry is optimized for the readout of gaseous detectors. A dedicated filter compensates the long hyperbolic signal tail produced by the slow motion of the ions and allows the shaper to achieve perfect return to the base line after 5 µs. Measurement of fabricated chips showed a noise performance of 530 e- rms at 0 pF external input capacitance and 1.2 µs peaking-time, with a noise slope of 11.2 e- rms/pF. The gain is 3.6 mv/fC over a linear dynamic range of 560 fC.<P><B>Summary:</B><P>The Gasplex is a 16-channel low noise signal processor built in a 1.5 µm technology and specially designed for gaseous detectors. Each channel consists of a Charge Sensitive Amplifier (CSA) followed by a filter, a Semi-Gaussian shaper and a Track/Hold circuit. The peaking time acts as a delay allowing an external trigger to store the information on a capacitor; finally, the 16 channels are multiplexed to one output. <P>The Gassiplex0.7-2 has been developed to fit the ALICE requirements, using the same circuitry principle, in a 0.7&nbsp; µm process. In gaseous detectors, the ion cloud released by the avalanche around the anode wire induces current as long as it drifts in the electric field from the anode to the cathode. The charge close to the anode can be approximated by the relationship </P><P ALIGN="JUSTIFY">q(t) = Q<SUB>0</SUB>Aln(1+t/t<SUB>0</SUB>) and the current by I(t) = I<SUB>0</SUB>B/(1+t/t<SUB>0</SUB>), where Q<SUB>0</SUB> is the total ionic charge and A, B and t<SUB>0</SUB> are constants depending on the detector geometry and the electric field.<P>The CSA stage consists of a folded cascode with a feedback capacitor of 1&nbsp;pF and an active feedback resistor of 20&nbsp;M<FONT FACE="Symbol">&#87;</FONT>. This 20&nbsp;<FONT FACE="Symbol">&#109;</FONT>s decay time constant is necessary in order to be sensitive to the largest possible fraction of detector current, which last over several tens of microseconds. <P>A deconvolution filter has been implemented to compensate the long hyperbolic tail resulting from the ion drift and convert the signal of the CSA into a quasi-step function. To perform the deconvolution, the transfer function of the deconvolver G(s) should be the exact inverse of the transfer function of the detector H(s), namely G(s) = H(s)<SUP>-1</SUP>.<P>The charge given by the detector is approximated by the sum of three weighted exponentials. Each exponential is modelled by a pole placed in the feedback of a summing amplifier to implement the inverse transformation: G(s) = Vout/Vin = A/(1+<FONT FACE="Symbol">&#98;</FONT>A); for A large G(s) ~ 1/<FONT FACE="Symbol">&#98;</FONT> and <FONT FACE="Symbol">&#98;</FONT>= K<SUB>1</SUB>/(1+sT<SUB>1</SUB>) + K<SUB>2</SUB>/(1+sT<SUB>2</SUB>) + K<SUB>3</SUB>/(1+sT<SUB>3</SUB>). After deconvolution the filter output looks like a step function with one pole given by the Rf.Cf decay time constant of the CSA. It allows the shaper to maintain a stable and precise return to the base line.<P>The Semi-Gaussian shaper has an original feature: the output of the filter go through two different integrating paths which are compared at the inputs of an OTA; it results in a S-G shape and thereby eliminates the usual differentiation capacitor. In this way the different blocks of the analog channel are DC coupled and permit a high counting rate without base line distortion. <P>The Gassiplex0.7-2 provides an individual channel calibration circuit with a precision of ±1It is also possible to switch off the filter for the readout of Silicon detectors at a gain of 2.1&nbsp;mv/fC and a dynamic range of 900&nbsp;fC. With the deconvolution filter in operation, the measured performances figures are the following: at a power consumption of 9&nbsp;mW/ch, we measured the noise as 530&nbsp;e<SUP>-</SUP> rms with 0&nbsp;pF input capacitance and a slope of 11.2&nbsp;e<SUP>-</SUP> rms/pF. The non-linearity is ±2&nbsp;fC over the 560&nbsp;fC dynamic range at a gain of 3.6&nbsp;mv/fC. The readout can be performed at 10&nbsp;MHz with a capacitive load of 30&nbsp;pF.