F.Faccio, P.Moreira, S.Marchioro, K.Kloukinas, M.Campbell
Cern, Ch-1211 Geneva 23, Switzerland
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The CMS tracker will use approximately 1000 digital optical links for the transmission of timing, trigger and control signals. These digital signals, transmitted serially at a frequency of 80Mbit/s, will be converted into electric signals by a PIN photodiode at the receiver end. The developed ASIC is part of the optical receiver system that will amplify the photocurrent, regenerate the timing and deserialize the signals. The communication channel end sitting inside the CMS detector will work in a harsh radiation environment, and should therefore be radiation hard. 

The front-end element of the optical receiver is an amplifier, which has to amplify the photocurrent and detect the presence of a reset, which is coded in the foreseen protocol as missing pulses (absence of signal for more than 2µs). The specifications of such amplifier circuit include radiation tolerance up to 10Mrad, a wide dynamic range (current signals between 10 and 500µA) and capacity to accommodate for a high leakage current in the photodiode (up to 100µA). The last two specifications come from the maximum photodiode degradation in quantum efficiency and leakage current induced by radiation. No commercial component satisfying these requirements is available, and the development of an ASIC was necessary.

To meet the speed, power consumption and radiation tolerance specifications, we have chosen a 0.25µm commercial CMOS technology. The systematic use of enclosed transistors and guardrings enables the circuit to achieve the required radiation tolerance for both total dose and Single Event Latchup (SEL). Moreover, the required bandwidth of 80Mbit/s can easily be achieved without large currents, with the result of limiting the power consumption below 35 mW per channel at the nominal voltage of 2.5V.

The amplifier circuit is DC coupled to the photodiode, and its architecture consists of a transresistance preamplifier with variable gain, a chain of limiting gain amplifiers, an LVDS output driver and a reset detector block. Automatic Gain Control (AGC) adjusts the transresitance between 200W for the maximum input signal and 16kW for the minimum input signal. Therefore, the preamplifier output signal has constant amplitude over the whole input dynamic range. This solution has the advantage of achieving the high gain, hence low input noise, for small input signals. Another loop was integrated in the preamplifier to enable the compensation of the photodiode leakage current up to 100µA.

To minimise the noise injection in the substrate and maximise the Power Supply Rejection Ratio (PSRR), a pseudo-differential structure has been chosen for the preamplifier; the signal path is then fully differential from its output to the ASIC output, which uses Low Voltage Differential Signaling (LVDS). The integrated reset detector block looks for missing pulses in the input signal, and enables a flag in case of reset.
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An 80Mbit/s optical amplifier was developed for the CMS tracker slow control optical link. The circuit has been integrated in a 0.25µm commercial CMOS process using radiation tolerant layout practices to achieve the required radiation tolerance. An Automatic Gain Control (AGC) loop allows for detection of wide dynamic range input signals (10µA to 500µA photocurrent) with minimum noise, compatible with the maximum expected radiation-induced drop in quantum efficiency of the PIN photodiode. A second feedback loop can accommodate for the photodiode leakage current up to 100µA.
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