Electrical-Optical Converter

The design criteria for the optical transmission system was low heat dissipation, minimum geometrical dimensions and immunity against radiation damage at the 100 krad level. Rise times of 5 ns for the digital control signals are required. Figure 7 presents our solution for the optical signal drivers. We use LED diodes with pigtails (see section 5), and a very simple amplifier for the analog driver. This amplifier together with the LED has a linearity of better than 1% for a current range mA and a temperature stability of 10% gain change for a temperature variation C. Gain variations of the BFR93 transistor are compensated by selecting R in fig. 7. The parasitic input capacity of the circuit is only 2.3 pF, which is small compared to the capacity of the signal line on the hybrid from the APC128 to the LED driver. This latter capacity limits the rise time of the analog pulse to about 200 ns and determines the maximum readout speed of the APC to about 2 MHz.

The optical receivers for analog and digital signals are displayed in figure 8. The bandwidth of the analog receiver is limited to 20 MHz to minimize noise. We measured the noise of the whole readout chain

where the PIN-diode current in the receiver is of order 10 A. This corresponds to a noise:signal ratio for a minimum ionising particle through the silicon sensor of 0.01. The noise voltage distribution has a purely Gaussian shape over a wide range as depicted in figure 9, which demonstrates the immunity of the optical signal transfer against pick-up noise.

 

device power per strip power for full detector

APC128 preamplifier 0.23 mW 18.8 W

current source 0.06 mW 4.9 W

Decoder and APC128 digital part 0.20 mW 16.4 W

LED driver 0.04 mW 3.2 W

PIN diode receiver 0.05 mW 4.0 W

total power 0.58 mW 47.3 W

The total power consumption at the detector is summarised in table 1. The power of all components is given both per silicon strip and for the full detector. Note that the LED driver is only active during data readout and idle during pipeline operation, and uses therefore less power during the latter. About 1.5 m away from the detector there are voltage regulators consuming 8 W.

The behaviour of the optical drivers/receivers has been tested after radiation damage applied with a Co source. For a dose of 100 krad the gain has changed by 3%. It can be recalibrated on-line whenever necessary.