3.4. Single chip realisations
An integrated optoelectronic receiver for a pulsed TOF laser radar is presented in paper VIII. The purpose of the circuit is to demonstrate the operation of the receiver with a photodetector integrated into the same chip. The chip includes an integrated pn photodiode (cross-section shown in Fig. 23), a transimpedance preamplifier, a high-pass filter for pulse shaping, current mode gain control, a post-amplifier, a timing comparator and a peak detector, as shown in Fig. 25.
The measured bandwidth and maximum transresistance of the amplifier channel are about 150 MHz and 280 kΩ respectively. The input referred noise, 2.1 pA/√Hz, is about three times lower than with the external photodiode realisation described in paper I. This was achieved because of the smaller capacitance of the photodetector and the absence of any parasitic capacitance or inductance of the photodetector and amplifier channel package, PCB wiring, bond wires, I/O cells and ESD protection structures. However, the advantage from the point of view of the minimum usable optical signal is questionable as the responsivity of the integrated photodiode (0.3 A/W) is about a half of that of discrete PIN photodiodes (0.6 A/W) and about 160 times smaller than that of APDs (50 A/W), which in any case present a drawback at a high supply voltage. Nevertheless, the measurements demonstrated that it was possible to integrate the whole receiver channel into a single chip without any process modifications and that the chip could be used for pulsed TOF laser rangefinding.
The performance of the optoelectronic receiver shown in Fig. 25 is summarised in Table 5.
Table 5. Performance of the receiver for a pulsed TOF laser radar.
| bandwidth | ~150 MHz |
| maximum transresistance | 280 kΩ |
| Input-referred current noise | 2.1 pA/√Hz |
| responsivity of the pn photodiode | ~0.3 A/W |
| capacitance of the pn photodiode | 0.54 pF |
| rise time of the pn photodiode | < 5 ns |
| walk error | ±10 ps, (±1.5 mm), (1:30 input signal) |
| single-shot precision | < 200 ps, (< 30 mm), (SNR > 10) |
| current consumption | 20 mA @ 5 V |
Paper IX presents a chip in which the level of integration is further increased by including the time interval measurement. The chip includes four identical optoelectronic receiver channels and a digital-mode three-channel TDC, as shown in Fig. 26. Only optics and a laser pulse transmitter are needed to measure distances in up to three directions with a single optical pulse. The receiver channels consist of an integrated pn photodetector, a transimpedance preamplifier, a high-pass filter for pulse shaping, a post-amplifier, a timing comparator and a peak detector which is common to all the receiver channels.
The chip was designed to demonstrate the feasibility of integrating a whole multi-channel receiver as a single chip in order to measure several distances with a single optical pulse. The photodetectors are circular, with a diameter of 100 mm, and placed in a row. A photograph of the chip is shown in Fig. 27.
The chip has been implemented in an AMS 0.8 µm BiCMOS process without any process modifications and is 4.6 mm x 3.3 mm in area. The measured bandwidth and transresistance of the amplifier channels are 140 MHz and 390 kΩ respectively and the input-referred noise is 2.2 pA/√Hz. The single-shot precisions are 51 mm and 13 mm with SNRs of 11 and 100.
Since the packaged version of the circuit was unstable because of the inductances of the bond wires and lead frames of the CLCC84 package, the unpackaged chip was bonded directly onto the PCB. A photograph of the chip on the PCB is shown in Fig. 28.
In the linearity measurements the start channel was used for the start signal and the stop3 channel for the stop signal. Fig. 29 shows a non-linearity of ±12 mm in the range from 0.27 m to 22 m. This non-linearity is caused by cross-talk from the start channel to the stop3 channel, which can be reduced by better design of the preamplifier, which had quite a poor PSRR.
The transmitted optical signal was divided between four fibres for measurement purposes and the reflected signal was focused on the chip. The efficiency of the division was poor and a great deal of power was lost. By designing photodetectors of a customised shape and placing them in a manner appropriate to the application, the efficiency can undoubtedly be improved. Rectangular photodiodes placed next to each other, for example, will effectively collect the signal from a target on which the laser stripe is focused and measure distances from given sectors. Measurements of this kind could be useful in anti-collision warning system for vehicles, for example.