3D Time-of-flight distance measurement with custom - Universität ...

156 CHAPTER 6
connected to the sensor’s pins by loose wires, which can be plugged very flexibly
into dedicated sockets. A photograph of the driver board is shown in Figure 6.3.
Lens holder
Filter holder
TOF-ASIC
Figure 6.3 Driver board. Left: top-view with mounted TOF-chip, filter holder on
top of the sensor chip (filter not inserted), lens holder, gate drivers
and shielded analog output; right: bottom view with flexible plugged
wire-connections, trigger circuitry for signal recovery and 40 Pin
connector to the sequencer board. (Size: 90 mm x 110 mm).
The task of realizing adjustable signal amplitudes, rise and fall times and pulse
widths is solved by the circuitry illustrated in Figure 6.4. First, the digital signal from
the sequencer is recovered by two inverters in series (74AC14 contains a Schmitt
trigger). The rise/fall time of the recovered signal is then synthetically enlarged by
an RC low-pass filter and the amplitude is adjusted by the potentiometer P1. With
the Schmitt-trigger in the gate driver, this circuitry allows the adjustment of the
signal’s pulse width within some nanoseconds. (For the operation of our 1-Tap pixel
at 20 MHz we found a performance optimum for a duty cycle of 1:1.)
The gate driver is supplied by an adjustable voltage (4.5 V to 15 V) that also
defines the coarse output voltage amplitude. The output amplitude adjustment with
this regulation alone is not sufficient, since the gate driver requires a minimum
supply voltage of 4.5 V and is not capable of generating smaller voltage amplitudes
necessary for high system flexibility. The fine adjustment is achieved by an ohmic
(DC) and capacitive (AC) voltage divider, also introduced in [TIE]. The slope of the
signal, fed to the CCD gates, can be varied by the absolute capacitive load of the