Lexicon
7-47
Fader Touch-sensing (sheet 6)
Each motorized fader is equipped with an electrically-conductive knob which connects to a terminal of the
fader through a sliding track. This feature makes it possible to sense when a knob is touched by an
operator.
When an operator touches the knob, the apparent capacitance at the terminal increases due to the added
capacitance of the operators body. This additional capacitance is detected in a bridge circuit where the
apparent capacitance of the touch track is compared with a reference capacitor.
The terminal from channel 1 (TOUCH1) is coupled to comparator U28 by capacitor C118 and current-
limiting resistor R103. The dual diodes of D22 are normally back biased; D22 protects the touch circuitry by
diverting high-voltage triboelectric charge (static) either to ground or into C120/C199, biased at 5V by R87.
In normal operation, D22 and R103 can be ignored.
The other comparator input is connected to a discrete 33pF capacitor (C119), and both comparator inputs
are driven through individual 39.2K resistors (R104,R105) by a bridge excitation waveform. The excitation is
produced from TCH_CLK, a 3.3V, 125kHz squarewave, buffered by emitter-follower Q8. Relative to the
period of the squarewave, the time constants of the bridge are long enough that the comparator inputs are
quasi-triangular. The relative amplitudes of the triangles depend on the relative capacitances at the two
inputs.
When the knob is not touched, its capacitance is smaller than 33pF and the corresponding triangle at U28-3
is larger than that developed at U28-2. At the positive peak of the triangle, the comparison will be positive,
and the output of U28 will therefore be high. At that point, the high level is latched by the TCH_CLK
squarewave into U35, which drives TCH_DET1 from its inverting output. In the untouched state,
TCH_DET1 is constantly low.
When the knob is touched, the capacitance becomes greater than 33pF, and the result of the comparison is
opposite at all instants of time. The latched value is therefore opposite, and in the touched state,
TCH_DET1 is constantly high.
The operation of the touch-sense circuits for the other odd-numbered faders is identical. The even-
numbered circuits operate similarly, but their excitation square wave is of the opposite phase. This helps
ensure proper touch sensing in the case where multiple knobs are touched simultaneously.
The states of TCH_DET[8:1] can be read by software via tri-state buffer U24 and the FPGA.
4.6. Joystick (sheet 5)
Channels 0 and 1 of A/D converter U31 are used to digitize the position of the two-axis joystick connected
to J7 and J8. As with the faders, the joystick is supplied with 5Vdc, to produce position-dependent X and Y
dc voltages, which are digitized by the two channels.
Piezo Transducer (sheet 5)
A piezoelectric ceramic transducer disk is mounted flat on the pc board, under the Lexicon button. When
the button is pressed, it not only makes contact with switch pattern SP47, it also produces flexure of the
piezo element. The resulting voltage is peak-detected and amplified by U29 (LM358), then digitized by A/D
U31. The amplitude of the piezoelectric voltage gives an indication of how hard the button was pressed,
which software can determine from the profile of the digitized waveform.
4.8. Notes on A/D Operation
The A/D reference and the potentiometer excitation voltages are the same, so the digitized position
information from the potentiometers is a ratiometric measurement, insensitive to the exact value of the 5V
excitation. In contrast, the accuracy of both the +12V sense and the piezo transducer channels do depend
on the accuracy of 5VA, which is +/5%.
The hardware-controlled interface implemented in the FPGA produces all the A/D control words, clock, and
chip-selects. The serial bit clock rate is 2MHz. The entire A/D scan cycle takes 200usec, so each channel is
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