DAC-Mod (optimizing the digital-analog-converter)
The audio DAC of the trunked (EVA5) version was originally designed to support data transmission only. The converter is build from discrete components consisting of a resistor network (R812 - R815) followed by an active low-pass filter (U801A, C804, C805, R821). The network is driven directly by two CPU pins (30 and 31).
The following tools are required for this mod:
- Soldering iron
- Solder wick, solder (if needed)
Original circuit
- Location in the schematic diagram
- Location on the board
- Unmodified DAC circuit
- Unmodified PCB, arrows indicating the required modifications
- Circuit with modifications in red
- Modified PCB
Modifying the circuit
The modification is simple:
Reposition R811 to connect 5V and node R812-R815.
Swap R815 and R814.
There are still only two pins of the CPU being used, but additionally to the output states "1" and "0" the high-impedance input state is also employed.
The re-ordered resistor network now allows 9 distinct states/voltages to be set (the symmetric nature of the original network (where both CPU pins had a series resistor of 200 kOhm) would only yield 6 distinct states/voltages).
The effective resolution of the DAC is thereby raised to an equivalent of 3.17 bit. The converter is now able to produce a signal (at full scale level) with more than 19 dB SNR.