Instead of using the MAX22088, which only ships as QFN package, this project uses a LM4871 as transmitter and a Raspberry Pi Pico as receiver.
Warning: Totally overengineered project ahead!
In version v0.1 and v0.2 the Ti LM4871 has been used.
Starting from version v0.3 the STM TS4990 is being used as
pin-compatible replacement.
See Ti LM4871 vs STM TS4990 for details.
The LM4871 is and dual
ouput 3W audio amplifier and the circuit around it is following the
design guidelines.
The input is an analog signal generated by a 3 level signal from GPIO
A_H and A_L. The resulting analog signal has
one of the following levels:
- 3.30V
- 1.65V
- 0.00V
The Z-Diodes D3 and D5 protect the device from voltage spikes that
could be introduced on the long P1/P2 lines.
R5 and R6 limit the output impedance to more than 4Ohms.
R11, R12, R10 and R9 pull the lines to idle position when the LM4871 is in
shutdown mode. They also provide a minimum load of 10KOhms if the P1/P2 bus is
not connected. The LM4871 tend to oscillate at several Mhz without a load.
The datasheet recommends 2KOhms as minimum load!
C4 prevents oscillations that can happen if the gain is too small.
Using 4.7pF and a gain of 5 seem to prevent most oscillations.
The LM4871 is able to drive 0.5Watts at 3.3V.
The LM4871 can drive a 3.0V amplitude signal from a 3.3V supply. Due to the dual-output the generated differential signal has a level of +/- 3.0V.
The Z-Diodes D106 and D105 protect the device from voltage spikes that
could be introduced on the long P1/P2 lines.
R109, R116 and R8 are providing a bias voltage of 2,5V (when driven by 3.0V ADC voltage)
and they divide the signal amplitude by 1.66.
R114, R110 and R7 are providing a bias voltage of 2,5V (when driven by 3.0V ADC voltage)
and they divide the signal amplitude by 1.66.
The total impedance of R109, R116, R8, R114, R110 and R7 is greater than 10KOhms.
C115 acts a low pass filter.
ADC_A and ADC_B should be connected to a differential ADC. In this case it's
a RPi pico using ADC0 and ADC1.
The signal has been measured on the Pico ADC behind the AC coupling capacitors and voltage divider:
The voltage on the ADC has an amplitude of 1.73V. Mutliplied with the voltage
dividor reciprocal gain this give a 2.87V amplitude after AC coupling.
When connected to the P1/P2 bus the amplitude is a actually a bit lower: 2.5V
It is suffcient to be detected by the remote P1/P2 transceiver circuit.
The P1/P2 remote transceiver drives a 5V amplitude on the bus.