Saturday, July 15, 2023

Gamma Dog hardware - PCB v4.0

I finished the design of PCB v4.0 for the Gamma Dog and OSH Park service came through again with excellent manufacturing quality boards.

The improvements in the schematics and layout are not huge but this version incorporates the progress in the development, which continued after PCB v3.0.

Gamma Dog Main PCB v4.0

Bottom layer with a Marie Curie quote.

Changes from PCB version 3.0:

  • changed the placement of the DAC board and EEPROM board connectors. The new version of the 12-bit DAC daughterboard is slightly larger due to additional QT connectors and it wouldn't fit in the old footprint. 
  • added HV voltmeter circuitry for measuring and adjusting the HV PMT Bias in the field - the measurement range is 0 - 1255V (max). The High-Voltage reading is displayed when entering the menu system's diagnostic screen and will allow for field adjustments when changing different detectors. The circuit provides HV PS status data for the Self-Diagnostics / Health-Check during startup. The accuracy is better than the on-screen resolution of 1V.
  • added the I2C digital potentiometer for the Digital Audio Volume Control to the main board (MSOP-10 package) - with the previous version it was an in-line add-on board as it was developed after fabrication of PCB v3.0
  • added support for the LED in the GREEN/DOWN button - this LED serves as "Charging" (flashing) and "Charge complete!" (solid) indicator during battery charging.
  • added an SMD jumper to configure the power source for the Audio Power Amplifier module - options now are either regulated 3.3V from the MCU board or direct battery power (4.1V max) (affording the highest possible audio volume).
  • re-organized (separated) and moved the DOWN Button and Audio Amplifier connectors - the GREEN/DOWN button now has a dedicated 3-pin connector (previously, shared it with the Audio Power Amplifier), and the connector is placed closer to the button's location.
  • removed all of the unnecessary component footprints, related to the old analog circuit for setting Minimum Pulse Height Threshold.
  • Improved the matching between the motherboard's pads and the corresponding HV PS module output and GND pads for easier installation of the piggybacked PS module.
  • Various small changes in component placement, trace routing and overall layout optimizations.

The PCB with SMDs, sockets, headers and connectors installed. Visible on the right is the HV voltmeter's VD string of precision resistors. 
The input impedance of the voltmeter is >1 GOhm to reduce any voltage drop caused by the measurement circuit.

DAC, EEPROM, Comparator and HV Power Supply + Pulse Amplifier 

The nRF52840 MCU module along with various sensors and RFI Shielding Can around the HV circuit

Latching Relay module and RFI shielding cover installed.

Completed, calibrated and tested V4 boards.

Firmware version is now v4.5, supporting the new hardware features and changing how unit-specific data is handled. 
All of the "Identity data", unique to each Gamma Dog unit, such as calibration offsets for various voltage dividers, pulse amplifier DC bias value, exact DAC reference voltage, serial number, etc is now stored in a dedicated "Read-Only" area of the EEPROM and not hard-coded in the firmware as before. 
Each unit is flashed with its "identity" during manufacturing, removing the need for unit-specific firmware code and simplifying firmware updates. 

The PMT Bias reading shown when entering the menu system. 
The value is constantly read and displayed at 4Hz until a button is pressed.