Assembling the V2.12 Programmable Voltage Reference Board.
It was homemade pizza night, so I got off to bit of a late start with board assembly.
After a thorough cleaning of the board with isopropyl alcohol and hitting all the pads with a flux pen, I applied solder paste using a blunt tip syringe. I am currently using MG Chemicals 4860P Sn63/Pb37 solder paste with good results.
Next came the component placement which went well except for the AD5060BRJZ which comes in a 8‑lead SOT-23 package with a 0.65 mm lead spacing.
Refow soldering on the Presto “Liddle Griddle” electric skillet was uneventful, and I manually followed the reflow profile provided by the solder paste manufacture using a non-contact infrared thermometer. I took special care not to overheat or stress the Texas Instruments voltage reference IC’s.
I wanted to easily remove the Teensy 3.2 processor board if needed, so I used some Mill-Max 350 series interconnects. Two 14 pin interconnects were used for the outer two rows and a single 5 pin interconnect with one pin removed for A10, A11, REF and vUSB for the inner row.
Even though the solder paste used is a no-clean type, I thoroughly cleaned all flux residue, finger prints, etc. from the main board with an isopropyl alcohol bath and light scrubbing with a brush to remove all contaminants, followed by an alcohol rinse and canned air drying.
Initial board testing went well, with the 5 volt rail reading 4.998 volts and the negative bias supply around ‑2.3 volts. Analog filtered 5 volt rail measured 4.82 volts which is well above the 4.6 volt minimum. Current draw was under 7 mA with the display attached and reading 1.000 V.
The 2.048 volt reference was spot-on, but when adjusting the trim pot for the 4.096 reference I was at the extreme end of the adjustment. A 11K ohm resistor in place of the 12K in position R1 put the adjustment back in the center of the adjustable range.
I did notice an error in the silk screen for the display connector which had the +5 and GND text reversed. This has been corrected in V2.13, along with some solder mask aesthetic issues and the value change in resistor R1.
I still have a bit of software to finish, but all of the major parts are completed.
One of the hardware features that I added to the V2.12 board was the ability to monitor the actual output voltage using the Teensy 3.2’s 16 bit analog to digital converters. My plan is to compare the programmed output voltage against the actual voltage and change a status LED from green to red if it is outside a specified range.
To maintain output measurement accuracy I am using a TI REF5020ID 2.048 V voltage reference in U4 connected to the Teensy external vref pin. Output voltage is supplied to the Teensy analog input via an OPA365 buffer op amp U3 and a Vishay foil resistor 25K-25K voltage divider IC2.
The buffer op amp for the AD5060 digital to analog converter was changed to a LTC1152 zero-drift op amp with a 10 mA sink and source capability.
Other hardware items added are a MAX14931 four channel digital isolator IC1 used for the SPI connection to the DAC, and a transient voltage suppression diode D3 for ESD protection across the output.
Next part is mounting the board in the case, aluminum voltage control knob modifications, user interface decisions, possible shielding for the analog side, BOM documentation, and power supply decisions.
Parts and files for the Programmable Voltage Reference V2.13:
OSH Park PCB board service Project page
Eagle CAD V7.5 PDF, schematic, and board ZIP file
Firmware for Teensyduino — Version 0.7 with output V check ZIP file
Front Panel Designer — Front panel for PV Reference2 ZIP file
Digikey cart for V2.13
Download Digikey CSV parts file