Programmable Voltage Reference V2.12 Completion…

Finishing up the Programmable Voltage Reference project …

Output set to 200 mV on the Programmable Voltage Reference and reading 200.009 mV
Output set to 200 mV on the Programmable Voltage Reference and read­ing 200.0098 mV. The blue LED dis­play actu­al­ly looks a lot bet­ter, when I don’t have the pho­tog­ra­phy lights point­ed at it.

Items that were left to com­plete from the last Programmable Voltage Reference V2.12 assem­bly post:

  • mount­ing the board in the case — Done
  • alu­minum volt­age con­trol knob mod­i­fi­ca­tions — Done
  • user inter­face deci­sions — Done
  • pos­si­ble shield­ing for the ana­log side — Done
  • BOM doc­u­men­ta­tion — Done
  • and pow­er sup­ply deci­sions — Done

Mounting the board in the case was the easy part of the com­ple­tion list. There was plen­ty of room com­pared to some of my past builds, and I could have eas­i­ly gone with the 4.72″ long ver­sion instead of the 6.3″ that I used. The main rea­son I opt­ed for the longer ver­sion is that it would be the same size as the Millivolt Meter and would eas­i­ly stack on top of it.

Mounting the Programable Voltage Reference inside the Hammond case and connecting wires
Mounting the Programmable Voltage Reference inside the Hammond case and con­nect­ing wires

I placed fer­rite toroid cores on both banana jacks, and held them in place with kap­ton tape. To pow­er the unit I will nor­mal­ly have it con­nect­ed to the com­put­er with a USB cable plugged into the Teensy 3.2 thru a rear mount­ed USB type B jack on the back pan­el. For the times that I need the unit to be iso­lat­ed, it will use an inter­nal bat­tery pack with 6 AA cells con­nect­ed thru the front pan­el switch.

Reflection of components in copper shield on analog side of circuit board
Reflection of com­po­nents in cop­per shield on ana­log side of cir­cuit board

I decid­ed on a flat cop­per sheet shield the width of the cir­cuit board and just over 3/4″ high, with notch­es on the bot­tom to clear board com­po­nents. I may print a small plas­tic cov­er for the 4.096 volt ref­er­ence IC to reduce air cur­rents across it and improve ther­mal stability.

Angled view of circuit board with shield mounted before installation
Angled view of cir­cuit board with shield mount­ed before installation
Front panel voltage adjustment knob modified for RGB LED view
Front pan­el volt­age adjust­ment knob mod­i­fied for RGB LED view

I mod­i­fied the alu­minum knob on the RGB LED rotary encoder with a light pipe pan­el ter­mi­na­tion. I cur­rent­ly have the indi­ca­tor blue when in cal­i­brat­ed off­set mode, red when cal­i­bra­tion off­sets are turned off, green when in man­u­al off­set mode, and vio­let when the pro­grammed volt­age dif­fers from the actu­al volt­age output.
To use an exter­nal volt­age ref­er­ence on the Teensy 3.2 board, a 470 ohm aref resis­tor tied to 3.3 V on the Teensy 3.X board needs to be removed.

Teensy3.2 Aref 470 ohm pullup resistor
Teensy3.2 Aref 470 ohm pullup resis­tor. Remove for exter­nal reference

I fin­ished up the BOM doc­u­men­ta­tion and added it to the previous
assem­bly post, with all parts being sourced from Digikey. I also includ­ed a CSV text file for the required board parts.
I per­formed an ini­tial cal­i­bra­tion on the 4.096 volt ref­er­ence, and then stepped thru all 4095 set­tings and not­ed the dif­fer­ence from the actu­al set­ting. The soft­ware off­set cal­i­bra­tion writ­ten by uChip uses run-length encod­ed data stored in the eep­rom. I only need­ed 8 off­sets for the entire range, so I must have lucked out on a good DAC chip. After about 250 hours of pow­er on time for the ref­er­ence, I will check the 4.096 volt ref­er­ence again and re-cal­i­brate if needed.
So far I am pleased with the func­tion­al­i­ty and accu­ra­cy, espe­cial­ly when com­pared to the total cost of the board, com­po­nents, and enclosure.
Total cost of the sys­tem using pre­mi­um com­po­nents, and cus­tom front pan­el was around $200 USD. It can be built for a lot less, but my goal from the start was to build the best device I could afford, not the least expen­sive sys­tem possible.

A big THANKS to uChip for the ini­tial design and post on the Sparkfun forum.

Link to uChip’s orig­i­nal project files with PDF doc­u­ments of ser­i­al port com­mand def­i­n­i­tions and cal­i­bra­tion notes.

6 Replies to “Programmable Voltage Reference V2.12 Completion…”

  1. Nice built and nice doc­u­men­ta­tion. Any chance of shar­ing who pro­duced the front pan­els? Are they laser cut?

    1. The front pan­el was pro­duced by “Front Panel Express”. I used their sup­plied soft­ware to design the pan­el. This is the third time I have used them for fab­ri­ca­tion, with excel­lent results each time.
      The pan­el design file is avail­able if requested.

  2. Hi Greg,

    Can I ask how much the front pan­el cost?
    Really like what you do with the Hammond enclosures.



    1. Hi Jon,
      The cur­rent pric­ing for a sin­gle pan­el from “Front Panel Express” is $42.81 USD, no dis­counts applied.
      Greg (Barbouri)

      1. Thanks Greg,
        That’s actu­al­ly quite rea­son­able. I thought it would be twice that amount.
        Think, I may using that ser­vice soon 


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