Programmable Voltage Reference Project

A pre­ci­sion pro­gram­ma­ble volt­age ref­er­ence cir­cuit capa­ble of 0.001 to 4.095 volt out­put in 1 mV steps with an accu­ra­cy of 100 uV.

Programmable Voltage Reference circuit board top
Programmable Voltage Reference cir­cuit board top com­po­nent side, Version 1.04

This project is based on a cir­cuit designed by uChip and pre­sent­ed in the Sparkfun forum. There is a Github page for the orig­i­nal design.

uChip PVR set to 4.000 volts

Original Uchip pro­gram­ma­ble volt­age ref­er­ence with HP bub­ble dis­play. Set for 4.000 volts

PVR circuit board with surface mount components soldered
Circuit board with sur­face mount com­po­nents attached
Programmable Voltage Reference board back side. Version 1.04
Programmable Voltage Reference board back side. Version 1.04

I have been want­i­ng to build an adjustable pre­ci­sion volt­age ref­er­ence for some time now, and was real­ly excit­ed to find this design by uChip on the Sparkfun forum. It had most of the fea­tures and fit the spec­i­fi­ca­tions that I was inter­est­ed in.

I made a few small design changes such as adding a wide­band noise reduc­tion capac­i­tor to the REF5040 cir­cuit, addi­tion­al board rout­ing to clear the area under the unused Vref pins, adding mount­ing holes, remov­ing the sol­der mask from crit­i­cal areas, and com­pact­ing the board a bit.

PVR solder paste
Adding sol­der paste to the Programmable Voltage Reference cir­cuit board

The orig­i­nal design is capa­ble of gen­er­at­ing an out­put between 0.001 volts and 4.095 volts with an after cal­i­bra­tion accu­ra­cy of 100 uV. It uses a REF5040 low-noise, very low drift, pre­ci­sion volt­age ref­er­ence IC by Texas Instruments. The datasheet spec­i­fies a tem­per­a­ture drift of 3 ppm/°C Max for the high-grade ver­sion, and a long term sta­bil­i­ty of 45 ppm/1000 hr after 1000 hours.

HP 7 segment by four LED bubble display
HP 7 seg­ment by four LED bub­ble dis­play
Removing failed atmega 328p processor chip from board
Removing failed atmega 328p proces­sor chip from board

The cir­cuit worked great for 1 week, and then I had prob­lems with the proces­sor start­ing up when pow­ered on. All volt­ages except for the neg­a­tive ana­log, that is gen­er­at­ed by the micro-proces­sor were spot-on. Usually after sev­er­al pow­er cycles the sys­tem would start up and I would see the dis­play and out­put read­ing 1.000 volts.
Later that after­noon the board start­ed pulling exces­sive cur­rent, and the out­put volt­age was drop­ping on the 5 volt reg­u­la­tor. The prob­lem end­ed up being the 328P micro-proces­sor.

Programmable Voltage Reference version 2.12 design
Programmable Voltage Reference ver­sion 2.12 design

I decid­ed to do a major redesign so that I could add more fea­tures such as out­put mon­i­tor­ing, off-board I2C dis­play, off-board rotary encoder with RGB LED, SPI iso­la­tor, and a LTC1152 op-amp for the out­put buffer. The proces­sor was changed to a Teensy V3.2 board using a MK20DX256VLH7 Cortex-M4.
More to fol­low.

7 Replies to “Programmable Voltage Reference Project”

    1. After assem­bly and test­ing of the ver­sion 2.12 board I will make it avail­able to order from the OSH Park print­ed cir­cuit board ser­vice, and will most like­ly have sev­er­al sin­gle bare boards avail­able. As far as a full kit, I cur­rent­ly do not have the avail­able time for the logis­tics of putting togeth­er a kit right now, but this may change in the future. If the 2.12 board works well, I will at least put togeth­er an easy order BOM from Digikey.
      Thanks,
      Barbouri

  1. OK, the rea­son we were inter­est­ed in a kit was;
    1) The dif­fi­cul­ty of hand sol­der­ing on such fine pin pitch­es
    2) The need to pro­gram a micro we don’t nor­mal­ly deal with

    Any sug­ges­tions on over­com­ing these hur­dles?

  2. What’s the point of rout­ing away the board under the ref­er­ence? This might make sense if board leak­age or high volt­age creep were an issue, but nei­ther of those will have any impact on the REF5040 so the ben­e­fit of the PCB “penin­su­la” escapes me.

    Nevertheless, a very cool project and way cheap­er than those Datel cal­i­bra­tors:

  3. Please let me know when the pcb and bom are avail­able, i need to get one of these asap. thanks (post would be to 4509 queens­land aus­tralia) Am extreme­ly inter­est­ed.

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