Voltage Reference Quad Project Version 2 — Part 1

Voltage Reference Quad Project ver­sion 2 board

An updated version of the
Voltage Reference Quad Project’s circuit board.

This is my ver­sion 2 board update, which incor­po­rates sup­ply volt­age reg­u­la­tion, over-cur­rent pro­tec­tion, and trim­mer for the 5.000 volt ref­er­ence all on the same board. The orig­i­nal ver­sion 1 board required an exter­nal reg­u­lat­ed sup­ply for the MAX6350‑5 5.000 volt ref­er­ence IC, but did have an on-board 5 volt reg­u­la­tor for the oth­er three ref­er­ence IC’s.

Quad Voltage Reference side view
Quad Voltage Reference side view

The new board uses a LM2937IMPX-10, which is a pos­i­tive lin­ear volt­age reg­u­la­tor IC with a 10V out­put at 400mA in a SOT-223–4 pack­age.
The 5 volt reg­u­la­tor is a LM2936MP‑5.0 with a 5V out­put  at 50mA also in a SOT-223–4 pack­age.
Included on the pow­er input sec­tion is a 100 mA PTC fuse for over cur­rent pro­tec­tion. A descrip­tion of PTC fus­es is includ­ed in the Milliohm Meter Version 1.5 post.
With a 10K ohm load on the ref­er­ence out­puts the 12 volt DC sup­ply cur­rent is less than 10 mA. The 5.000 ref­er­ence is capa­ble of sourc­ing or sink­ing up to 15 mA, and the three MAX6126 series ref­er­ences can Sink/Source 10 mA each.
An onboard trim­mer poten­tiome­ter was also added for fine adjust­ments to the 5.000 volt ref­er­ence. I chose a Vishay Ultra High Precision trim­ming poten­tiome­ter with a ±15ppm/°C tem­per­a­ture coef­fi­cient part num­ber Y005610K0000K0L. Vishay also makes this poten­tiome­ter in the 1285G Series with a ±5ppm/°C tem­per­a­ture coef­fi­cient.
Some minor lay­out changes were made on the board to improve ground­ing, iso­la­tion, parts spac­ing, and the added com­po­nents which increased the cir­cuit board size.

Quad Voltage Reference Version 2 Eagle board
Quad Voltage Reference Version 2 Eagle board
MAX6126 Typical operating circuit diagram, showing 4 wire Kelvin sense connections
MAX6126 Typical oper­at­ing cir­cuit dia­gram, show­ing 4 wire Kelvin sense con­nec­tions

The three 6126 ref­er­ence IC’s (2.048, 2.500, 4.096) all have inde­pen­dent con­nec­tions for the pow­er-cir­cuit out­put (OUTF) sup­ply­ing cur­rent into a load, and for the cir­cuit input reg­u­lat­ing the volt­age applied to that load (OUTS). This con­fig­u­ra­tion allows for the can­cel­la­tion of the volt­age drop on the lines con­nect­ing the ref­er­ence and the load. The 6126 ref­er­ence also has the same type of Kelvin con­nec­tion to can­cel drops in the ground return line (GND, GNDS).
The cir­cuit board has 4‑wire con­nec­tions for each of 6126 ref­er­ence cir­cuits, which I have ter­mi­nat­ed at the con­nec­tor to a sin­gle pair of jacks for each ref­er­ence out­put.
This could be eas­i­ly mod­i­fied in the pan­el design to sup­port a 4‑wire (force/sense) con­nec­tion for each of the MAX6126 ref­er­ence out­puts, by adding an extra pair of con­nec­tors for these ref­er­ence out­puts.
I used sil­ver coat­ed 20 AWG Belden 83007 hook-up wire with Teflon TFE insu­la­tion, for all con­nec­tions from the board to the front pan­el jacks.

Reference output and sense wires being soldered to the circuit board
Reference out­put and sense Teflon insu­lat­ed wires being sol­dered to the cir­cuit board

Again I used Front Panel Express to fab­ri­cate the front pan­el for the enclo­sure, using their Front Panel Designer soft­ware. The pan­el fits a stan­dard Hammond 1455N1601 extrud­ed box with met­al end plates 6.299″ L x 4.055″ W x 2.087″ H.

Quad Voltage Reference V2 front panel
Quad Voltage Reference V2 front pan­el

There is now a heater indi­ca­tor includ­ed on the front pan­el which will be cov­ered in a Part 2 post, along with asso­ci­at­ed cir­cuit­ry.

Back view of front panel with jacks, switch, and indicator inserts mounted
Back view of front pan­el with jacks, switch, and indi­ca­tor inserts mount­ed
Front panel with components mounted
Front pan­el with com­po­nents mount­ed

I will be exper­i­ment­ing with a heat­ed enclo­sure for the ref­er­ence board using a PID tem­per­a­ture con­troller and a Maxim DS18B20 dig­i­tal ther­mome­ter with 12-bit res­o­lu­tion. I am still in the pro­to­typ­ing stage, but plan on imple­ment­ing this on an Atmel ATtiny85 8‑bit AVR RISC-based micro­con­troller.

The Voltage Reference Quad V2 cir­cuit board is avail­able thru OSH Park.

EagleCAD7.5 schemat­ic and board files for the Voltage Reference Quad V2

BOM and final assem­bly will be includ­ed in the Part 2 post.

5 Replies to “Voltage Reference Quad Project Version 2 — Part 1”

  1. Please hur­ry and post sec­ond half 🙂 The devices your cre­at­ing are works of art.
    I’m real­ly inter­est­ed in how you will heat the ref­er­ences.
    You real­ly should do YouTube videos on the devices you make.
    Keep up the great work.

  2. We’d love to see part 2, this device will help a lot in our high pre­ci­sion mea­sure­ment pro­to­typ­ing. Keep up the good work

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