Voltage Reference Quad Project

Vref4BoardFrontPanel01Precision Reference Standard Quad front panel

After see­ing sev­er­al oth­er volt­age ref­er­ence projects online and many more assem­bled boards on Ebay, I decid­ed to put togeth­er my own ver­sion of a mul­ti-out­put mod­ule that I could use for pro­to­typ­ing ADC, and oth­er projects.
I need­ed at least a 2.048 volt and a 4.096 volt pre­ci­sion ref­er­ence. An extreme­ly high accu­ra­cy 5.000 volt ref­er­ence was also on my list. I already had an enclo­sure on hand that I want­ed to use, but in my ear­ly mock ups the three out­puts on the front pan­el just did­n’t look right. So for good mea­sure a 2.500 volt ref­er­ence was added to the project.

The 4.096 volt and low­er ref­er­ence IC’s only required a sta­ble 5 volt sup­ply, but the 5.000 volt ref­er­ence IC required a min­i­mum of 9 volts and prefer­ably 10 volts for max­i­mum sta­bil­i­ty. So a sep­a­rate add-on board was designed to sup­ply 9 or 10 volts along with a warm-up timer and bat­tery low volt­age alert.

All the volt­age ref­er­ence IC’s are pro­duced by Maxim Integrated.
The 5.000 volt ref­er­ence is a MAX6350, and is a low-noise, pre­ci­sion volt­age ref­er­ence with an extreme­ly low, 0.5ppm/°C typ­i­cal tem­per­a­ture coef­fi­cient and ±0.02% ini­tial accuracy.
The three low­er volt­age ref­er­ences are all MAX6126A series ultra-low-noise, high-pre­ci­sion, low-dropout volt­age ref­er­ences, that fea­ture high-sta­bil­i­ty, laser-trimmed, thin-film resis­tors that result in 3ppm/°C (max) tem­per­a­ture coef­fi­cients and ±0.02% (max) ini­tial accu­ra­cy.Vref4Board01Quad volt­age ref­er­ence board fab­ri­cat­ed by OSH Park

StandardEagle schemat­ic for Quad Vref board

StandardBoardQuad Vref board lay­out in Eagle

Vref4BoardSpaste01Board with sol­der paste and com­po­nents placed

Vref4BoardReflow01Vref board after sol­der reflow. Had a cou­ple of capac­i­tors that did­n’t straight­en out as much as I had hoped, but no issues oth­er than looks.

RefPowerEagle schemat­ic for pow­er and sta­tus mon­i­tor board

Power and sta­tus board lay­out in Eagle CAD
Shared project Power & Monitor board on OSH Park

This board con­nects the two 6 volt AAA bat­tery packs in series. and also has con­nec­tions for the front pan­el “On” switch. There are jumpers for select­ing 9 or 10 volt sup­ply out­put for the 5.000 volt ref­er­ence. I includ­ed both so I could test the dif­fer­ence in sta­bil­i­ty between the two, as the spec­i­fi­ca­tions from Maxim lead me to believe the 10 volt pow­er would give a typ­i­cal Line Regulation of 2 ppm/V vs 10 ppm/V for a low­er voltage.
An Atmel Tiny85 proces­sor turns a LED from red to blue after 120 sec­onds has elapsed from pow­er on, and also mon­i­tors that the bat­ter­ies are sup­ply­ing a min­i­mum of 10.5 volts to the reg­u­la­tors. If the volt­age drops below the thresh­old, the front pan­el LED blinks red at a 1 pps rate to indi­cate low battery.

Atmel Atiny85 Arduino code:

// Precision Reference Standard
// 2016 Greg Christenson (Barbouri)
// Dual LED driver for system warmup and battery 5V Ref 12volt battery low voltage
// LED is solid red upon power on for 2 minutes then switches to solid blue after 2 min. timeout
// If 12 volt battery goes below 10.5 volts then LED flashes red one time per second for 1/10th second

int BlueLED = 4; // Blue LED resistor and Anode
int RedLED = 3; // Red LED resistor and Anode
int Voltage = 1; // Battery voltage from divider R3, R4 - 1/3 of 12 volts nominal Analog pin1
int val=0; // Setup variable val for analog read
unsigned long time; // Setup variable for time as unsigned long to read milliseconds

void setup() {
// Set pin locations
pinMode(BlueLED, OUTPUT); // sets the digital pin as output
pinMode(RedLED, OUTPUT); // sets the digital pin as output
digitalWrite(RedLED, LOW);
digitalWrite(BlueLED, LOW);


void loop() {
// Check current conditions

time = millis(); // read current time from power up
val = analogRead(Voltage); // read current battery voltage

if (val <= 715) // 1/3rd of 10.5 volts * 0.0049 volts per unit (5 V ref)
digitalWrite(BlueLED, LOW); // Turn off Blue LED if 2 min. timeout has elapsed
digitalWrite(RedLED, HIGH); // sets the Red LED on
delay(100); // keeps the Red LED on for 0.1 seconds
digitalWrite(RedLED, LOW); // sets the Red LED off
else if (time < 120000) // Check for warm-up time
digitalWrite(RedLED, HIGH); // sets the Red LED on until 2 min. timeout
digitalWrite(RedLED, LOW); // sets the Red LED off
digitalWrite(BlueLED, HIGH); // sets the Blue LED on

delay(900); // keeps the Red LED off for 0.9 seconds or delays loop



Vref4BoardCaseFit01Initial test place­ment of boards and bat­tery packs

Vref4BoardInternalWire01Lots of Teflon wires run­ning to the front panel

The Maxim MAX6126A IC’s have the capa­bil­i­ty of Output and Ground sense leads which I con­nect­ed to the front pan­el terminals.
Also used were wide­band noise reduc­tion capac­i­tors on the NR pin of the IC’s.
I also uti­lized 10μF con­duc­tive poly­mer sol­id elec­trolyt­ic chip capac­i­tors for the out­put bypass capacitors.
Ferrite toroid cores were placed on all banana jacks, along with fer­rite bead cores on all force and sense wires to reduce EMI.

Vref4BoardFrontOpen01Front pan­el con­nec­tors wired to board and ready to test

I used “Front Panel Designer” to lay­out the front pan­el, and the file was sent to Front Panel Express for pro­duc­tion. Panel thick­ness is 1.5 mm and is black anodized aluminum.
The mon­i­tor LED is con­nect­ed to the pan­el using a fiber optic light pipe.

Vref4Board49999MVM01Testing using my recent­ly con­struct­ed Millivolt meter

I also test­ed using my Fluke bench meter and all out­puts were well with­in the stat­ed ini­tial volt­age accu­ra­cy. I had includ­ed con­nec­tions on the cir­cuit board for adding an exter­nal trim poten­tiome­ter to the 5.000 volt ref­er­ence, but cur­rent­ly do not see the need to imple­ment it. I will mon­i­tor the drift with age­ing and add if need­ed in the future.

Link to EagleCAD schemat­ic and board files download

Link to Front Panel Designer pan­el files for use with Hammond 1455N1202 box

An updat­ed ver­sion of the Voltage Reference Quad Project:
Voltage Reference Quad Project Version 2 – Part 1

9 Replies to “Voltage Reference Quad Project”

  1. thank for shar­ing, I will try to build one.
    can you give the ic3 ref­er­ence please on Power & Monitor board

    1. IC3 on the pow­er and sta­tus mon­i­tor board is a 78L05 reg­u­la­tor in a TO-92 pack­age. It sup­plies a reg­u­lat­ed 5 volts to the TINY85 IC and LED.

  2. Thank again, I realise, I did not have fer­rite toroid cores. I quick­ly check on ebay but I did not found. do you have a ref­er­ence of your ferrites ?

    I just com­plet­ed the pcb sol­der­ing. I will also order the same box as you pro­pose is a nice design.

    1. Hi nechry,
      I used both:
      399–10822-ND B‑20L-44 FERRITE BEAD CORE EMI 1.6MM
      on the force and sense con­nec­tions, and
      on each of the jacks, from Digikey.

      1. is pos­si­ble to add a pic­ture of the jacks, see from inter­nal. I did not realise you put also some at this place.

        So I order every­thing form digikey, box and ferrites

        And Still remain a question:
        The Four out­put volt­age board is pow­ered with J1 from Power & Monitor board (5ref+/5ref-) but the oth­er side +9Batt comme also from the Power & Monitor board ? of a 9v battery ? 

        And last think in pic­ture the bat­ter­ies hold­er seem twice 6xAAA but you wrote about 6V bat­tery pack. if I’m cor­rect 6x1.5 = 9V but 2 in packs in series will result 18. and the Power & Monitor board sup­posed to get max­i­mum of 12V. so I’m lit­tle bit con­fuse for pow­er­ing the 2 boards.

        Sorry if my ques­tion same to be, too basic.

        1. Hi nechry,
          The best pic­ture of the inter­nal jacks is labeled “Lots of Teflon wires run­ning to the front pan­el”. The fer­rite toroid’s are held in place by the yel­low kap­ton tape, and are just the right size to slide over the extra thread­ed part of the jacks. The fer­rites are not manda­to­ry, just an extra lay­er of noise protection.
          The 6 cell AA bat­tery (9V) pack pow­ers the three MAX6126A Vref’s only (-BATT +9BATT) and is switched sep­a­rate­ly from the first side of the dou­ble pole front pan­el pow­er switch.
          The MAX6350 is pow­ered by reg­u­lat­ed 9 or 10 volts thru J1 from the “Power and Status” board using two sep­a­rate 4 cell AAA (6V) packs con­nect­ed in series thru the board for 12 volts total.
          There is a sep­a­rate con­nec­tion on the “Power and Status” board that con­nects to the sec­ond side of the dou­ble pole front pan­el pow­er switch.

          1. Thank Barbouri for expla­na­tion, I’m pre­vi­ous­ly think­ing the “Power and Status” board mon­i­tor the entire Voltage Reference IC not just the 5V. I think­ing to only use a 12V bat­tery pack for the “Power and Status” board and also the +9Bat with the 12V to. With this, all forth ref are mon­i­tored. Do you have a rea­son to sep­a­rate and only mon­i­tor a bat­tery pack?

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