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