# Voltage Reference Resistor Calculation (Shunt)

I am trying to design a circuit using this LM4040-5 Voltage Precision Reference for my DAC8554, but I can't for the life of me figure out how to properly calculate the resistor value to be applied. Right now the LM4040-5V (5V reference) is giving me voltages ranging between 4.77v to 5.11v. Obviously not very precise!

This is the basic schematic for the LM4040 via its datasheet, as well as the formula they suggest for calculating the Resistor value:

So for my particular design, VS == +12v, VR == +5v, but where/how can I obtain the values for IL (load) and IQ (operating current)?

• 4.77v to 5.11v. - what resistor value gave this and how stable is your 12 volt supply? Apr 21, 2020 at 16:59
• I would like to think my 12v supply is stable, though it will not always be exactly 12v. More like 12v +- 0.5v. The resistors I have tested range between 5k and 10k Apr 21, 2020 at 18:11
• Does the DAC take current surges? Apr 21, 2020 at 18:28
• @Andyaka I am not familiar with the term, would it say in the datasheet? Apr 21, 2020 at 22:08
• it looks to be no more than 250 uA. What supply do you feed to AVdd - is this from the reference voltage too? Apr 22, 2020 at 8:37

These are values that come from the datasheets for the respective parts. The DAC shows the following:

So you need to supply 250uA to the DAC.

Then for the LM4040 to operate correctly it needs a reverse current of:

So at least 74uA for the -5 version.

So the minimum current through the resistor has to be 324uA. Unless you're counting every uA I'd go up to 500uA at least to be sure you have enough current over input supply tolerance, etc.

With 12V on one side of the resistor and 5V on the other then, the maximum resistor value you can have is (12-5)V/500uA or 14K. Anything higher may not regulate properly.

This is of course assuming you don't have any other load connected to the reference.

• Basically, he can use a 10k or a 12k resistor. 15k should be OK as well, it would still be within the safe area because the DAC's requirement is its maximum possible, but that might be pushing it because we don't know the stability of his 12V source. Apr 21, 2020 at 17:11
• Ok if you look on page 24 of the DAC8554 datasheet it says you can both power and ref the DAC with the voltage reference, which is what I am doing. So based on the DAC datasheet I need to supply 0.95mA(power input) + 0.25mA (ref input) which gives me 7v/1.2mA == 5.83kOhm resistor value. This does bring the Vref to exactly 5v, but it does not stay there. Sometimes it goes up to 5.02, sometimes 4.98... Apr 21, 2020 at 18:02
• ALSO, supplying the lm4040 with a 10k resistor via +12v causes a voltage output of about 4v... I am starting to think this component could be damaged, because none o this makes sense. Apr 21, 2020 at 18:02
• @scottc11 The 0.95mA spec for the DAC is with no load on the DAC output. If you have a load on the output you will need more current. If you're powering the DAC and the ref input from the LM4040 then at 10K resistor clearly won't be small enough. If you have any load on the output of your DAC even the 5.83K resistor might not be small enough. Apr 21, 2020 at 18:31
• @JohnD so each output of the DAC is being sent to the + input of an op-amp, but I haven't wired that part up yet on my breadboard. Currently just trying to get consistent values. Would the op-amp datasheet tell me the "load" for its inputs? Apr 21, 2020 at 22:12