1/25/23, 10:06 PM
Dave Er i ck s on L M399 Pr es i s i on 10V R ef er ence
LM399 Reference: Precision 10V voltage source.
The Schematics, PCB f iles, and BOM are here
Introduction
The LM399 voltage reference is used for many precision instruments like multimeters up to 6.5 digits. These are simple to use thermally regulated voltage
references that can provide better than 1ppm/C stability. The problem with these is that they use a 7.1V buried zener diode with an accuracy of about
+/3%, not precise by any means. But they are very stable. The challenge is how to build a precision voltage reference from these parts.
For the ultimate stability, 7 or more digit instruments use the LTZ1000 reference. This part is more complex and more expensive.
The issues with the LM399:
Inconvenient voltage 7.1V. 10.000V or 5.000V would be more useful.
Poor accuracy +/- 3%
To generate 10V from 7.1V requires an amplifier with a gain of 10.0 / 7.1 or a gain of 1.408. For this, two precision resistors with a ratio of 1:0.408 or
2.45:1 are needed plus a precision, low-drift opamp. But that isn't enough. The ratio needs to be both adjustable and stable. These are conflicting
requirements. Also the ratio of these resistors is more important than the absolute values. And temperature drift causes changes to this ratio. This is a
fundamental problem affecting precision circuitry. The fix is to not use individual resistors but to use a resistor network with both resistors on the same
substrate. By building them on a common substrate, the advantages are that the two resistors:
Are built with the same formulation of materials
Are built at the same time
Share the same temperature rise
Have similar tempcos, so the ratio tempco is lower than the individual resistors.
Precision resistors are usually 5 to 20 ppm/C. The best resistors, Metal Film types, are 2ppm. But when built as a network, the ratio match can be as low
as 1/10 or less, so 1 to 2 ppm or better. Resistor networks have critical specs called ratio match and ratio tempco. Because the two (or more) resistors
are built on a common substrate and a generally much better than the tempos of individual resistors.
The problem is that precision networks are often custom designed and have high cost and long lead times.
The next problem is how to correct the 3% error. Variable resistors (trimpots) are notoriously not stable for tempco or for mechanical stability.
Design
The answer to these design problems is to use three resistor networks in parallel:
A precision, matched network with 2.44:1 ratio and very low ratio tempco (<= 2ppm/C)
Additional 25ppm precision fixed resistors to pull the output voltage closer to 10.00V: Hard trim
A trimpot to pull in the last error to 0.
Maxim makes off-the-shelf resistor networks in an SO23 package with various common ratios and 2-1ppm ratio match. Many of these are common integer
ratios, but one model is 8.571K / 21.43K which is 2.500 : 1.00. This is close to the desired 2.44:1
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1/25/23, 10:06 PM
Dave Er i ck s on L M399 Pr es i s i on 10V R ef er ence
The additional fixed resistors are selected as a function of the LM399 actual voltage. This technique, also known as a 'hard trim', uses a spreadsheet to
determine the trim resistor to use for a given LM399 voltage. One resistor provides a correction of about +/- 3%. By using a commonly available 0.1%
25ppm resistor here, these contribute only a few percent of the total ratio. Lets say it adds 2% of the ratio. That means that its tempco of 25ppm is
multiplied by about 2% or about 0.5 ppm/C. These are crude calculations.
Then the last maybe .1% of the ratio can be corrected with a trimpot plus a fixed 25ppm high-value resistor.
If you are building only a few of these, you would measure your LM399's voltages, determine which resistors you need with a spreadsheet, and order them.
If you are building lots (10's or 100's) of these, you would order a range of resistor values, and have them on hand.
Why does this work? It works because the main resistor network contributes most of the precision ratio and therefore most of the tempco ratio error, the
'hard trim' provides a few percent LM399 accuracy correction and therefore only a few percent of the tempco error. Finally, the trimpot provides about
0.1% of the trim and therefore only .1% of the tempco.
Trimpots tempcos are generally specified as a change in resistance vs temperature. Makes sense since they are variable resistors. 20 turn cermet and
wire-wound trimpots are generally 100ppm/C. However if a trimpot is wired as a voltage divider, it is similar to a matched resistor set, and its resistance
change is not so important. What is important is its ratio change (tempco), which should be better than the resistance tempco.
Resistor Network Version
I happen to have some surplus precision resistor networks on hand. These are shown in the circuit below in RN1. They are in a small SO16 package and
contain two identical 6 resistor networks, each with a common pin. Each network has 2x 10,000, 2x 1,000, and one each 3,332 and 3,224 ohm resistors.
If I add a 1K to the 3332, I get a 4332 : 10K or 2.308:1 ratio. By using a 3224 instead of the 3332, I get 2.367:1. Both of these ratios are close to the
ideal 2.41:1 ratio needed to convert 7.1V to 10V.
I don't have specs for these and so didn't know their tempco match, so I soldered three parts to SO16 adapters and tested them in a breadboard. All 3
measured about 1 ppm ratio match, which is excellent. Here is the hand-wired prototype.
Here is a version of the LM399 circuit that uses this RN. The hard trims R1A and R2A are still needed as is the trimpot R10.
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1/25/23, 10:06 PM
Dave Er i ck s on L M399 Pr es i s i on 10V R ef er ence
Test Results
The first hand-wired prototype gave excellent results. I tried several of the resistor networks and they all provided < 1ppm/C temperature drift.
Test Fixture
To test up to 4 of these at a time, I wanted a quick way to select them. This setup uses a 2 pole rotary switch to select one of up to 4 boards. This would
allow the boards to be in the temperature chamber and the selection and output jacks to be outside.
The Blog f or this project
Dave's Home Page
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