LM399 Reference: Precision 10V voltage source.
The Schematics, PCB files, and BOM are here

lm399

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:
 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:
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 1ppm 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. Only a few common values and ratios are available off-the-shelf.

The next problem is how to correct the 3% error. Variable resistors (trimpots) are notoriously not stable for tempco or for mechanical stability. Using a single trimpot to pull the output the full 3% will introduce the trimpot's temp drift.

Design

The answer to these design problems is to use three resistor networks in parallel:
 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

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 as important. What is important is its ratio change (tempco), which should be better than the resistance tempco. Unfortunately trimpots do not specify a ratio match, so this is an assumption. I or someone should verify this. Someday.

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.

proto

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.


rn version


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.

fixture

To be continued...

Having a simple +10V reference is not very exciting. I decided to update my 18b DAC Project to use a more stable LM399. Its improved LM399 design is powered from an isolated USB +5V power source. Check it out.

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Last Updated: 9/20/2024