Hello,
Reading many (many) posts on this forum, I got interested in making a voltage reference for my own (hobbyist) use.
As always, I try to do something new, and the fun is in learning from it.
The purpose of this post is to describe my thoughts and ideas, and even show an initial schematic,
in order to receive your feedback regarding improvements and corrections. (I am sure this initial design is _not_ perfect!
)
I would like to make a voltage reference that is stable, with a fair amount of accuracy, say, 4 to 5 digits.
The challenge is to reach that goal on a relatively small budget.
I do happen to have an LM399H reference, which I bought some time ago, expecting it would come in handy someday.
So this reference is a natural point of departure. One of the messages in the posts on this forum is that
for a decent reference, the required precision resistors easily outweigh the reference in terms of cost.
So in he background (I do have a job!
) , I started to think about how to achieve stable and accurate voltage division.
The other day I got the idea to use a multiplying DAC for that purpose. When I checked the specs,
the internal resistor chains have accuracy and tempco numbers ranging in the low ppms, so that fits my target nicely.
The resolution would also have to be in that general range, so I started looking around for a 16-bit multiplying DAC.
Comparing specs and prices here and there, in the end I converged on the AD7849 (2 ppm FSR/C, 6 LSB accuracy @ 25C).
Over the entire temperature range, the numbers are worse (but still quite decent!), but I do not intend to use the reference in my freezer.
Well, one thing led to another, and the result at this point is an initial schematic design in Kicad.
The design is a bootstrap configuration: the reference is biased based on its own output (I like that!).
The DAC makes a nice round 10 V from the irregular zener voltage, and a few opamps are used to transport the voltages back and forth.
No discrete precision resistors, no trimpots!
The part I did not put in the design is the microcontroller needed to load the appropriate value in the DAC.
Just about any micro with four GPIO pins available can do that job. I plan to use an Arduino Uno to get going,
and perhaps later once the design has settled, I will switch o a smaller device.
Of course, the trick is to load the 16-bit value that makes the output as close as possible to 10 V,
and finding that value is the equivalent to trimming the pots. For that, I will need access to a good reference!
I happen to have a nice enclosure of what used to be an OCXO, about 5x5x2 cm, with 2 ground pins (connected to the case)
and 4 signal pins. I intend to use it to contain the part in the dashed area in the schematic.
Last but not least, I do have some questions at this point, and I would highly appreciate your responses to them:
-The bootstrap loop to bias the zener has quite some delay in it: two opamps and the DAC itself. Does it need filtering for stability?
I would guess not, as I expect the gain to be less than 1, due to the zener. Is that correct?
-There is still one signal pin left unused on the enclosure. What to do with it?
For example, it could be used for a temp sensing diode, or it could be used to make the 3k resistor external
(that would lower the power dissipation in the box)
-Speaking of which, the 3k resistor is used only to offload the biasing opamp, as it supplies the bulk of the current
needed to regulate the zener. I would expect the circuit to work (almost) as well without it. Is it a good idea to leave it in?
-Are the LT1013 opamps ok to use, or should I use some other type? (I know, this is a loaded question...
)
My preference is to use one dual type in an 8-pin package, but I will use two single types if recommended.
-Is the 79L15 good enough to feed the negative supply for the DAC, or should I upgrade to something better?
(two cascaded LM337 perhaps?)
-Is the LT3042 overkill? It does supply +15V for the DAC and indirecly supplies the LM399H, so I will use it if it makes a difference.
-Would placing the LM399H in a shielded enclosure make a difference, temperature-wise or EMC-wise?
And of course any and all other comments are very welcome too!
Evert-Jan