Old fashioned zener 10V reference

[Conrad Hoffman]

So as not to OT the other device specific threads, here's a very conventional zener reference I built some years ago. It has stayed within a couple ppm and isn't terribly temperature sensitive. The trick is fine tuning the zener current for your particular zener, not just using the data sheet value. If the board were put in a simple oven it would be near immune. The capacitor needs to be tantalum for low leakage, or probably better yet, ceramic. I don't have data logging set  up, nor anything more than a 6.5 digit meter, but looking on the high gain scope, the noise level is similar to my 731A. Feedback values are based more on what I had, rather than perfectly optimized, but the performance suggests they're not far off.

You can also do the same trick with a 731A and similar, fine tuning the reference current for a much lower tempco.

[mmagin]

I like it.  Might build some of these with the new-old-stock 1N829A I've obtained and the 2DW233 I'll be getting from the group order.
Now I really need to build a little temperature-testing chamber.

I kind of wonder, are the better grades of 1N82x just ones which have a zero TC point that falls closer to the datasheet spec? 

[Conrad Hoffman]

Good question and I don't know the answer. It seems like any of them can be trimmed to zero TC. I do the whole board, so if some resistor tempcos get trimmed out in the process, all the better. It's the end result that matters. I posted this to show that very good results can be achieved with inexpensive and unsophisticated methods. The same circuit should be ideally suited to the other reference devices.

[enut11]

Hi Conrad
Thanks for sharing. What type/tolerance resistors did you use and what was the nominal zener voltage?
enut11

[Conrad Hoffman]

It's a 6.4 volt zener. Back when I bought them they were inexpensive, but today they start at about $9. The resistors are ordinary Mepco or similar RN55 1% parts. I probably selected  the feedback resistors for value, to get the trim centered correctly. Also, one changes the current setting resistor to the zener for zero TC, which shifts the voltage. Then the feedback to get back to 10 volts output. It may be necessary to go back and forth a couple times if you want perfection.

[RandallMcRee]

Conrad,
How do you determine where the zero TC operating point is? In particular, do you need an oven? Can it be done quickly?

Thanks in advance,
Randall

[zhtoor]

So as not to OT the other device specific threads, here's a very conventional zener reference I built some years ago. It has stayed within a couple ppm and isn't terribly temperature sensitive. The trick is fine tuning the zener current for your particular zener, not just using the data sheet value. If the board were put in a simple oven it would be near immune. The capacitor needs to be tantalum for low leakage, or probably better yet, ceramic. I don't have data logging set  up, nor anything more than a 6.5 digit meter, but looking on the high gain scope, the noise level is similar to my 731A. Feedback values are based more on what I had, rather than perfectly optimized, but the performance suggests they're not far off.

You can also do the same trick with a 731A and similar, fine tuning the reference current for a much lower tempco.

Hello Sir,

always a pleasure to read your articles on metrology.

1. can you tell me about the long-term drift characteristics of this design?
2. it seems that the while tuning the current through the temperature compensated zener, you would also change the voltage gain,
    how about decoupling the current through the zener and the voltage gain?
3. what would be the performance of a zener connected bjt in series with a diode connected bjt instead of the zener, the bjt can be selected for
    noise performance.

regards.

[Conrad Hoffman]

If one worries about all the possible sources of drift, using 1% metals and an IC socket should doom this circuit from the start. OTOH, my experience is once it settles down for a few weeks, it stays within a couple ppm for several years at a time. FWIW, my 731A uses metal films for the zener current and manganin wound resistors for the dividers (I think). The 731A has it's last official cal more than 10 years ago and in comparison with three other reference units, it hasn't moved more than 1-2 ppm. A lot of the drift specifications you see become much better after things have aged a few years. I'd rather have an ancient reference with a lot of history, than a brand new one with zero history!

My "oven" was nothing more than a cardboard box over the board with a lightbulb shining on it. Warm the board up a dozen degrees, cool it down (all slowly) while monitoring the voltage. Change current until you can cycle it with no significant change. If you temperature controlled the circuit, it probably wouldn't be capable of sub-ppm performance, at least not without better resistors. It's very good, but can't break the laws of physics. Or Ohm. IMO, sub-ppm performance is vastly harder to achieve and much more expensive as you need better meters and comparison standards. that's why we have threads on the LTZ parts! This circuit is a great way for somebody to get their feet wet with a discrete design, and for many it's all they might need.

As for decoupling TC and gain, you have to change the zener current to find the zero TC point, and that changes the zener voltage. The zener curve isn't a straight vertical line. Thus, you have to change the gain to get back to 10 volts, and that's just the nature of the thing. Adding more complexity than the minimum shown, isn't going to improve things. Certainly though, you can use this circuit with any sort of two terminal reference device.

[Andreas]

I kind of wonder, are the better grades of 1N82x just ones which have a zero TC point that falls closer to the datasheet spec?

Hello,

according to a post on volt-nuts board the zero T.C. current is usually from 4-11 mA for any 1N82x.
The 1N829(A) is only closer to the 7.5 mA test current.
And of course the A-types are selected for lower dynamic resistance. (which should influence noise).

With best regards

Andreas

[Mickle T.]

Here is a simple zener references I built some years ago.

[Kleinstein]

...
according to a post on volt-nuts board the zero T.C. current is usually from 4-11 mA for any 1N82x.
The 1N829(A) is only closer to the 7.5 mA test current.
And of course the A-types are selected for lower dynamic resistance. (which should influence noise).

With best regards

Andreas

A low dynamic resistance is also important to get a low sensitivity to resistor drift. I have not seen very much on the correlation with noise. They sometimes show noise as a factor relative to the thermal noise of the dynamic resistance, but I am not sure this is more than a way of plotting things. Especially with non buried zeners noise is more a question of defects causing 1/f noise and popcorn noise.

[enut11]

@Conrad Hoffman
Quote
"My "oven" was nothing more than a cardboard box over the board with a lightbulb shining on it. Warm the board up a dozen degrees, cool it down (all slowly) while monitoring the voltage. Change current until you can cycle it with no significant change. If you temperature controlled the circuit, it probably wouldn't be capable of sub-ppm performance, at least not without better resistors. It's very good, but can't break the laws of physics. Or Ohm. IMO, sub-ppm performance is vastly harder to achieve and much more expensive as you need better meters and comparison standards. that's why we have threads on the LTZ parts! This circuit is a great way for somebody to get their feet wet with a discrete design, and for many it's all they might need."
Quote

Conrad
Did you measure the temperature under the 'cardboard box'?
enut11

[Gyro]

One thing to remember wth 1N82x zeners (and all axial zeners) is the need for stress-free mounting. This is specified in the IN82x datasheets and also mentioned in the LM399 datasheet* (by way of comparison). Datron used to handle this by putting a large complete loop in each lead before soldering, basically using most of the supplied lead length.


Edit: * Actually, not the datasheet, the App note, NS AN161.

[Vtile]

Isn't that stress relief a thing in every single component - the PCBs included - when someone tries to archieve zero TC. I would be suprised if IC chips wouldn't suffer it at some degree also.

[Should be taken with grain of salt]1n821 I can't recall the datasheet, but on that were a one sentence that said that best voltage for buried zener is 6.5 to 6.8 volts (1N829 it were one of these zero drift voltage ref. IC datasheets and comparison to traditional zeners, if I did read between the lines they basicly say their IC is not better, but easier for production). [/Should be taken with grain of salt]

What I'm wondering atm. is that

A: How good TC one could archieve with the 1n821..1n827 series, by trimming the current.
B: How much of the noise of components are infact coused by the background radiation from ie. radon.

[Kleinstein]

With the correct current one can bring the linear TC to zero. What is left is mainly the second order TC. That is a parabola like dependence. So depending on the temperature range, one could get the TC to below something like 5 ppm/K over a 10 or 20 K range.
Compensating this second order effect is possible but difficult and may introduce extra drift. However having a stable temperature helps a lot against the square law error.

At the usually rathe high current, the noise due to radiation could not be a really big part of the noise. Radon is more a problem to very high impedance circuits with high voltage. 

[Conrad Hoffman]

I did have my darkroom thermometer stuck in a hole in the box, but this was so long ago I don't remember the exact temperatures.

Stress was a  huge issue for long term stability when I did my voltage reference for the MML articles, and I never had much success with the plastic packages in that regard. Three lead mounting of the device to the board was important, as well as floating the board from it's mounting wires. In this case, the fact that I wired it up on Vector board might be a positive. The leads underneath can easily flex for all the parts and don't put much stress on the bodies the way a PCB might. Stress relieving loops for critical parts sound like a good idea in that case.

One thought occurred to me about making up your own TC zeners from two diodes that I realized from working with a power zener attached to some other parts. Almost all the heat transfer is through the leads. If you solder two diodes together in fairly close proximity and maybe put a Styrofoam sleeve on them, they can't help but be at almost the same temperature. Conduction through metal is way more important than any other thermal path.

Mickle- "simple"? Really nice job!

BTW, I've always thought a perfect case for these references would be to put it in an old Eppley or Weston standard cell case, powered from a wall wart plug on the back.

[Conrad Hoffman]

Nice and similar to the circuit above. I used parallel resistors entirely to get the values I needed, not for any special stability or tempco reason. It was easier to get the trim within range by padding. I stock a lot of values, but not every one ever made! I assume your circuit values were chosen for similar reasons?

[mmagin]

Perhaps this deserves its own thread (maybe once I have a chance to try the idea):
I'm wondering about replacing the trimmer in a circuit like this with a AD5270 digital pot (and some kind of little PIC circuit to generate the SPI adjustments from up/down/save buttons or something).  The specs look quite good.

[Kleinstein]

Digital pots are often limited to relatively low voltage like 5 V. So not many types are actually very useful for adjusting the voltage. Usually the rheostat chips are less suitable, as the include the switch in the active path and thus tend to have a higher TC. The true pots used as a voltage divider can make use of good resistor matching. However they still have the bond wires at the ends.

[Conrad Hoffman]

You can do so much with a PIC these days! I'd think more in terms of injecting a voltage to create an offset, such that the full range of the control was only 5 ppm or so. That way even if the control circuit drifts, it doesn't have much effect. Push it into the negative input via a very big resistor or add a couple ohms on the ground leg of the feedback and again, drive it with a big resistor.

[nctnico]

Interesting circuit! 
Just one remark about a low leakage capacitor: a polyester (PET) capacitor will have an even lower leakage than ceramic X7R (which is somewhere in the several nA range at a few Volt).

[Vtile]

So as not to OT the other device specific threads, here's a very conventional zener reference I built some years ago. It has stayed within a couple ppm and isn't terribly temperature sensitive. The trick is fine tuning the zener current for your particular zener, not just using the data sheet value. If the board were put in a simple oven it would be near immune. The capacitor needs to be tantalum for low leakage, or probably better yet, ceramic. I don't have data logging set  up, nor anything more than a 6.5 digit meter, but looking on the high gain scope, the noise level is similar to my 731A. Feedback values are based more on what I had, rather than perfectly optimized, but the performance suggests they're not far off.

You can also do the same trick with a 731A and similar, fine tuning the reference current for a much lower tempco.

I'm not particularly good at op-amp circuits and analysis of them and my math seems to be off **. I wonder what is the purpose of the 8.3k resistor and tantalum on the non-inverting leg. Is it just a regular RC filter on the input?

I noticed somewhat similar circuit on the OP07 datasheet, but there were a pull up resistor from the zener cathode node to the positive supply voltage. I wonder what is the purpose, my educated guess is that it provides a run up voltage for the reference, resulting faster (but worse? *) stabilization of the system.

**The typical current for 1N4568 seems to be only 500uA so that part were actually correct. What seems to be an series RC-filter is still question mark for me.
* Edit. Added a question mark, to indicate that it is also a wild guess.

[Kleinstein]

The 8.3 K and the capacitor are just an RC filter. Zeners tend to be noisy and filtering helps a little - at this position filtering comes at low extra cost.

Depending on the supply for the OP, they might need a kind of extra power to make the circuit start up well. With just a single supply, the OP07 should be OK without an extra resistor. If at all a resistor to the OPs output might help a little.

[guenthert]

Here is a simple zener references I built some years ago.

  I was browsing the 'tubes for voltage references based on the 1N829 temp. compensated Zener as the prize for the LTZ1000 reaches stratospheric regions and couldn't help but noticing that the first one looks like a well executed  re-implementation of the circuit of the Solartron 7081.   

Can you share how it turned out, i.e. have you noise and drift figures?


(those interested in the original circuit google for SolartronReadme.txt, SolartronRef-1.PDF, and SolartronRef-2.PDF.)

[Tj138waterboy]

Have to resurrect old post to ask if one wanted to make a good current source to tempco test a 1n829, which circuit  construction would be more temperature stable? Opamp current source, wildar current source 2 or 3 transistor version or wilson current mirror 3 or 4 transistor version, or would basic single transistor current source be just as good? Will be powered by stable dc power supply hp6111a @ 12v. Will be cycling temperature of only 1n829a zener attached by wirewraped leads inside insulated cookie tin.  Current control circuit will be built on solderless breadboard or project board in room that temperature varies between 50-65degrees.