Electronics > Metrology

Lowest drift, lowest noise voltage reference (ADR1000AHZ)

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The resistors for the 6.6 to 7 V gain would still be the most critical one when it comes to drift. They enter with a 6% factor, or a bout a facto 17 attenuation. The resistors at the ADR have a larger attenuation, like 200 times and higher.
For the gain stage one may just get away with an LT5400-7 as a maybe better available resistor.

Az OPs can react to the input impedance and details in the supply decoupling. So there is a potenetial for them to show more than the datasheet noise. They are also a possible source for EMI so that other parts than give the extra voltage.

As a lower noise OP at the ADC I would consider the OPA202 as a possible candidate.


--- Quote from: Dr. Frank on December 06, 2021, 10:04:44 pm ---And no, the LTZ1000 does not necessarily need more stable resistors. The timely stability of the circuit is mainly determined by the chip itself, the T.C. drift can be trimmed to near zero. Exactly the same goes for the ADR. So, please, no Vodoo stuff here, again. The ADR has much better noise performance, definitely, and that's the main reason to go for it.

--- End quote ---

It is true that the LTZ1000 CAN be trimmed to ZTC, but without trimming the ADR1000 is much better in terms of TC. In one project exactly that was the reason to move to the ADR1000.

Yeah the lead time on these VPG resistors is horrible but ok, metrology is no hobby for the hasty ones, trying to improve my patience here :D
I am gathering test gear for this project since nearly two years now (orignally planned with the LTZ of course)...

Thanks for the tipp, didn't thought on that one...in my head I needed a external source (of heat) to do it, not that I don't have a 2510, but I wasn't ready to put the efford in.
So I will think about a proper setup to do it (waiting on some new PCBs anyways which one of them will be modified for that).

Yes I am aware of the trimming, however i would like to get the drift characteristics first (and see how the tempco behaves) and then tweek the refs. Indeed i was wondering about the low setpoint, but I think its worth a shot.

Regarding the exchange of U160 and U165: I assume (not checked) there supply is bipolar +-15V, so a LT1128 would work fine and it has very very low noise...considered it instead of the LT1013 but the bipolar power supply needed to get to (or near) 0V keeped my from implementing it. Didn`t seem necessery at the time, but after this discussion about the really low noise floor this is worth a rethink.



There should be no absolute need to get really zero TC for the unheated reference. If possible it would help to get the TC without the heater to below 20 ppm/K. Something like the 10 ppm/K range should be perfectly OK with a reasonable stable temperature. A slightly higher current for the zener and/or transistor could do the job.

The LT1128 is low noise for a very low impedance ( e.g. 100 Ohms) source, but not with some 10 K like in the 3458 reference amplification. For the ADR1000/LTZ1000 reference the OPs see some 50-70 K and thus even more impedance. The OP is also behind the gain of the transistor and thus less critical. So the LT1013 should be good enough there.

So if at all the LT1128 may have a small chance to work in the 6.6 to 7 V gain stage, but even there I would prefer a lower noise current OP. The chopper artifacts from a chopper OP could be an issue.
The ADC in the 3458 should react a little to reference noise in the 150 kHz range. The noise in this range may depend on the compensation (including OP speed) around the reference. The curve in the DS looks good for the ADR1000, though not too sure which OP is used and there may be limit of the test system.

Trying to get even lower noise from the ADC in the 3458 is a different topic. Only U160 would see the same frequency band as the reference. For U165 and U110 the chopper action of the AZ mode is effective and thus a relevant frequency of more like 2.5 Hz with an often relatively low BW and some extra gain (around 2.25 for the more critcal U110, should be 1.25 for U165).  For the actual use reference noise at lower frequencies than 0.1 Hz may be relevant. The 0.1 to 10 Hz range is more like a range still easy to measure. It would be mainly U160 to worry about for low frequency noise.


working on alternatives to the statistical divider with a LTC1043 cirquit.
The main amplification is done by a 3 to 2 (factor 1.5) charge pump.

The remaining part can be easily done with standard resistors.
Either up or in a additional buffer stage with a voltage divider down.

Temperature compensation is done with a diode cirquit.
The typical +220 ppb/K can be compensated with the following cirquit.
R8+R9 are meant to be a 50K trim pot where the T.C. can be trimmed from around -300 to +300 ppb/K.

Unfortunately there is a unwanted offset along with the T.C. trimming where I am looking for a solution.
Any ideas?

with best regards



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