LM399 based 2.5V reference - Page 1
Forum section: electronics / general

author: MarkKruetzmann
author_rank: Contributor
timestamp: April 04, 2019, 01:06:11 pm
content: Hello Forum!While being a reader for quite some time now, I decided to join in here and post my clever or not so clever ideas and thoughts. While thinking about building my own 6.5 digit multimeter (and quickly realizing I won't top my 3456A anytime soon) I got some ideas for that. I state upfront, however, that I will not attempt to actually build such a thing in the near future. It just amuses me to think about the problems and maybe help others and you guys are probably on this forum for the very same reasons.In many attempts on building a DVM people are using off-the-shelf chips instead of running their own ADCs because that is much, much easier. They have quite good reasons in doing so because these ADCs claim to provide enough precision for use in a 6-digit-multimeter (LM2400 states that in the datasheet, the AD7177-2 even claims 24 noise-free bits and 1ppm INL typical, 3.5ppm max). However, these chips require a stable 2.5V reference. Most, if not all, 2.5V-references do have quite some temperature dependance and higher drift than the ever popular LM399 or even LTZ1000. Often a simple resistive voltage divider is then used to get to the needed 2.5V, destroying all that nice stability over time and temperature. That is why I came up with something else:A LM399-based 2.5V-reference (2.55V +-2%, but it should be quite usable and better than most approaches)Starting with a modified "portable calibrator" from the datasheet of the LM399. Instead of the resistors used, one should use a resistor network consisting of a resistor ratio of 1:10. Most extreme would probably be the Vishay Z-Foil networks, but let's just settle with a LT5400 with dual 10k/100k resistors. One could also use the 1k/9k version of that to probably achieve lower (Johnson) noise and use a 1k in parallel with a 9k or something. These devices have a 0.2ppm/K matching temperature coefficient. In a configuration, where the voltage of the LM399 is boosted by 10%, the impact of these resistors would be only 0.02ppm/K. That is a decade lower than the typical LM399 spec. The offset drift of a LT1001 with 1µV/K adds another 0.13ppm/K to that. So with a LM399, you probably get away with worse (cheaper) resistors. This now 7.65V is used to generate the current through the buried Zener via a resistor of, say, 680 Ohms, and to feed a divide-by-three voltage divider built around a LT1043. These devices do have single-digit ppm offsets (datasheet says +-3ppm for the division by three) but if I recall correctly, "Andreas" made some measurements and they have practically no drift.So, this is my idea of a precision 2.5V(-ish) voltage source with very low drift over time and temperature. I do have some questions though:1: The LM399 datasheet lists conditions between 0.5 and 10mA of Zener current. I would like to use the LT5400 in the double 1k/9k version as to reduce noise though and use a feedback path of 1k|9k from output to inverting input and 9k from there to ground. The remaining 1k I would use for a stable current for the buried Zener, however, in this configuration that current would be only about 0.7mA. I am not quite comfortable with that, but is it enough?2: The op-amp (LT1001 in this example, there may be superior ones) states an offset of 60µV maximum. If this is stable over time, I do not really mind, but they do not specify that. So: Does that offset voltage drift over time or does it stay stable where it is, and if yes, can I assume that for all op-amps?3: Am I right that the LT1043 divider does not drift over time and temperature (or just very, very little?) I do not mind the uncertainty of 3ppm, if it just stays stable.4: What caps to use with LTC1043?If my assumptions are correct, I would have a 2.5V reference with just 0.15ppm/K added temperature dependence to the 0.3ppm typical/1ppm max of the LM399. I would need a buffer though to feed the voltage to the ADC, so with the LT1001 for example this would add another 1µV/K or 0.4ppm/K. A better op amp would be helpful here.5: Are there better op-amps usable for this circuit? A dual precision op amp would be quite nice.

author: ycui7
author_rank: Contributor
timestamp: April 04, 2019, 01:15:16 pm
content: I used LT5400 (1K: 5K) and a dual 2x2K vishay SMD pack to drive the LM399. The LT5400 set the zener current to 1mA (1+1+5K). The drive amplifier is OPA2277. I was able to get 0.5ppm/K tempco (tested by ADCMT 7480T, did not bother to get better tempco, because 0.5ppm/K was good enough for me).This give me 5V reference. I used another LT5400 to divide the 5V to 2.5V.Since I need to use my reference to drive SAR ADC, I added LT3042 to buffer the reference with OPA2277 to servo the LDO.The overall noise level of the reference is around single digit uV level.I also run a few months long stability test with 34970A, the drift is less than 10 ppm for the first few months, most variation was actually due to seasonal room temperature change. I get roughly 1 ppm variation within day due to room temperature change.I do get a few 5 ppm spike every few days, I suspect they are due to EMI, but no proof.Beyond reference, I used AD5781 (the 18-bit version of AD5791) for analog output and LTC2378-16 for analog input. I could not measure any drift between AD5781 and LTC2378-16 up to the sixth digit. The analog readback from LTC2378-16 was digitally averaged to get more resolution, the readback was stable on the sixth digital over days of measurement.The output of AD5781 was also monitored by my 7480T for days, it showed similar drift to LM399, less than 1ppm/K tempco. So, I would say LTC2378-16 can perform better than 10 ppm with digital averaging.I would expect the 20-bit version AD5791 and LTC2378-20 perform at single digit ppm level stability.

author: iMo
author_rank: Super Contributor
timestamp: April 04, 2019, 05:33:44 pm
content: In case you are LTspice positive you may play with it..The precise 2.5V tempco depends on the LM329 (it is not the 399), LT1043 and the LT1001 opamp models used (doublecheck).The LT5400 resistors are within 0.2ppm matching as an example.The simulation sweeps through 5 temperatures.PS: Added an "ideal LM399" with OPA177 as an example.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 04, 2019, 06:49:52 pm
content: There is no need to use super low TC resistors with the current source for the LM399. So no need for the LT5400 here. "normal" 25 ppm/K resistors should be good enough, as the effective impedance of the LM399 is very low (e.g. < 1 Ohms). So at 1 mA and even a 100 ppm/K TC for the current would only give some 100 nV/K. Similar the OP is usually not that critical - so an OP07 should be good enough here.The LT1043 should use physical relatively small film caps to avoid extra parasitic capacitance. This usually are polyester (e.g. MKS).There could be quite some spikes in the supply current to the LT1043 - it needs good decoupling and some care.If one needs exactly 2.5 V and not 7 V / 3 one could use the amplifier at the LM399 to go from 7 V to 7.5 V first. In this case slightly better resistors may be needed.The OP to buffer the ADC reference may have to be relatively fast to cope with fast current spikes. It may also need extra circuitry, e.g. extra cap + resistors to make the OP happy with the load.The OPs offset is usually relatively stable over time, especially with BJT based OPs. Precision OPs may specify the drift over time: e.g. the OP177 gives 0.4 µV/month as a typical drift limit.

author: Andreas
author_rank: Super Contributor
timestamp: April 04, 2019, 07:02:21 pm
content: Hello,1. 0.7 mA will be enough although the ageing drift is specced only for 1.000 mA2. the LT1001 is specced with 1uV/K temperature drift against a LTC2057 with below 0.05uV/K.3. The LTC1043 drift that I measure is only the drift of the buffer OP-Amp(which is necessary and which should have a low input bias current)the 3 ppm are a phantasy number. The absolute Error is higher. On a 2:1 divider I typicallymeasure a 25-35 uV too low value with 5V input which is stable over temperature and time.4. I use Mylar (MKS02 to fit into a D-Sub connector housing) although in theory polypropylene should be better.with best regardsAndreas

author: MarkKruetzmann
author_rank: Contributor
timestamp: April 04, 2019, 09:53:44 pm
content: So, to keep things a bit more reasonable, I could definitely go with some cheaper resistors. The overall concept seems to be quite okay. A reasonably priced precision dual op amp woul be nice, single ones for more than 10 bucks a pop would be suitable for something with a LTZ1000 and then possibly a 7-digit-multimeter.I will still work out a schematic for this as it might be useful to others to have a good 2.5V-Source for their projects. And I do learn quite a bit myself designing these things.

author: splin
author_rank: Frequent Contributor
timestamp: April 05, 2019, 12:04:50 am
content: Quote from: MarkKruetzmann on April 04, 2019, 01:06:11 pmNo - both of the above can use an external reference voltage up to the supply voltage which can be up to 5.5V. This is true for many if not most high performance ADCs - those with internal references tend to have 2.5V references (like the AD7177-2) but can use higher voltages.In general you want to use the highest reference voltage possible to achieve the best noise performance, but note that some ADCs, like the AD7768-4 and AD7768-8 show increased INL for Vref > 4.096V in fig 48 in datasheet (but not in the newer single channel version AD7768-1 which shows similar INL for all reference voltages - which may be the result of silicon differences or just random chance when characterising small samples). Many ADC datasheets include several graphs showing various characteristics plotted against Vref, including noise, so check them out when selecting the optimum reference voltage.If you are using a resistive divider, the smaller the reduction the better from a drift perspective so 7.65V to 5.5V would be much better than 2.5V. If you want to use the LT1043 then dividing by 2 would be a better option.Better still might be to use decent, but relatively cheap, resistors to produce a 5.5V nominal reference for the ADC for best noise performance and periodically calibrate itself by measuring an LM399/LT1043 generated 3.825V calibration reference. Use the high quality 5.5V reference to regulate the ADC supply voltage (with a jellybean opamp) to keep it as close to the maximum as possible. You don't need a high performance opamp to buffer Vref in this case and you will presumably already have a low input bias amplifier before your ADC driver which won't load the LT1043 when reading the calibration. This has the additional advantage of calibrating the gain of most of the signal chain up to the ADC including the input amplifier and ADC driver.

author: iMo
author_rank: Super Contributor
timestamp: April 05, 2019, 05:29:50 am
content: Quote from: MarkKruetzmann on April 04, 2019, 09:53:44 pmThe resistor's tempco of any divider you may use there is critical. The only not critical resistor is the one feeding/biasing the LM399. A cheapo OP07 with 1uV/C is much less critical than say a cheapo 15ppm/C resistor's based divider with 5ppm/C mismatch.