What kind of reference is this?? - Page 1
Forum section: electronics / general

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 16, 2017, 08:05:13 pm
content: Hi,today I received an old data precision 8200 calibrator in quite bad condition. Everthing was flying around in the housing. Therefore, I opened the box before switching it on. There were some modifications made.Most interesting one is the aluminium box. This box was connected to the place, where the old reference diode was. Inside this box there are 4 "things" with 4 pins. I can't see any labeling on these 4 parts.Does anyone know what kind of reference this is? And where does it come from?ThanksPhilipp

author: Vgkid
author_rank: Super Contributor
timestamp: February 16, 2017, 08:15:56 pm
content: Possibly a naked lm399. I have an ancient zener GE 4-pin voltage reference floating around somewhere, I will see if I can find it tonight.

author: Andreas
author_rank: Super Contributor
timestamp: February 16, 2017, 08:16:45 pm
content: Hello,Could be naked LM399s like here:http://www.gellerlabs.com/MGTA%20kits.htmWith best regardsAndreas

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 16, 2017, 08:20:34 pm
content: I opened one of my LM399 and the size and look will fit. Thanks!The box, with the PCB and the foam doesn't look like selfmade. Any ideas where it can come from?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 16, 2017, 09:47:36 pm
content: Don't know, but I have an 8200 and it's one of the more useful things on my bench. Well worth putting into good shape. Mine uses a garden variety LM399, but it stays within a couple ppm for years at a time. The rotary switches on mine are a nightmare to keep working. DeOxit hasn't helped much. My next attempt will be total immersion of the switch board in the ultrasonic cleaner. I'm assuming it's the switches, not the following circuitry, but scoping the switches hasn't convinced me they're as bad as they seem.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 16, 2017, 09:53:54 pm
content: I had only a quick test until now. 10V and 1V Range seems to work. The 100mV Range has some problems. 100mA isn't tested yet. One digit is totally off.I think this should'nt be a very big problem to get this thing up and running.About stability: I would prefer an original shape of the unit. Therefore, I will install a new LM399 in the right place.The 20bit DAC is really interesting. I've never thougt that this will work for 6.5 digits

author: lowimpedance
author_rank: Super Contributor
timestamp: February 16, 2017, 10:41:39 pm
content: So the PCB looks like some commercial product, even has a QA stamp !?. Any way if it is indeed 4 X lm399 (averaging ?) maybe it would be worth evaluating it first to see if it is actually going to be better than a single new LM399 especially since they would be nicely aged and also nicely housed.

author: Vgkid
author_rank: Super Contributor
timestamp: February 17, 2017, 12:09:32 am
content: Quote from: lowimpedance on February 16, 2017, 10:41:39 pmI only see 1 LM399 sized component(to-5)I do see 4 to-18 transistors, possibly for the zener current reference.

author: lowimpedance
author_rank: Super Contributor
timestamp: February 17, 2017, 01:47:56 am
content: It could be just the one, not having seen one without its coat on.I based previous my comment from viewing the Geller link posted and the rather similar looking lm399 part there without a casing to the four shown in the picture.Perhaps a closer picture of each part might shed more light. bit hard to tell anything from the one posted. (particularly with my aging eyeballs)

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 17, 2017, 06:38:50 am
content: As already mentioned the size of these 4 "elements" will perfectly fit to a nacked LM399.I will try to make better pictures this evening and I will power it up without this case. If it is really a LM399 it should run quite hot. I will also have a look at the traces and measure some voltages.

author: eurofox
author_rank: Unknown
timestamp: September 11, 2017, 08:25:59 am
content: Quote from: e61_phil on February 17, 2017, 06:38:50 amIt is a LM299.

author: e61_phil
author_rank: Frequent Contributor
timestamp: September 11, 2017, 08:39:28 am
content: Quote from: eurofoxNo

author: branadic
author_rank: Super Contributor
timestamp: September 11, 2017, 10:26:24 am
content: QuoteI'm missing the better pictures and the promissed voltages-branadic-

author: e61_phil
author_rank: Frequent Contributor
timestamp: September 11, 2017, 12:21:38 pm
content: Quote from: branadic on September 11, 2017, 10:26:24 amSorry, I already replaced this DIY reference with a LM399 and brought everything to the original state.Voltage was in the range of the LM399. Nevertheless, I had to change the resistors (R9 & R10), because the new reference brought it out of the trimming range.

author: eurofox
author_rank: Unknown
timestamp: September 11, 2017, 01:11:38 pm
content: Quote from: e61_phil on September 11, 2017, 12:21:38 pmI bought the same instrument with some problems, I have the same problem with the resistors 9 and 10, I cannot get the voltage in range with R12 and the reference is the originalLM299that is quite similar to the LM399.I have problems as well with this par that I cannot get into range, I suspect the operation amplifier, I order new parts before continuing my investigation.

author: e61_phil
author_rank: Frequent Contributor
timestamp: September 11, 2017, 02:01:34 pm
content: Quote from: eurofox on September 11, 2017, 01:11:38 pmIn my unit somebody replaced the original reference by an DIY construct. Therefore, I said no to the LM299. (Was a bit short, sorry)I calculated R9 and R10 (which are in series) in respect to the measured 10V output.If I can help you with any measurement in my working 8200, please contact me.

author: branadic
author_rank: Super Contributor
timestamp: September 11, 2017, 05:32:36 pm
content: QuoteWhy? 4x LM399 should result in half the noise of a single LM399, shoulnd't it? Or did you carefully select a LM399 for good stability and smallest noise?At the end a LM299 and a LM399 are nothing but a LM199 with downmarket specifications aka selection.-branadic-

author: Vtile
author_rank: Super Contributor
timestamp: September 11, 2017, 05:58:10 pm
content: Would be interesting to get the circuit drawing of the supposed DIY reference. For my eyes it looks anything else than DIY.Could it be 4 zeners (of some type) and OP(2)07 (in hermetic TO-99) circuit?

author: MK
author_rank: Regular Contributor
timestamp: September 11, 2017, 06:25:19 pm
content: It looks to me like it is 4 of the same reference in a set, LM199, LM299, and LM399 are all the same basic die from the same wafers, just selected for a different spec. If they do not have any popcorn noise they will all settle down to LM199 stability when old enough.

author: e61_phil
author_rank: Frequent Contributor
timestamp: September 11, 2017, 08:28:27 pm
content: Quote from: branadic on September 11, 2017, 05:32:36 pmI want to have it in an original state. And look onto my first picture. There was a very nasty perfboard (?). The board was already cooked under the rectifiers. Nothing, I would like to run over night in my house with my family.Quote from: Vtile on September 11, 2017, 05:58:10 pmWith DIY I meant the whole setup. The reference module itself might be taken from anywhere.

author: branadic
author_rank: Super Contributor
timestamp: September 12, 2017, 06:16:19 pm
content: Hopefully you have used the board to set up a Quad-LM399-Standalone-Voltage-Reference?-branadic-

author: branadic
author_rank: Super Contributor
timestamp: October 06, 2017, 10:23:10 pm
content: I received this unkown reference by Philipp yesterday and had the time to analyse it. If it's a 4x LM399 this is how the circuit looks like.The +15V and ground are given, as the board is supplied via some prototyp board with a MC7815CT on it.-branadic-

author: Tektron
author_rank: Contributor
timestamp: October 07, 2017, 03:31:16 am
content: Quote from: branadic on October 06, 2017, 10:23:10 pmJust opinion, I can be wrong.If npn type of transistor is correct, then with +15V at the emitter it will not work. Perhaps collector and emitter must be exchanged?And connect common point of R(2.7k) and emitter to 4xR(160 Ohm)?

author: floobydust
author_rank: Super Contributor
timestamp: October 07, 2017, 03:54:02 am
content: In OP's pic, the tabs are rotated 90 degrees, like a two-on-two arrangement for the mystery cans. To me it does not look like a foursome as the pins are not parallel connected?

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2017, 09:03:40 am
content: QuoteThe BC182B in the circuit is made by ITT, but I wasn't able to find a datasheet from ITT so I took the datasheet from OnSemiconductor. Maybe ITT used a different pinout for the transistor? EBC instead of CBE?The rest is just as shown as I draw the layout for veryfication.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2017, 09:09:25 am
content: The circuit shown does not make sense this way. My guess would be transistor collector and emitter swapped and the connector not going to the other string of resistors (810 Ohms) instead. This would make it the more or less standard circuit for a parallel connection with external force and sense connections on both sides.The 810 Ohms are for delivering a supply current to the LM399 and the 190 Ohms (a little on the low side) are for averaging the voltages.

author: 2N3055
author_rank: Super Contributor
timestamp: October 07, 2017, 10:03:29 am
content: I'm confused..Where is the zener current coming from ?Regards,Sinisa

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2017, 10:42:36 am
content: Updated the circuit diagram and replaced the values measured by the color code. Also changed pinout of npn transistor to EBC.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2017, 11:04:55 am
content: The updates circuit still does not make sense. The external connector could need to be at a different position, more like where the other resistors meet, to make the OP a non inverting amplifier with the external (force - sense connection).Also the new resistor values don't make that much sense: With only 212 Ohms (instead of 10 K before), the voltage would be in the 7.2 V range and thus too little current for the refs. if the 2.7 K are used to deliver the current.With 100 nF at the output, the circuit might need an extra direct capacitive feedback to prevent it from oscillating.

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2017, 11:53:28 am
content: As said I retraced the layout to make sure that my circuit equals the board. So the connections are given as drawn. I will power some +15V to the board and probe around tonight, maybe we are getting some more information?I'm currently not sure what to do with the two connections designated "yellow" and "red".-branadic-

author: 2N3055
author_rank: Super Contributor
timestamp: October 07, 2017, 12:02:51 pm
content: Transistor emitter is connected back to 15V+ via resistor... What is current flow here..Output will be at 15V constant.. There is nothing to pull current to make voltage drop on resistor.There is no feedback path from inputs of opamp to output.Zeners are connected between opamp inputs, with no current source apart from opamp input current bias.You need 1mA per zener to function properly.There is no current path to return, they are floating between inputs..If you at first simply remove all parts of circuit connected to heaters, it will be clearer what I mean.Regards.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2017, 01:47:35 pm
content: Is it possible to have an error in the drawing below the OP ?With Red connection towards pin 2 instead of pin 3 the circuit would make much more sense:The external connections would than be the NC and "red" connection to wards the positive output as a kind of drive and sense. The negative side would be GND and yellow.The only other point in this case would be the value for R11, that should be more like 10 K like in the first drawing.

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2017, 05:58:20 pm
content: I checked the circuit again except the resistor values. Couldn't find a mistake-branadic-

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2017, 07:01:20 pm
content: On the picture Philipp posted one can see, that the red cable is connected to pin 1 of the original populated LM399, the yellow cable to pin 2 and GND to pin 4, while pin 3 of LM399 was left open. Thus the reference voltage is available via red and yellow cable.The reference is additionally connected to another part of the former circuit via the pin left over. Maybe some additional voltage?-branadic-

author: branadic
author_rank: Super Contributor
timestamp: October 07, 2017, 10:53:24 pm
content: Update of the schematic. I read R11 color code in the wrong direction, so yes it is 11k. I also designated the external connections in to the schematic.Zener current comes from an external resistor via red cabel (analysed by the pictures Philipp made) to the circuit.What is still unknown is connection B as I can't see were it was connected to.BTW: This reference was intended to be a replacement for original LM299. External resistor for the zener current is a combination of 1k + 220R + 3.01k connected to +15V as can be found in Data Precision 8200 manual. For 4x LMx99 the resulting current seems to be somewhat low and should be something around 1.875k.http://www.ko4bb.com/getsimple/index.php?id=download&file=06_Misc_Test_Equipment/Data_Precision/DataPrecision_8200_6.5_Digit_Calibrator_Service_Manual.pdf-branadic-

author: zhtoor
author_rank: Frequent Contributor
timestamp: October 07, 2017, 10:59:37 pm
content: helloservo-ing the heater to maintain instantaneous-average ref-voltage against averaged ref-voltage time constant determined by R11, R12 and C6?maybe kleinstein can elucidate?regards.

author: branadic
author_rank: Super Contributor
timestamp: October 08, 2017, 11:22:44 am
content: Taking into account that this circuit was a direct replacement for a LM299 and that there is external circuitry around LM299 this is what the schematics looks like. Still don't know what connection B is for and where to connect it to.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: October 08, 2017, 05:38:41 pm
content: Meanwhile I put a R14=1.8k resistor from point E (+15V) to point D to allow about 4mA (1mA for each zener), connected point A to C (commmon ground) as by the external connection in the Data Prescision 8200 and powered everything up.I can measure a voltage of 7.24379V at point D and 10.44496V at point B. The circuit draws about 400mA on startup which reduces down to 34mA after some stabalization time (510mW) with the circuit in its styrofoam package inside the aluminium case.The naked TO-46 cans are getting quite hot, to hot to touch. But 90°C without its thermal shielding of LM399 is nothing you want to touch right?So yes, it's working right know.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: October 08, 2017, 07:11:20 pm
content: The 10.44 V to 7.24 V voltage ration suggest that the amplification is set by the 11 K and 24.9 K resistors just like in the normal circuit. This somewhat suggests the circuit is just like the standard circuit, with the normal operation with the current supplied not from an 1.8 K resistor to the +15 V but a smaller resistor (could be essentially a short) to the 10.44 V output - thus connecting B and D as a kind of force and sense to the output.

author: branadic
author_rank: Super Contributor
timestamp: October 08, 2017, 07:29:46 pm
content: So if I get you right I should tie B and D together and see what happens? Could make sense as this board presumably was designed for another application where this kelvin connection was necessary.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: October 08, 2017, 08:36:50 pm
content: To prevent excessive current, I would test the connection B to D first with a resistor in series. Maybe in steps like 1 K - 500 Ohms - 200 Ohms - 0.

author: branadic
author_rank: Super Contributor
timestamp: October 09, 2017, 06:37:46 pm
content: Tried that today using a decade resistor box and measured zener voltage as well as amplified voltage:ResistorZener voltageConnection B1.5k7.13109V10.28271V1k7.18656V10.36264V8207.22024V10.41127V5607.31231V10.54395V3307.53338V10.86274VSeems that this is the wrong approach. If this were real Kelvin connections I would expect necessary resistors to be onboard and no additional external components needed. For comparison with 1.8k @ +15V the zener voltage measures 7.24393V and 10.44509V at connection B.Any suggestions?-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: October 09, 2017, 07:11:41 pm
content: The measurements seem to match the circuit diagram, which would be odd, as this would mean that only one of LM399 is actually used.To change to the more normal circuit would be by not using the connection D, but connect a separate cable to the input side of the divider towards "B" (either on the board or as an external sense). This should be the via between the divider and the LM399s.The D connection would than be the average LM399 output.

author: branadic
author_rank: Super Contributor
timestamp: October 09, 2017, 07:40:09 pm
content: @ KleinsteinThanks. So you suggest a modification like this with B and D as Force + Sense (High) and A and B as Force + Sense (Low)?-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: October 10, 2017, 10:20:26 am
content: That modification was my first ideal. It should work to give a stable reference, but the force/sense function is rather limited as the there would a relatively high current (e.g. 3-5 mA range) through the sensing connections.One one might as well have the connections B to D' and A to C one the board.Anyway the 7 V to 10 V step does not look like being very stable (those 11 ad 24.9 K resistors). So one might as well have the connection B to D' (the modified one) and A to C on the board and read the 7.x V reference form A and D (old position). B would be only an approximate 10 V, not very stable.

author: branadic
author_rank: Super Contributor
timestamp: October 10, 2017, 07:11:03 pm
content: I will think about that modification. Maybe I leave it as it is, which means with an external 1.8k resistor for the zener current.However, what I did meanwhile was to replace the broken feedthroughs by new ones Type KMD 508 I received a few weeks ago. These little ceramic parts perfectly fit the place of the original ones. They are coming without a wire installed. All you need to do is to solder some 1.5mm² copper wire or bigger from both sides into it.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: October 10, 2017, 07:44:43 pm
content: With the external 1.8 K resistor, the output voltage is not only due to the zener refs, but there is also a contribution from the 200 Ohms resistor. So this circuit would be a very poor reference: like 100% LM399 + 2.5% of the supply (15 V) multiplied with a not very stable factor.One can see the effect from the series with the different resistors towards the output. The LM399 alone is very insensitive to a change in current.To make it a useful reference would be more like the extra links (B to D') and A to C and than a 7.xx V output from D and A. There will be an approximate 10 V output at B, but not very stable. With better resistors the 10 V output might be useful.

author: branadic
author_rank: Super Contributor
timestamp: October 10, 2017, 08:26:16 pm
content: So what you suggest are some bodge wires running as shown below. Nothing fancy to do and pretty short in length.Tryed that and here are the short results: D is reading 7.02631V while B is reading 10.13142V (A=1.4419 which equals 1+11k/24.9k quite well). After some warmup time the circuit still draws 34mA, so nothing damagedReplacing the gain setting resistors could be difficult as only 0207/10 like Vishay PTF56 do fit.-branadic-

author: AG7CK
author_rank: Regular Contributor
timestamp: October 11, 2017, 02:01:45 am
content: IMO the reference circuit probably is functionally similar to this circuit from the LM399 datasheet:Referring to:point B should be connected to the left side of R11 providing negative feedback for the OP07/BC182 buffered amplifier thereby delivering fixed current to the zeners via the 4x 2k7.R14 is a high value start-up bias resistor similar to the 200k in the Portable Calibrator.Point D is sense Vref. Vref is the junction of the 4x 200R.Point A is sense Lo. Point A is furthermore connected via a (zener current) return wire to a ground point for the +15V supply.The inverting amp OP07/BC182/R11/R12 is not intended for a 10.xx volt output - it is a precision voltage regulator for obtaining approximately constant current through the 4x 2k7/zener configuration independent of the stability or load conditions of +15V. [The 4x 2k7 play the role of the 5k bootstrap resistor in the datasheet.]Choosing something different fom 11k/24.9k in the feedback divider will yield another buffer voltage and hence a different zener current. The 10.xx volt reflects a a choice of ca. 1mA pr. zener and is imo accidentally (but not intentionally) close to the standard 10.0.The label "Heater Current @ D" which persists even in the revised schematic in the post above is misleading. The heaters are connected directly to +15V.R10 is just a pull-up resistor to avoid power-up interference with R14 initial bias.

author: branadic
author_rank: Super Contributor
timestamp: October 11, 2017, 08:34:26 pm
content: Thanks for this explanation.-branadic-

author: Vtile
author_rank: Super Contributor
timestamp: October 11, 2017, 10:47:04 pm
content: Now as AG7CK writes about it, the schematic indeed looks a lot of the "precision reference" aplication circuit found on ie. old OP05/7 datasheet.. Or from Conrads "oldschool reference" or similar thread.

author: AG7CK
author_rank: Regular Contributor
timestamp: October 12, 2017, 01:03:44 am
content: If indeed the circuit turns out to be of the bootstrap / auto-supply servo type in the datasheet (except from the multiple refs and follower buffer), there is a bit of useful explanation here:https://electronics.stackexchange.com/questions/329661/why-is-there-an-extra-resistor-on-the-opamp-in-the-lm399-datasheetIn my post #48 above I write "inverting amplifier". With a positive input and a positive output this should of course be non-inverting (which I am sure everyone has discovered already).

author: branadic
author_rank: Super Contributor
timestamp: November 04, 2017, 11:20:01 am
content: Did some first measurements and found, that the reference as is is pretty sensitive to humans hand. If I touch the grounded case and thus the connection "A" the output voltage shifts. This sensitivity also represents as what looks like big noise. If the case is untouched the noise is quiet high, but touched and holding hand to it the noise decreases besides a bigger voltage shift. Need to further dig in to it.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: November 27, 2017, 08:20:35 am
content: After some minor modification (connect "A" as short as possible to GND), the hand sensitivity vanished. I started measurements on that reference and here are the first results.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: November 29, 2017, 09:12:30 pm
content: With the current data given, some dependancy can be plotted:1. voltage vs. temperature2. voltage vs. humidity3. voltage vs. dew point temperature (includes temperature and humidity)4. voltage vs. ambient pressure-branadic-

author: branadic
author_rank: Super Contributor
timestamp: December 03, 2017, 06:17:11 pm
content: Updated the diagrams above. I wonder why noise is still in the same order of a single LM399 instead of being halved as this is a 4x LM399 reference.-branadic-

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: December 03, 2017, 07:30:55 pm
content: Noise adds as square-root of number of sources only for uncorrelated noise.So, if noise is not halved that does suggest that there is a correlated noise source dominating the performance.Temperature?Another possibility is that one of the sources is much worse than the others. You would need to measure them individually to quantify this.

author: branadic
author_rank: Super Contributor
timestamp: December 08, 2017, 04:31:57 pm
content: I had to interrupt the measurements today. The graphs above are updated. Seems like it took quite a while for the reference to acclimate. However, the noise is still bigger than expected.Measuring each single reference is somewhat complicate and time consuming, so this is a task for later. Next step is to turn back to my other LM399 and to measure the difference between reference and gained output voltage.Even with first 100.000 measurement values hidden I can't find a correlation to temperature, humidity, dew point temperature or pressure. So it seems that this is just the raw behavior of the reference. Attached Allan diagram without the first 100.000 values and all data.Any further comment on interpreting the data above is welcome.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: December 08, 2017, 07:53:05 pm
content: The Allan variance plots look like there is quite some 1/f noise, which is no a surprise for the LM399. The 1/f noise can scatter quite a lot - at least it does it with amplifiers. So it could be just one bad LM399 in the set, or maybe a lot of not so good ones. If the unit was not used for a long time, there can be some drift due to humidity effects (board stress) - this can be delayed quite a bit and thus a direct correlation with measured humidity might not be visible.For measuring single references it should still be OK to use the circuit with 4 refs, just take the signal directly from one reference. There will be a little noise from the other refs. Mixed in but due to the very low impedance of the LM399 this should be only a minute part (e.g. less than 1%).

author: branadic
author_rank: Super Contributor
timestamp: December 08, 2017, 08:44:13 pm
content: I don't think that this reference has increased 1/f noise (see FFT plot of all data points). It took the frist 100.000 samples @ 0,25Hz thus around 5 days to stabalize the conditions in the aluminium case. Since that noise is superimposing stability I think. I will investigate each reference output noise in near future.I'm not sure about the history of the reference, maybe Philipp can leave a comment on this?-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: December 09, 2017, 08:58:25 am
content: The spectrum looks quite a lot like 1/f noise, though the exponent seems to be a little less than 1. So more like E_N = const * f^-0.9.Having higher 1/f noise would be having the constant factor high - so hard to tell from the spectrum. If at all it would show in the higher frequency part (e.g. 1 Hz - 1 kHz).

author: e61_phil
author_rank: Frequent Contributor
timestamp: December 09, 2017, 05:36:34 pm
content: Quote from: branadic on December 08, 2017, 08:44:13 pmSorry, I don't have any further informations. The reference was sitting inside a broken Data Precision calibrator instead of a LM399.

author: branadic
author_rank: Super Contributor
timestamp: December 09, 2017, 11:47:39 pm
content: Quote from: Kleinstein on December 09, 2017, 08:58:25 amWell, comparing the spectrum of this 4xLM399 with the one of my single LM399 at its reference voltage output:I can't see any difference. And as they are both fully different voltage references with zeners from fully different batches but both based on LM399 I assume that this is a typical spectrum for LMx99 zener references.-branadic-

author: Andreas
author_rank: Super Contributor
timestamp: December 10, 2017, 09:28:21 am
content: Hello,do I understand it right?The FFT is calculated from the 100NPLC measurement values?In this case most of the relevant noise should be filtered out.I would rather compare 1/f (0.1 - 10 Hz) and eventually wideband (10 Hz - 100kHz) noise.On the 1/f noise I usually find a large stray between different LM399.See difference between my LM399#1 and LM399#2 with 1uV/Div and 1sec/divwith best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: December 10, 2017, 10:25:55 am
content: QuoteThat's right, but as far as I understood you can't filter 1/f noise? How does that fit togehter?-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: December 10, 2017, 11:02:43 am
content: For comparing noise data, the 0.1 Hz to 10 Hz band is a standard frequency range. Using 100 PLC mode of the DMM will filter out noise above about 0.25 Hz as it should only give one value every 4 seconds. So there is not much left from the more standard 0.1-10 Hz band. It is still OK to use such slow sampling, but one gets different data and thus difficult to compare too others who are less patient.To get closer to the normal 0.1 -10 Hz Band it would need the 1 PLC speed setting.If a DMM is used for the measurement, I would suggest using 1 PLC speed and the Allan variance curve to judge noise. It has kind of similar information as the FFT. The very long time scale might add some extra noise from the DMM due to effects like thermal variations - thus not much sense in using a 100 PLC mode, unless the DMM is old and does not perform well faster.

author: branadic
author_rank: Super Contributor
timestamp: December 10, 2017, 12:17:17 pm
content: Andreas,have you tried using your Picoscope plus 0,1 - 10Hz noise amplifier to plot spectrum?Kleinstein,wouldn't we expect noise below 0,1Hz to still have a negative slope like in the 0,1 - 10Hz band? Do we know how this slope changes for very low frequencies? Are there any references or publications available?-branadic-

author: Andreas
author_rank: Super Contributor
timestamp: December 10, 2017, 02:13:44 pm
content: Quote from: branadic on December 10, 2017, 10:25:55 amHello,at least it is very difficult to set up a analog filter below 10 Hz which does not introduce additional noise in this frequency range.The integrator in the DMM in a multislope ADC works different: part of the integrator is in the digital domain which does not add additional noise.Quote from: branadic on December 10, 2017, 12:17:17 pmnot up to now and I have not much measurements from LM399 with PicoScope.When I take a old measurement from LM399#CH6 with my usual setpup I get this:But this is mainly the filter characteristic with 10Hz upper bandwidth.I guess for a FFT I would need a different setup to see the lower frequency parts.with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: December 10, 2017, 05:47:06 pm
content: The AC coupling could fool you when doing the measurement with the scop. So it could be compensator for the 1/f slope of the noise.

author: branadic
author_rank: Super Contributor
timestamp: May 20, 2018, 08:46:12 pm
content: QuoteDid that today and measured low frequency noise with Pipelines amplifier. Here is the result. The gain should be A=10.000 so one vertical division corresponds to 500nV.-branadic-

author: chuckb
author_rank: Frequent Contributor
timestamp: May 20, 2018, 09:07:26 pm
content: I noticed in the scope photo that it was AC coupled during the measurement. The Tek2465 has a 10Hz (-3db) lower frequency limit when AC coupled.With the 0.1-10Hz noise filters/preamp the scope needs to be DC coupled. The noise filters/preamp has an internal AC coupling that works down to 0.1Hz.I expect this combination of equipment and settings provided you with a tuned filter at 10Hz that was -6dB down from the actual 10Hz noise.

author: branadic
author_rank: Super Contributor
timestamp: May 20, 2018, 09:37:48 pm
content: Thanks for the hint chuckb, I corrected that.-branadic-