7V to 5V stage design? For LTZ1000 or other reference - Page 1
Forum section: electronics / general

author: aronake
author_rank: Frequent Contributor
timestamp: April 26, 2023, 08:43:07 am
content: I been searching for design of 7V voltage reference to scale it down to 5V, 2V and maybe 1V to make a calibrator for lower voltages, but have not found anything. 7V to 10V is very well analyzed.Anyone have any designs to share on 7V to 5V? Maybe even Kicad (or other electronics CAD) project? Or Gerber files.My own idea would be to use an OP as buffer, voltage divider with precision resistors and another OP as output buffer. But there may be lots of details on how this is best done.

author: r6502
author_rank: Regular Contributor
timestamp: April 26, 2023, 11:51:09 am
content: Hello aronake,I think, it is done in the same way, as if you go from 7V to 10V. The difference is the position of your voltage divider. in the 7V to 10V it is located in the feedback loop of the OP in the 7V to 5V (or lower) it is in-between the reference and the buffer, or in-between two buffers. 1.st for the ref voltage followed by the voltage divider and than the 2nd buffer.Each Reference like ADR1399, LM399 LTZ1000 ... have a quite large tolerance of the absolute ouput voltage, but high stability. This makes it not so easy to use of the shelf components if you want a defined output voltage. So the question her is what do you want 5.000000V at the output, this will be difficult to reach, or something close to 5V or what ever you like, and quantity this value, and take this measured value for your reference. The 2nd option would be mutch easier to realize, as you could use of the shelf components like precise foil resistors or even ready to use voltage dividers, with a very good tracking of the TC of both internal resistors.If you give us more hints, what you want it will be easier to answer.Guido

author: branadic
author_rank: Super Contributor
timestamp: April 26, 2023, 07:40:16 pm
content: You can go the PWM DAC path, if you feel smart enough. Here is an example for 7 V --> 10 V, that can be adopted to your requirements.https://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/msg1955149/#msg1955149-branadic-

author: Echo88
author_rank: Frequent Contributor
timestamp: April 26, 2023, 08:04:37 pm
content: There are several ways:resistive divider/network:https://www.analog.com/media/en/technical-documentation/application-notes/an86f.pdfpage 46https://edms.cern.ch/ui/file/2820336/1/EDA-04061-V4-1_sch.pdfpage 3, more recentPWM-divider:like branadics example or as done with way more expense in the Datron 4910/11/12https://xdevs.com/doc/Datron/4910_4911/Datron_Wavetek_4910_4911_sm_service_manual.pdfpage 33https://xdevs.com/doc/Fluke/5700a/sm_rev9/sch_scan.pdfpage 48 Overkill and of course not as low noise as the Datron, but very nice as a reference and to learn from itcapacitive divider:LTC1043 or discrete implementationhttps://www.eevblog.com/forum/metrology/5v-adc-reference-board-attempt-based-on-ltz1000-and-ltc1043/transformer based:https://www.eevblog.com/forum/metrology/datron-calibrator-7v-to-10v-conversion-with-a-transformer/attempt by doktor pytaJust a few suggestions. All depends on budget, specs, etc.Edit: While were at it, are there any known transformer based voltage dividers/multipliers projects? I think this technique was used in the Datron 1281 or similar, but i havent looked into it so far.

author: iMo
author_rank: Super Contributor
timestamp: April 27, 2023, 07:35:58 am
content: Would the LTC1043 work with a transformer instead of capacitors?? Something like a synchronous detector with M:N transformer divider in the middle? Just a crazy idea

author: Kleinstein
author_rank: Super Contributor
timestamp: April 27, 2023, 08:31:59 am
content: A transformer would need more than just switches. It is more like a separte drive and output windings and a feedback loop to control the drive strength. The circuit in the Datron 1281 DMM could be a possible starting point for such a circuit.I would consider a PWM DAC the easier way, even with the correction of the switch resistance for good linearity. It still needs some extra effort to get the offset right, as the PWM DAC part does not work all the way to 0 V. For calibration purposes good linearity may help, so one could have voltges like 5 V, 3V, 2 V , 1 V , 0.5 V with only a signle external adjustment point. For just a stable ratio one would not need the linearity improvement part.

author: r6502
author_rank: Regular Contributor
timestamp: April 27, 2023, 11:54:48 am
content: Hello,when I read LT AN86, there is a schismatic available on the xDevs side:DAC_A00.pdfbased on the LT app note with more modern parts.Guido

author: aronake
author_rank: Frequent Contributor
timestamp: April 27, 2023, 04:15:40 pm
content: Quote from: r6502 on April 26, 2023, 11:51:09 amHi Guido,Thanks for this. What you describe is what I am targeting reference -> unity gain op -> voltage devider -> unity gain op.Then measure reference output, calculate voltage divider and construct the voltage divider with each "leg" as a couple of resistors in series and parallel to be able to come pretty close to 5V or 1V or what ever lower than voltage reference output I may want. Some trial and error likely needed with resistors to come very close to the wanted output. Alternatively add a "scaled down" potentiometer for final finetuning.My hope would be that someone have something similar as a gerber, ki-cad or other format that could be reused.

author: aronake
author_rank: Frequent Contributor
timestamp: April 27, 2023, 04:23:13 pm
content: Quote from: branadic on April 26, 2023, 07:40:16 pmThis is a very elaborate and interesting approach, like many other of the cool stuff you are doing. But I think a bit too elaborate for my needs...

author: aronake
author_rank: Frequent Contributor
timestamp: April 27, 2023, 04:26:01 pm
content: Quote from: Echo88 on April 26, 2023, 08:04:37 pmBase case would be to go the voltage divider route.Usage will be to try to get it very close to 5V (or some other voltage I want a decent reference for) then have someone with a good calibrated meter measure it so I know better what voltage it actually outputs. Then use it for things like testing linearity of multimeters 10V range. Maybe make a 1V version for calibration of 1V range of multimeter etc.