I am thinking about building a voltage reference, using TL431 as the voltage standard and OP07C as the precision amplifier. How good will that be?Good or not depends what you need. TL431 would be about same level of accuracy as voltage standards 150 years ago ;)
And since I don't have those sexy decade switches, would using reed relays affect the precision? How to select the resistors and trim the voltage standard? Will the microcontroller controlling the relays affect the output, since the control to the reed relays obviously have to come from a string of 74HC595s on the microcontroller's SPI bus? Is the "dial in a voltage on a keypad and hit go" a good user experience?
One step further, since a microcontroller is called for already, does it make sense to make it computer controlled? Is there any existing protocol available?
I am thinking about building a voltage reference, using TL431 as the voltage standard and OP07C as the precision amplifier. How good will that be?Good or not depends what you need. TL431 would be about same level of accuracy as voltage standards 150 years ago ;)
And since I don't have those sexy decade switches, would using reed relays affect the precision? How to select the resistors and trim the voltage standard? Will the microcontroller controlling the relays affect the output, since the control to the reed relays obviously have to come from a string of 74HC595s on the microcontroller's SPI bus? Is the "dial in a voltage on a keypad and hit go" a good user experience?
One step further, since a microcontroller is called for already, does it make sense to make it computer controlled? Is there any existing protocol available?
There is probably about a metric ton of better voltage references than TL431.
Please suggest a part. I have checked recently that even from Shenzhen suppliers selling parts salvaged from other boards, LM399 and LTZ1000 are expensive like hell, costing US$10 and US$50 each.The whole point of having a voltage reference to use as a standard is stability. You are not going to get anything standard-worthy out of a 431. Unfortunately, better things usually come with a heftier price tag. There are reasons why the 399 and LTZ are such well-established go-to reference parts.
REF02 is pretty common and you should be able to find some for 1-2usd/piece or less. Even brand new should be like 3-5 usd.
Please suggest a part. I have checked recently that even from Shenzhen suppliers selling parts salvaged from other boards, LM399 and LTZ1000 are expensive like hell, costing US$10 and US$50 each.
REF02 is pretty common and you should be able to find some for 1-2usd/piece or less. Even brand new should be like 3-5 usd.
Please suggest a part. I have checked recently that even from Shenzhen suppliers selling parts salvaged from other boards, LM399 and LTZ1000 are expensive like hell, costing US$10 and US$50 each.
It's nowhere as good as LM399 or LTZ but much much better than TL431
Some others that you might find: ADR4550, REF192, MAX6050
Well its something like 10 to 100 times better than TL431 so a definite improvement.REF02 is pretty common and you should be able to find some for 1-2usd/piece or less. Even brand new should be like 3-5 usd.
Please suggest a part. I have checked recently that even from Shenzhen suppliers selling parts salvaged from other boards, LM399 and LTZ1000 are expensive like hell, costing US$10 and US$50 each.
It's nowhere as good as LM399 or LTZ but much much better than TL431
Some others that you might find: ADR4550, REF192, MAX6050
How good is MC1403? The datasheet quotes a 10ppm tempco.
If you can't spend $10 on an LM399, you probably don't need a voltage reference at all.
Well its something like 10 to 100 times better than TL431 so a definite improvement.REF02 is pretty common and you should be able to find some for 1-2usd/piece or less. Even brand new should be like 3-5 usd.
Please suggest a part. I have checked recently that even from Shenzhen suppliers selling parts salvaged from other boards, LM399 and LTZ1000 are expensive like hell, costing US$10 and US$50 each.
It's nowhere as good as LM399 or LTZ but much much better than TL431
Some others that you might find: ADR4550, REF192, MAX6050
How good is MC1403? The datasheet quotes a 10ppm tempco.
Probably good enough to start playing around. If I remember correctly your workbench didn't include fancy 8,5 digit meters or anything like that. :)
Many of the listed references are available with 0,02% accuracy. That should be more than enough to check your meters even without sending to calibration lab.Well its something like 10 to 100 times better than TL431 so a definite improvement.REF02 is pretty common and you should be able to find some for 1-2usd/piece or less. Even brand new should be like 3-5 usd.
Please suggest a part. I have checked recently that even from Shenzhen suppliers selling parts salvaged from other boards, LM399 and LTZ1000 are expensive like hell, costing US$10 and US$50 each.
It's nowhere as good as LM399 or LTZ but much much better than TL431
Some others that you might find: ADR4550, REF192, MAX6050
How good is MC1403? The datasheet quotes a 10ppm tempco.
Probably good enough to start playing around. If I remember correctly your workbench didn't include fancy 8,5 digit meters or anything like that. :)
No all I have is 3.5 digit old ones. If I am to send one down to a calibration lab, should I finalize the design and send down that digitally controlled voltage standard assembly, or just the reference chip itself? It is just a working standard I will be used to calibrate my gear against.
Many of the listed references are available with 0,02% accuracy. That should be more than enough to check your meters even without sending to calibration lab.Well its something like 10 to 100 times better than TL431 so a definite improvement.REF02 is pretty common and you should be able to find some for 1-2usd/piece or less. Even brand new should be like 3-5 usd.
Please suggest a part. I have checked recently that even from Shenzhen suppliers selling parts salvaged from other boards, LM399 and LTZ1000 are expensive like hell, costing US$10 and US$50 each.
It's nowhere as good as LM399 or LTZ but much much better than TL431
Some others that you might find: ADR4550, REF192, MAX6050
How good is MC1403? The datasheet quotes a 10ppm tempco.
Probably good enough to start playing around. If I remember correctly your workbench didn't include fancy 8,5 digit meters or anything like that. :)
No all I have is 3.5 digit old ones. If I am to send one down to a calibration lab, should I finalize the design and send down that digitally controlled voltage standard assembly, or just the reference chip itself? It is just a working standard I will be used to calibrate my gear against.
I want to test the low voltage ranges too so I will need that accuracy and I probably will have to build an adjustable supply out of a reference chip, and that (as well as my old meters) will have to take a trip to the cal lab.
I want to test the low voltage ranges too so I will need that accuracy and I probably will have to build an adjustable supply out of a reference chip, and that (as well as my old meters) will have to take a trip to the cal lab.
If you feel that you need calibration (repeatedly, if you are really serious about it?), the few extra $ for a better part should be negligible compared to the overall cost of the module and calibration service, shouldn't they?
Maybe when I decided to get a 8.5-digit DMM I will need that. However before that, are you suggesting that the accuracy of MC1403 as posted in the datasheet is more than good enough for me to calibrate my gear against?
REF102 is 0.025% and has very good stability over temperature and time. It is a good candidate to build a 3.5 digit standard that doesn't need any initial calibration, just build and use. If you add trimming capability and calibrate it, you can get much better accuracy that should hold true for a long time.Maybe when I decided to get a 8.5-digit DMM I will need that. However before that, are you suggesting that the accuracy of MC1403 as posted in the datasheet is more than good enough for me to calibrate my gear against?
The MC1403 is only specified at +- 1% absolute accuracy, it seems. So if you want an "absolute" standard, rather than just checking your DMMs for drift, that may not be enough for you. Some of the other references mentioned in the thread fare a factor of 10 better in this respect, e.g. the REF192 or the MAX6050 at 0.1% and 0.2%, respectively. At first glance, I have not seen one with the 0.02% accuracy mentioned by mzzj, however.
Maybe when I decided to get a 8.5-digit DMM I will need that. However before that, are you suggesting that the accuracy of MC1403 as posted in the datasheet is more than good enough for me to calibrate my gear against?
The MC1403 is only specified at +- 1% absolute accuracy, it seems. So if you want an "absolute" standard, rather than just checking your DMMs for drift, that may not be enough for you. Some of the other references mentioned in the thread fare a factor of 10 better in this respect, e.g. the REF192 or the MAX6050 at 0.1% and 0.2%, respectively. At first glance, I have not seen one with the 0.02% accuracy mentioned by mzzj, however.
If I want more accuracy how many digits can I squeeze out of a REF102 with calibration capabilities? And how to design calibratable relay operated decade switches?REF102 is 0.025% and has very good stability over temperature and time. It is a good candidate to build a 3.5 digit standard that doesn't need any initial calibration, just build and use. If you add trimming capability and calibrate it, you can get much better accuracy that should hold true for a long time.Maybe when I decided to get a 8.5-digit DMM I will need that. However before that, are you suggesting that the accuracy of MC1403 as posted in the datasheet is more than good enough for me to calibrate my gear against?
The MC1403 is only specified at +- 1% absolute accuracy, it seems. So if you want an "absolute" standard, rather than just checking your DMMs for drift, that may not be enough for you. Some of the other references mentioned in the thread fare a factor of 10 better in this respect, e.g. the REF192 or the MAX6050 at 0.1% and 0.2%, respectively. At first glance, I have not seen one with the 0.02% accuracy mentioned by mzzj, however.
isl21009bfb850 : 0.01% @ 5V
max6350: 0.02% @ 5V
ref102c: 0.025% @ 10V
With best regards
Andreas
| Part # | Nominal voltage | Initial accuracy | Tempco | Drift | Price (from Shenzhen, unit: CNY) |
| ISL21009BFB825 | 2.5V | +/- 0.5mV (+/- 0.02%) | 3ppm | 50ppm/1000hrs | 15/each |
| REF02AU | 5V | +/- 0.015V (+/- 0.3%) | 4ppm | 100ppm/1000hrs | 5/each |
| MC1403 | 2.5V | +/- 25mV (+/- 1%) | 10ppm | No data | 4.25/each (8.5/pack of 2) |
| MAX6350CPA | 5V | +/- 1mV (+/- 0.02%) | 1ppm | 30ppm/1000hr | 30/each |
REF102 is 0.025% and has very good stability over temperature and time. It is a good candidate to build a 3.5 digit standard that doesn't need any initial calibration, just build and use. If you add trimming capability and calibrate it, you can get much better accuracy that should hold true for a long time.Maybe when I decided to get a 8.5-digit DMM I will need that. However before that, are you suggesting that the accuracy of MC1403 as posted in the datasheet is more than good enough for me to calibrate my gear against?
The MC1403 is only specified at +- 1% absolute accuracy, it seems. So if you want an "absolute" standard, rather than just checking your DMMs for drift, that may not be enough for you. Some of the other references mentioned in the thread fare a factor of 10 better in this respect, e.g. the REF192 or the MAX6050 at 0.1% and 0.2%, respectively. At first glance, I have not seen one with the 0.02% accuracy mentioned by mzzj, however.
Scored a few used LM399 at US$2.5/each, end of discussion. I am going to design my standard project around these.
Scored a few used LM399 at US$2.5/each, end of discussion. I am going to design my standard project around these.
Congratulations -- good to see that your project is making progress. With that choice it will make sense to have your voltage reference calibrated once it's built, since the LM399 has great stability, but a rather large 2% initial tolerance.
Yes, the status as used affects it... for the better. It is well known that these heated zeners drift the most in the first few hundred hours of use, then level off to a much better long-term stability. A used device is actually best, otherwise you need to 'burn in' the new device for several months in order to get over that initial period of drift.Scored a few used LM399 at US$2.5/each, end of discussion. I am going to design my standard project around these.
Congratulations -- good to see that your project is making progress. With that choice it will make sense to have your voltage reference calibrated once it's built, since the LM399 has great stability, but a rather large 2% initial tolerance.
So your suggestion here is go ahead and build the voltage reference out of LM399, and send it on a trip to the cal lab? Will its status being a used part affect its accuracy?
Yes, the status as used affects it... for the better. It is well known that these heated zeners drift the most in the first few hundred hours of use, then level off to a much better long-term stability. A used device is actually best, otherwise you need to 'burn in' the new device for several months in order to get over that initial period of drift.Scored a few used LM399 at US$2.5/each, end of discussion. I am going to design my standard project around these.
Congratulations -- good to see that your project is making progress. With that choice it will make sense to have your voltage reference calibrated once it's built, since the LM399 has great stability, but a rather large 2% initial tolerance.
So your suggestion here is go ahead and build the voltage reference out of LM399, and send it on a trip to the cal lab? Will its status being a used part affect its accuracy?
The LM399 has excellent stability, but poor initial tolerance. That means that it any two of them could be at significantly (a few %) different voltages, but they will each be extremely stable, over a range of temperature, and in the short term and long term. After building your device, you will need to get it calibrated so that it is putting out exactly some value that you choose. Then (if you did it right) it should remain quite stable over time.
Only you can decide what amount of accuracy is needed, and you need to decide this before starting the design of the project. After you go beyond about 0.01% (100 ppm), you get into volt-nut territory where everything matters. Choice of components becomes critical: low drift, precision DC op-amps, resistors need to be special expensive low tempco ones. And the layout of the PCB becomes important too.
If you want to make it adjustable, you might try building a couple of decades of Kelvin-Varley dividers. You could use relays in place of the switches, but I'd suggest using latching relays (less self-heating) and make sure that they are signal (or telecom type) not power relays. Depending on your specifications, you might look for ones with low thermoelectric contacts.
There are various threads here on eevblog and other forums discussing building precision references. Search and read, and try not to lose your mind in all the volt-nuttery.