As a hobbyist I picked up a 6.5 digit Rigol 3068 to chase the precious PPMs and started playing with precision references. It was eye opening how much temperature plays a role and to actually measure this.
For star grounds I like to join the grounds by a zero ohm resistor. That way you can have all the grounds as different nets. Helps me with the layout as the wire nest shows different wire for each ground. Just helps in reminding to keep the grounding as a star. Just a thought 🙂
I found that using precision, stable resistors for 2-8 and 2-Gnd is the most important thing. Choose values that are very close to 10 V without the trimmer, then the multi-turn potentiometer can be 100 or 200 ohms to truly "trim" the resistance. I keep seeing these circuit boards with interruption between the board and the LM399, but really the interruption should be between the op-amp, it's resistors, and the rest of the board. The LM399 is already thermally isolated and stable, it's these other components that will add PPM to the overall circuit.
Best is to enclose the whole thing in insulation. Long term drift till warm, then stable, which if you ovenise the entire reference circuit, and keep that at 40C, makes it much more stable as well.
If you ask nicely, ADI will sample that board to you gratis. Go to customer support, sample components and submit request. I have a ton of really tricky Analog Devices kit that they sent for free. Some of the list prices made me almost wet my self until I saw it printed , “ this is not an invoice, nothing is due”. With your channel, you should be able to get whatever you want.
LM399 is really one of the legendary references. It is used in HP3458A, Fluke 732 (reference) and some other high end applications. But to obtain such performances, the application you used, and/or components used just won't do.
The best voltage for calibrating ( most ) digital voltmeters is ... 19 . 000 Volts ( ? ) ... this seemingly peculiar choice sends the digital voltmeter to ( almost ) FSD and enables the greatest accuracy when (re)setting ....... DAVE™🛑
Testing as a single value only guarantees accuracy at that specific value. For true accuracy you need to check over a range of values so you can also check linearity.
This is something that fascinates me, I'm on the search for the truth but what I have found so far suggests in favour of the pour ! and that's where my money is !
Padding resistors also have the bonus of a better TC, as they are going to be a lot lower TC than the cermet pot, if you choose the right family of low TC resistors, which the pot is almost guaranteed not to be great for TC. Padding also means the TC can be tuned as well, as the ratio of fixed resistor to variable makes the TC of the pot that much lower.
Slight asterisk - it's often better to "limit the influence" of a pot by using a series resistor from its wiper to limit the current driving it can steer an adjustment with. Especially if its top rail is already from a stable reference. This is because yes, the pot has a large TC, but usually inside of the pot - *both sides of it will have relatively the same TC*. So both sides will drift the same way. That's not true if you add padding resistors on either side.
@@ivolol Padding resistors ideally you want the pot in the middle of range, and thus nulling out TC to a great extent. The padding resistors it is much cheaper to get ones with low TC so the TC of the pot contributes less, though if you are going to ovenise the complete reference, including the opamp and trimmer, you get better overall drift. Military I used to use resistor boxes to set offsets and nulling on the bench, then used fixed resistors to set the actual value, often meaning I was using the ratio to get both to be close to an E96 value in use, and if they were more than 2:1 ratio that opamp get changed out for excessive drift. I changed lots of SFC2741T opamps, as they had drifted offset past 10mV over time. Next most common were 5474's, they did not like being used for input data recovery, logically fine just input threshold drift outside of TTL levels.
I've been wanting to build one of these 10V reference boards. I'll get back to it as soon as I deal with my economic emergency and the Fall of the Republic.
What CAD program is Imsai guy using? I really enjoy my daily class her and ongoing learning on this channel. Many other great channels too but few offer a daily format, at least as far as I have found. Thanks
Now if you could take things one step further: Create a reference current source? BTW, I have found this circuit only works on high impedance volt meters. Drag down the output using lower impedance voltmeters, and the voltage is no longer accurate. What is the limitation on current from the op-amp before inaccuracy occurs?
It's a single *point* ground where each conductor or wire arrives separately. So that the current on one circuit element doesn't affect the others. Same thing can be done for non-Grounds too, when you need (for example) a single point voltage reference. It's somewhat related to 4-wire measurements, so you can look that up for more explanation.
If I wanted to build it I couldn't calibrate it because I don't have precision instruments. I would like to build it specifically to test a home-built voltmeter. It seems like a kind of "circular reference" to me. I can't test the voltage reference without a voltmeter, I can't test the voltmeter without a voltage reference. However, this is useful information.
One thing you *can* know is that the LM399 or ADR1399 (if you are buying new, better to buy the latter) is likely to drift extremely little after you leave them turned on for a week. So for everything but crazy precision instruments you know that the voltage you measured from it will not have changed at all in a year or two years from when you last measured it.
What you could do though is check whether your cheap voltmeters drift relative to a fairly precise voltage reference. This assumes that the circuit around the LM399 is properly designed. But for normal people mostly not really worth it.
@@ivolol For economic-practical reasons I bought a voltage reference based on the AD584 already "calibrated" by the Chinese, with a report of dubious validity, but it was the best I could do. But since I would like to build my own I can read up on the ADR1399. Thank you.
@@sebastian_harnisch I haven't tried the voltage reference "toy" I bought yet, because I'm way behind on making the voltmeter, but I think it's a good idea. The voltmeter is analogue therefore the precision is relative. I only need it to calibrate the various trimmers. Thanks
Kind of bothers me, not that anyone cares, that the regulator is kind of in-line with the opamp and resistors, so that any heat generated generates a thermal gradient across the opamp and resistors... Would it be feasible to locate the regulator "to the left" of the resistor/opamp, to minimize the thermal gradient, especially across the gain setting resistors... Its ground would then be kind of centered on the circuit, too... Otherwise, I guess a small heatsink would help...
Наука
As a hobbyist I picked up a 6.5 digit Rigol 3068 to chase the precious PPMs and started playing with precision references. It was eye opening how much temperature plays a role and to actually measure this.
when I watch every video of this channel,I aways PRESS ”LIKE“！every video is so good!
For star grounds I like to join the grounds by a zero ohm resistor. That way you can have all the grounds as different nets. Helps me with the layout as the wire nest shows different wire for each ground. Just helps in reminding to keep the grounding as a star. Just a thought 🙂
I found that using precision, stable resistors for 2-8 and 2-Gnd is the most important thing. Choose values that are very close to 10 V without the trimmer, then the multi-turn potentiometer can be 100 or 200 ohms to truly "trim" the resistance. I keep seeing these circuit boards with interruption between the board and the LM399, but really the interruption should be between the op-amp, it's resistors, and the rest of the board. The LM399 is already thermally isolated and stable, it's these other components that will add PPM to the overall circuit.
Best is to enclose the whole thing in insulation. Long term drift till warm, then stable, which if you ovenise the entire reference circuit, and keep that at 40C, makes it much more stable as well.
LT1021-5 and INA105. You can get 5V and 10V. Nice thing is the LT1021 has a defined range from the factory.
If you ask nicely, ADI will sample that board to you gratis. Go to customer support, sample components and submit request. I have a ton of really tricky Analog Devices kit that they sent for free. Some of the list prices made me almost wet my self until I saw it printed , “ this is not an invoice, nothing is due”. With your channel, you should be able to get whatever you want.
IMSAI Guy: ...LTZ1000...
Marco Reps enters the chat 😂
Marco Reps: My precious PPMs
LM399 is really one of the legendary references. It is used in HP3458A, Fluke 732 (reference) and some other high end applications. But to obtain such performances, the application you used, and/or components used just won't do.
The best voltage for calibrating ( most ) digital voltmeters is ... 19 . 000 Volts ( ? ) ... this seemingly peculiar choice sends the digital voltmeter to ( almost ) FSD and enables the greatest accuracy when (re)setting ....... DAVE™🛑
Testing as a single value only guarantees accuracy at that specific value. For true accuracy you need to check over a range of values so you can also check linearity.
@@JaenEngineering Would help if it had multiple calibration points. single point cals are never the greatest for accuracy.
Analog Devices/Burr Brown with batteries
After listening to Rick Hartley's advice and recommendations, I wonder: is there a reason to not use a ground plane / copper pour in a case like this?
This is something that fascinates me, I'm on the search for the truth but what I have found so far suggests in favour of the pour ! and that's where my money is !
Besides what he said in the video you mean?
4 layer, everyone gets good grounds then
@@charlesdorval394 Kind of. I mean compared with a star topology, is ground plane a drawback? That's the point that's not quite clear...
@@nashaut7635 Oh okay, sorry I misunderstood what you meant
Padding resistors also have the bonus of a better TC, as they are going to be a lot lower TC than the cermet pot, if you choose the right family of low TC resistors, which the pot is almost guaranteed not to be great for TC. Padding also means the TC can be tuned as well, as the ratio of fixed resistor to variable makes the TC of the pot that much lower.
Slight asterisk - it's often better to "limit the influence" of a pot by using a series resistor from its wiper to limit the current driving it can steer an adjustment with. Especially if its top rail is already from a stable reference.
This is because yes, the pot has a large TC, but usually inside of the pot - *both sides of it will have relatively the same TC*. So both sides will drift the same way. That's not true if you add padding resistors on either side.
@@ivolol Padding resistors ideally you want the pot in the middle of range, and thus nulling out TC to a great extent. The padding resistors it is much cheaper to get ones with low TC so the TC of the pot contributes less, though if you are going to ovenise the complete reference, including the opamp and trimmer, you get better overall drift. Military I used to use resistor boxes to set offsets and nulling on the bench, then used fixed resistors to set the actual value, often meaning I was using the ratio to get both to be close to an E96 value in use, and if they were more than 2:1 ratio that opamp get changed out for excessive drift. I changed lots of SFC2741T opamps, as they had drifted offset past 10mV over time. Next most common were 5474's, they did not like being used for input data recovery, logically fine just input threshold drift outside of TTL levels.
@@SeanBZA Ye Olde 'SOT' Select On Test !
@@andymouse No drift either, once set and new opamp it never came back. At least not for that channel, the rest and the power supply though....
@@SeanBZA :)
I've been wanting to build one of these 10V reference boards. I'll get back to it as soon as I deal with my economic emergency and the Fall of the Republic.
nice project! - no LM723 super low noise power supply for this?
Better still Batteries.
What CAD program is Imsai guy using?
I really enjoy my daily class her and ongoing learning on this channel. Many other great channels too but few offer a daily format, at least as far as I have found. Thanks
Kicad
The legend that is 'KiCad'
Hope all is well with you!
Now if you could take things one step further: Create a reference current source? BTW, I have found this circuit only works on high impedance volt meters. Drag down the output using lower impedance voltmeters, and the voltage is no longer accurate. What is the limitation on current from the op-amp before inaccuracy occurs?
Now do mV reference
Would you explain star ground what is it
It's a single *point* ground where each conductor or wire arrives separately.
So that the current on one circuit element doesn't affect the others.
Same thing can be done for non-Grounds too, when you need (for example) a single point voltage reference.
It's somewhat related to 4-wire measurements, so you can look that up for more explanation.
PPM's
If I wanted to build it I couldn't calibrate it because I don't have precision instruments. I would like to build it specifically to test a home-built voltmeter. It seems like a kind of "circular reference" to me. I can't test the voltage reference without a voltmeter, I can't test the voltmeter without a voltage reference. However, this is useful information.
One thing you *can* know is that the LM399 or ADR1399 (if you are buying new, better to buy the latter) is likely to drift extremely little after you leave them turned on for a week.
So for everything but crazy precision instruments you know that the voltage you measured from it will not have changed at all in a year or two years from when you last measured it.
What you could do though is check whether your cheap voltmeters drift relative to a fairly precise voltage reference. This assumes that the circuit around the LM399 is properly designed. But for normal people mostly not really worth it.
@@ivolol For economic-practical reasons I bought a voltage reference based on the AD584 already "calibrated" by the Chinese, with a report of dubious validity, but it was the best I could do. But since I would like to build my own I can read up on the ADR1399. Thank you.
@@sebastian_harnisch I haven't tried the voltage reference "toy" I bought yet, because I'm way behind on making the voltmeter, but I think it's a good idea. The voltmeter is analogue therefore the precision is relative. I only need it to calibrate the various trimmers. Thanks
@@leonardochiruzzi7642 I also bought this device from Amazon and it was spot on but I was worried before it arrived!
How does R1 and R3 improve line regulation? line regulation = the 12 volt regulator changing???
Kind of bothers me, not that anyone cares, that the regulator is kind of in-line with the opamp and resistors, so that any heat generated generates a thermal gradient across the opamp and resistors... Would it be feasible to locate the regulator "to the left" of the resistor/opamp, to minimize the thermal gradient, especially across the gain setting resistors... Its ground would then be kind of centered on the circuit, too... Otherwise, I guess a small heatsink would help...
👍👍👍👍👍
This is not the right design of the cutout that will create mechanical stresses when heated by LM399.