Electronics > Metrology
What source to use with old multimeter calibration if I have nothing else.
David Hess:
I finally collected and repaired enough old multimeters that I now require an absolute voltage reference standard for calibration. Usually I might use my HP 3478A, however it is currently unavailable. My next best option would be my pretty old Tektronix DMM916.
I have these meters to calibrate:
2 x Tektronix DM502 3-1/2 digit with 2 volt and 200 millivolt basic ranges
3 x Tektronix DM501 4-1/2 digit with 2 volt basic range
2 x Tektronix DM501A 4-1/2 digit with 2 volt and 200 millivolt basic ranges
Calibration requires a less than 2 volt reference, and the meters with 200 millivolt ranges also require a less than 200 millivolt reference because they reduce their reference voltage to increase sensitivity rather than using a x10 amplifier.
I was thinking of buying an LTC6655 1.25 volt 0.025% 2 PPM/C bandgap reference and then using an LT5400BIMS8E-8 0.025% 1:9 resistor array to divide down to 0.125 volts for the 200 millivolt calibrations. I will pick up some OPA186 chopper stabilized operational amplifiers for buffering if required; they are not the best but they do have 5 picoamp input current and are low cost.
Are there any better options that I should consider?
At some future time I will design and build something better which perhaps I can send out for calibration. At one time I would have bought a PentaRef with the proper voltages, however they are not available.
beenai2018:
My bench meter is broken currently and one of the occasions where I needed an absolute voltage ,I used the resistance functionality of my handheld to create one. ie pass a known current from the handheld via a precision resistor to get a voltage reference.
Since you have multiple tek units and they have precision lower value resistors in the DC Amps range ,there may be a way to cross -calibrate against one known source (the 916).
the 916 is .06%+XX on DC V but only 0.3% on resistance ranges . The tek is .05% +XX on DC V and 0.2% on DC Amps (and has a nice round 1K for the 200uA range).
This is just a fun experiment while waiting on the precise parts though .
David Hess:
--- Quote from: beenai2018 on January 10, 2025, 07:50:08 pm ---My bench meter is broken currently and one of the occasions where I needed an absolute voltage ,I used the resistance functionality of my handheld to create one. ie pass a known current from the handheld via a precision resistor to get a voltage reference.
Since you have multiple tek units and they have precision lower value resistors in the DC Amps range ,there may be a way to cross -calibrate against one known source (the 916).
the 916 is .06%+XX on DC V but only 0.3% on resistance ranges . The tek is .05% +XX on DC V and 0.2% on DC Amps (and has a nice round 1K for the 200uA range).
This is just a fun experiment while waiting on the precise parts though .
--- End quote ---
My DMM916 is just the best meter I happen to have available and while it has good specifications and is in excellent shape, it has not been calibrated in 20+ years and it is a handheld instead of bench instrument.
The resistance calibrations on these meters depend on the basic DC volt calibrations, so I have to do the DC volt calibrations first. Then I just need a precision resistor to calibrate the ohms converter output on any range.
The DM501s are calibrated against themselves and my DMM916 because I use them for temperature measurement, but one of the DM501s has just failed with high input leakage so it is time to rebuild them. One of the DM502s is out of calibration enough to notice and I could calibrate just them against my DMM916. The DM501As both have input multiplexer leakage and will be getting rebuilt and improved.
Somewhere I have an ESI Kelvin Varley divider that should be good to 0.003%, but that is the problem ... somewhere. Combined with an accurate reference and some buffering, it would make an excellent calibration source over a wide voltage range.
I just thought that there might be a better option than to start with a 1.25 volt 0.025% LTC6655 and precision resistor network for division, like a more accurate reference that is easily available.
iMo:
I would recommend you to go for LM399 or ADR1399 when targeting 4.5digits meters adjusting. Perhaps buy 2-3 of them, put it in parallel, with an opamp and the LT5400 4x10k divider you will get something around 10.5V out. You will need to calibrate it after some 3-4months burn-in.
Avoid the epoxy case references, like the one in the PentaRef..
PS: an example below with LT5400-3 divider
David Hess:
--- Quote from: iMo on January 11, 2025, 08:10:12 am ---I would recommend you to go for LM399 or ADR1399 when targeting 4.5digits meters adjusting. Perhaps buy 2-3 of them, put it in parallel, with an opamp and the LT5400 4x10k divider you will get something around 10.5V out. You will need to calibrate it after some 3-4months burn-in.
--- End quote ---
The problem with that is that the initial accuracy of the LTC6655 is 0.025%, or 312.5 microvolts out of 1.25 volts, while the ADR1399 is 275 millivolts out of 7.05 volts, or 49 millivolts out of 1.25 volts, making the initial accuracy 150 times worse. I do not have any way to do the initial calibration of a LM399 or ADR1399 based reference standard, and if I did, then I could do the same with a simpler LTC6655 based reference standard, or more likely an LT1021 or similar.
There are some calibrated 10 volt reference standards I could buy, however the calibration voltages I need are below 2 volts and below 200 millivolts, so any likely divider would ruin the accuracy without itself being calibrated. I could work my way around this by matching a set of precision resistors to make a divider, and I might do this to replace the expensive LT5400BIMS8E-8 with something that performs even better. 40 0.1% 15ppm/C resistors cost the same as 1 LT5400BIMS8E-8 so I need to look into that.
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