# Precision multimeter calibration reference

8 Steps
WARNING / DISCLAIMER: If it ain't broke, don't fix it. Tampering with the settings inside your multimeter will void the warranty, could mess up it's accuracy permanently, risks causing mechanical failure, and could feasibly cause actual danger from high voltages. I accept no responsibility for any damage caused by following these instructions.

About 10 years ago I bought a Mastech 3.5 digit + analogue multimeter when they were in the sale at Maplin. I like this meter, but I've lost confidence in it's accuracy, and thought that rather than buy a new one, or pay for professional calibration, I would have a go at DIY calibration. I'd like to share the results, so here it is.

The voltage ranges of my particular meter are: 200mV, 2V, 20V, 200V, 600V. On the 200mV range it has 0.1mV resolution, and tolerance of ±(0.5% +1 digit)

Before I go any further, I would like to direct you to this excellent guide to multimeter accuracy: http://www.designworldonline.com/articles/5416/260/How-to-Determine-Digital-Multimeter-Accuracy.aspx. I learnt far more from it than I thought I knew.

After many hours of playing with various numbers, I settled on using a 5v reference with a tolerance of ±0.5mV, equivalent to ±0.01%. Even though the meter's lowest range is 200mV, I chose to use this reference and divide down the output by about 26 (actually in practice it was about 28) using precision (±0.01%) resistors, since as you divide the output voltage, you also divide the absolute error voltage, which moves down the significance of the digits.

I eventually made a spreadsheet to work out the various scenarios of the reference and resistors being out by ±0.01%, showing best and worst cases. The error is now ±0.052mv at worst, which in terms of the resolution of my meter is insignificant if positive, or within the manufacturers tolerance if negative. It is 0% at best, and out of over 24 possible combinations the error was better than ±0.01% for 12 of them, and exactly ±0.01% for 4 of them. I've uploaded the spreadsheet as a .pdf as well as .xls.

I think you'd have to be quite unlucky to get the worst cases. The actual voltage used is slightly unfortunate as it is rounded up by 0.036mV by the meter - a figure ending in a whole 0.1 of a millivolt would have been better, however having a very tight budget, I had to use the cheapest parts available. I'm sure you can choose better values than I did.

If you download the spreadsheet and want to play with the numbers, you only need to enter any value once - red figures at right for tolerances, black bold figures near top for inputs V, R1 and R2 - pretty much all the rest is formulas so don't change anything until you understand them. It will show volts or millivolts automatically, ranged on 199.9mV. Second page shows meter tolerances, again, you only need to change anything once. It shows "E" for over range.

The logic of choosing this method was in the tolerance of the voltage references I could actually afford. There is a range of references with a tolerance of ±0.5mV made by Intersil. For a 5v reference this is ±0.01%. Although these references go down to 1.25V. the tolerance is still ±0.5mV, which is ±0.04%
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## Step 1: Parts and tools

Parts for basic version:
U1:  Intersil ISL21009BFB850Z precision voltage reference 5V ±0.5mV (you can find the datasheet here)
Rx:  806 ohm ±0.01% (Vishay)
Ry:  29.4 ohm ±0.01% (Vishay)
Vero board or similar
SOIC - DIL conversion board
(You could actually use an Arduino project board as these have 0.1" matrix and SOIC pads on them. I already had Vero board which is why I used it)
Battery clip, wires, test sockets (sprung contact type), case

D1 and D3 are arranged so that they can be replaced by a single common cathode bi-colour LED if desired. If you want the regulator's voltage to be fixed at 6.8V, omit VR1 and R9 and connect the op-amp's non-inverting input directly to Q4 drain.

Parts for enhanced  version:
Q1:  MOSFET - Si9430DY - chosen because it is logic level, has low Rds(on), and was the cheapest I could find on Ebay!
Q2, Q3, Q5: General purpose transistor, PNP
Q4:  MGSF1P02LT1 chosen because it was in my junk box. Any low Rds(on) P-Type enhancement mode logic level MOSFET will do
Q6:  2N2905 (Intersil recommendation, but I just used another general purpose PNP)
D1:  LED, Green
D2: 1N4148
D3:  LED, Red
Z1:  Zener diode, 6.8 volts
C1, C2: 10µF capacitor (I actually used 22µF)
R1:  47R
R2:  390R
R3:  10K
R4:  10K
R5:  1K
R6:  100K
R7:  10K
R8:  470R
R9:  56K
R10: 200 ohm (I actually used 220 ohm)
R11: 306R ±0.01% (Vishay)
R12: 29.4R ±0.01% (Vishay)
VR1: 22K preset
VR2: 10K potentiometer (lin) if "fine" adjustment required
VR3: 100K potentiometer (lin)
(NB, a single multiturn precision potentiometer would be far better, but expensive)
U1:  Intersil ISL21009BFB850Z precision voltage reference 5V ±0.5mV (you can find the datasheet here)
U2: Op amp suitable for single supply (I used a LM358 because I happened to have one)
pfred2 says: Apr 15, 2012. 8:16 AM
I've never seen a meter like that one with a mechanical movement, and a digital read out before. Do they both work at the same time or do you switch between the two?
throbscottle (author) in reply to pfred2Apr 15, 2012. 1:55 PM
They both work together, but the analogue scale is only useful to show rapidly changing readings. This whole project has got me thinking about building a better meter myself, since this one although versatile isn't very good - I have something to calibrate with now!
pfred2 in reply to throbscottleApr 16, 2012. 12:19 AM
I have seen multiple meters influence each others readings when used together in circuits. It could be why you have been having some difficulties calibrating your unusual instrument. No meter is completely disassociated from the circuit it is connected to. Why some even say that just the act of watching an event influences its outcome. All of that is too philosophical for me though.

I'm curious as to why you need such precision in your instruments. Are you doing original science or something? I never encounter situations where I need very accurate readings personally. With respect to electronics I guess you could say I'm rather high tolerance.
throbscottle (author) in reply to pfred2Apr 16, 2012. 12:16 PM
Interesting point.

Multimeters drift over time, and I was pretty certain mine wasn't accurate any more. Since I was going to do this, I thought I may as well build the best possible reference I could within my means. Actually, when you do the sums, it needs to be this accurate since my meter will resolve 100 microvolts - just not very well, it turns out.

I have a vague plan to make a better incarnation of this - the veroboard construction doesn't do justice to the reference chip.
pfred2 in reply to throbscottleApr 16, 2012. 10:48 PM
Often sensitive electronics needs to be designed so it is guarded from interference. Search the term ground guard ring and it may bring up some information on the topic.