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Check your DMM’s accuracy with this: MiniCal 5V Meter Calibration Standard How accurate is your digital multimeter? Find out with this simple yet accurate DC voltage reference. If your meter fails the grade, the reference can be used as the calibration standard too. And as a bonus, we’ve thrown in a crystal-locked frequency reference which doubles as a crystal checker. R ECENTLY, THE NEED arose to recalibrate an expensive digital multimeter. As the job seemed quite straightforward, I decided to tackle it myself. Like most hobbyists, I don’t have access to the high-accuracy voltage standards used in calibration labs. Nevertheless, I came up with a scheme that I thought would be accurate enough for general hobbyist work. By hooking up five multimeters and two panel meters to a voltage divider across a battery, I figured that the mean reading should serve as a reasonable “standard”. However, I was amazed to see that no two meters read the same and the range of values was much greater than I had anticipated. Although the readings were proba70  Silicon Chip By BARRY HUBBLE bly within the specs for each meter, it was a sobering demonstration. In the absence of anything better, I calibrated my upmarket digital meter to the mean value but was determined to find a more accurate method that would give me some confidence. down the MAX6350’s +5V output to generate a 192.3mV reference. In addition, the board includes a crystal-locked oscillator for checking meters, oscilloscopes and the like. The frequency of the oscillator is determined by crystal selection. The MiniCal solution How it works The Maxim range of IC voltage references proved ideal for this purpose. In particular, the MAX6350 +5V DC reference boasts a very impressive untrimmed accuracy of ±0.02%, with an extremely low temperature coefficient of 0.5ppm/°C. Generally, voltmeters are calibrated on their lowest DC range (200mV for 3.5-digit meters). The “MiniCal”, as this new project is called, divides Fig.1 shows that the circuit consists of two completely separate sections. With slide switch S1 in the lefthand ABOVE: our Tektronix 4.5-digit meter is pretty much spot on, especially when the 0.02% accuracy of the MiniCal voltage reference is considered. Other (cheaper) meters might not be as accurate. www.siliconchip.com.au Fig.1: the MiniCal consists of independent oscillator and voltage reference circuits. To minimise noise on the voltage reference, only one of the circuits can be powered at a time, selectable via slide switch S1. position, battery power is applied to the oscillator section. Some readers may recognise this circuit and, in fact, it’s based on the “Simple Go/ No Go Crystal Checker”, originally published in the August 1994 edition of SILICON CHIP. The basic Colpitts oscillator used in the original design proved ideal for the frequency reference section of the MiniCal. Although not strictly necessary, the circuit has been reproduced in its entirety, meaning that it can also be used as a crystal checker if so desired. Crystal X1, the 150pF capacitor between Q1’s base and emitter, and the 100pF capacitor to ground together form the feedback network. The output from Q1’s emitter is AC-coupled via a 1nF capacitor to the “FREQ” test pin. Although we’ve specified a 10MHz crystal for X1, the circuit should work with values from 1MHz to at least 21MHz without modification. The remaining circuitry connected to Q1’s emitter performs the crystal “go/no go” function. Diodes D1 & D2 and the 100nF capacitor rectify and filter the AC signal from the emitter. The resultant DC voltage is applied to the base of Q2, switching it on and lighting the “OK” LED whenever oscillation is present. Voltage reference With switch S1 in the righthand position, the voltage reference section of the circuit is powered. This section is very simple and consists of only a www.siliconchip.com.au voltage reference IC, three capacitors and two resistors. The MAX6350 (IC1) can operate with an input range of 8-36V, providing an untrimmed output of 5V ±0.02% (4.999V - 5.001V). Small tantalum capacitors on the input, output and “NR” (Noise Reduction) pins reduce circuit noise to just 3.0µVp/p (typical) in the 0.1Hz to 10Hz spectrum. Battery-powered operation ensures that this is not degraded by external (conducted) noise sources. Note: the MAX6350 is available in both 8-pin DIP and SO (surface mount) packages. The PC board design accommodates both package styles. We expect that most constructors will opt for the surface mount device, as it is cheaper and easier to obtain (see parts list). Resistors R1 & R2 divide down the MAX6350’s +5V output to obtain the 192.3mV calibration voltage. At a minimum, these resistors need to be Main Features • 5.000V ±0.02% voltage standard • 192.3mV ±0.2% voltage standard (optional ±0.1% or ±0.04%) • Two ±0.1% resistor standards (optional ±0.01%) • Crystal-locked frequency reference • Crystal checker 0.1% types (see parts list) to achieve the specified 0.2% voltage tolerance. As you can see, the use of 0.1% resistors degrades circuit performance somewhat. However, the result is a good compromise between accuracy and cost, and is sufficient for meter checking. If you want to use the MiniCal for calibration, then you will need to upgrade to tighter-tolerance resistors in order to meet the basic accuracy specs of your instrument. Two alternatives for R1 & R2 are shown in the parts list. The 0.01% resistor pair gives a ±0.04% tolerance on the 192.3mV output but will set you back about $77. Alternatively, you can install the 0.05% 25:1 divider network for a tolerance of about 0.1% and a much lower cost of just $18. Note: the 25:1 divider network consists of two 0.1% resistors (1kΩ & 25kW) with a ratio accuracy of 0.05%. The device is supplied in a 3-pin surface-mount (SOT-23) package. So why did we choose an odd calibration voltage of 192.3mV instead of a nice round figure? Well, it was simply a convenient choice using available resistor values. Other division ratios could be used but for best results the reference voltage must be close to (but not exceeding) 200mV. Construction All parts mount on a single PC board coded 04112031 – see Fig.2. If you have surface-mount devices for IC1 and/or R1 & R2, these should be installed first (see Fig.3). You’ll need a temperature-controlled soldering iron with a fine chisel tip and small-gauge solder for the job. A bright light, magnifying glass and 0.76mm desoldering December 2003  71 Fig.2: follow this diagram closely when assembling the board. Take care with the orientation of the diodes (D1 & D2) and tantalum capacitors. Note: this final version of the PC board differs slightly from the early version shown in the photographs braid (“Soder-Wick” size #00) will also prove useful. Next, on the top side of the board (see Fig.2), install all components in order of height, starting with the wire link, resistors and diodes (D1 & D2). Obviously, if you’ve mounted the R1/ R2 divider on the bottom side, then you shouldn’t install anything in the R1 & R2 positions on this side! Note that all the tantalum capacitors are polarised devices and must be inserted with their positive leads Fig.3: the PC board design can accommodate both conventional (DIP-8) and surface-mount (SO-8) package types for IC1. If you have the SO-8 type, then mount it on the copper side of the board as shown here. The optional 25:1 resistor network (R1/R2) also goes on this side. aligned with the “+” symbol marked on the overlay. Install the battery holder last of all. It should be fixed to the PC board with No.4 x 6mm self-tapping screws before soldering. To complete the job, attach small stick-on rubber feet to the underside of the PC board to protect the assembly as well as your desktop. Operation Due to the expected intermittent use of the MiniCal, a power switch has not been included. Simply plug in a battery and use the slide switch to select between the oscillator function (“FREQ”) or voltage reference function (“VOLTS”). Note that the battery voltage must be at least 8V for correct operation of the reference IC. When measuring the oscillator frequency, the crystal checker function must be disabled by removing the jumper from JP2. This is necessary because the checker circuit loads the Fig.4: this oscilloscope shot shows the signal on the “FREQ” test pin with a 10MHz crystal installed. Fig.5 (right) shows the full-size etching pattern for the PC board. 72  Silicon Chip www.siliconchip.com.au Parts List 1 PC board, code 04112031, 71mm x 88mm 1 10MHz crystal (X1) (user select, see text) 1 3mm green LED (LED1) 5 PC board pins (stakes) 2 2-way 2.54mm SIL headers (JP1, JP2) 2 jumper shunts 1 miniature DPDT PC-mount slide switch (Altronics S-2060, Jaycar SS-0823) 1 9V PC-mount battery holder (Altronics S-5048, Jaycar PH9235) 3 No.4 x 6mm self-tapping screws 4 small stick-on rubber feet 1 9V battery The MiniCal is powered from a 9V battery to ensure low-noise performance. The inset shows how the surface-mount version of IC1 is mounted. oscillator, reducing the signal on the “FREQ” test pin below the sensitivity level of most multimeters. Follow the instructions provided with your multimeter regarding calibration. In general, most multimeters should be calibrated on their lowest (basic) range, which is normally 200mV for 3.5 digit models. As described earlier, accuracy will be about ±0.2% using ±0.1% resistors for R1 & R2. This figure is good enough for many general-purpose instruments, which typically specify an accuracy of ±0.25% at best. Note that calibration instructions usually specify a standard of ±0.1% or better. Calibration is normally only applicable to the basic range, with all other ranges depending on that calibration. The 5V output and 0.1% resistors should therefore only be used to check the accuracy of your meter, not to calibrate it. Note that, in use, the jumper shunt (on JP1) must be removed before measuring the 0.1% resistor values. Note also that some meters may require special tools and/or know­ ledge for successful calibration. When in doubt, read the (service) manual first! Meter loading effects A resistive divider was chosen to www.siliconchip.com.au Table 1: Capacitor Codes Value 470nF 100nF 10nF   1nF 150pF 100pF μF Code 0.47µF 0.1µF .01µF .001µF    –    – EIA Code IEC Code   474 470n   104 100n   103   10n   102    1n   151 150p   101 100p generate the millivolt source because it’s simple and requires no adjustment. However, the down side to this simplicity is that the meter’s input impedance loads the divider network and therefore reduces the reference accuracy. For example, when a meter with a 10MΩ input impedance is connected, the reference voltage will fall by about 0.02mV. This corresponds to a 0.01% reduction in accuracy. Assuming you know your meter’s input impedance, the loading effect can easily be factored into the calibration where maximum accuracy is required. Further reading Detailed technical information on the MAX6350 voltage reference IC can be downloaded from the Maxim web SC site at www.maxim-ic.com Semiconductors 1 MAX6350CPA (DIP) or MAX6350CSA (SMD) voltage reference (IC1) (Farnell 162-097, also available from www.futurlec.com) 1 BF199 NPN RF transistor (Q1) 1 BC548 NPN transistor (Q2) 2 1N4148 diodes (D1, D2) Capacitors 1 10µF 16V tantalum 1 2.2µF 16V tantalum 1 1µF 16V tantalum 1 470nF 16V tantalum 1 100nF 63V MKT polyester 1 10nF 63V MKT polyester 1 1nF 63V MKT polyester 1 150pF ceramic disc 1 100pF ceramic disc Resistors (0.25W, 1%) 1 47kΩ 1 2.2kΩ 1 10kΩ 1 1kΩ 1 25.5kΩ 0.1% (R1) (Farnell 340-522) 1 1.02kΩ 0.1% (R2) (Farnell 339-180) -OR1 25:1 0.05% resistor network, Vishay MPM series (Farnell 309-8576) -OR1 25kΩ 0.01%, Vishay S102J series (Farnell 309-8175) 1 1kΩ 0.01%, Vishay S102J series (Farnell 309-8114) Note: items listed with Farnell catalog numbers can be ordered direct from Farnell, phone 1300 361 005 or visit www.farnell.com December 2003  73