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Low-cost precisi 10V DC referenc checking DMMs Ever checked the calibration of your digital multimeter? OK, we know . . . you haven’t because there’s no easy or cheap way of doing it. But now you can, with this low-cost precision DC voltage reference. Without any adjustment it will provide you with a source of 10.000V DC accurate to within ±5mV or ±0.05%. By JIM ROWE M OST OF US DON’T ever get our DMMs calibrated, though we know that they do drift out of calibration over years of use. However, if you are using them during the course of your work, they should be checked every year or so – otherwise how can you trust the readings? The problem is, it can cost quite a lot to send a DMM away to a standards lab for calibration – more than many DMMs are worth. So generally we either hope for the best or simply buy a new DMM if we suspect that our existing meter has drifted too far out of calibration. +VIN 2 AD587 RS A1 NOISE REDUCTION 6 VOUT RF 8 RT 5 TRIM (OPTION AL) RI 4 GND 44  Silicon Chip Fig.1: block diagram of the AD587 10V voltage reference. It consists of a buried zener diode and its associated current source, plus op amp IC1 which operates as an adjustable gain buffer stage. Buried zeners have their avalanche zone several μm inside the oxide layer and so do not suffer from long-term drift or ‘walkout’. Back in the 1970s, when DMMs first became available, the only practical DC voltage reference was still the Weston cell. This wet chemical ‘primary cell’ had been developed in 1893 and subsequently became the international standard for EMF/voltage in 1911. It produced an accurate 1.0183V reference which could be used to calibrate DMMs and other instruments. Unfortunately, Weston cells were fairly expensive and few technicians had direct access to one for meter calibration. As a result, a reasonablyfresh mercury cell was often used as a kind of ‘poor man’s’ voltage reference. Fresh mercury cells have a terminal voltage very close to 1.3566V at 20°C and the voltage falls quite slowly to about 1.3524V after a year or so. Silver oxide cells were also used for the same purpose, having a stable terminal voltage very close to 1.55V. Of course, batteries have a tendency to obey ‘Murphy’s Law’ and usually turn out to have quietly expired just before you need them. And although siliconchip.com.au on e for If you have access to a high-precision bench multimeter like this one, you can tweak the output of your 10V Reference so that it is really close to 10.00000V. Mind you, the last one or two digits will always “bobble about” due to residual noise superimposed on the 10V Reference’s output and also due to the normal digital uncertainty of the last digit in such a precision instrument. The bench multimeter also needs to have been calibrated within the last year or so in order to be absolutely certain that its readings are as accurate as possible. mercury and silver oxide cells have quite a long life, especially if you use them purely as a voltage reference, they certainly aren’t immune to this problem. So these batteries make a pretty flaky voltage reference, at best. Fortunately, in the 1980s, semiconductor makers developed a relatively low-cost source of stable and accurate DC voltage: the monolithic voltage reference (MVR). This is basically a very accurate voltage regulator. It produces a precise regulated DC output voltage when fed with unregulated DC power but unlike the more familiar 3-terminal regulators, it can supply very little current. The Analog Devices AD587 device used in this new Precision 10V Reference Mk.2 incorporates a number of recent advances in MVR technology. These include an ion-implanted ‘bursiliconchip.com.au ied’ zener reference diode plus high stability thin-film resistors on the wafer. These resistors are laser-trimmed to minimise drift and provide higher initial accuracy. The AD587 also operates from an unregulated input voltage of between +15V and +18V, with a quiescent current of just 4mA. This is somewhat lower than earlier MVRs, making it very suitable for battery-powered operation. Block diagram Fig.1 shows what’s inside an AD587. The voltage reference cell itself is at upper left, consisting of the ‘buried’ zener and its current source. The other main circuit section is op amp A1, used as an adjustable gain buffer. RF, RI & RT are high-stability thin-film resistors, laser trimmed to allow the gain of A1 to be set with a high degree of precision. The output voltage (between pins VOUT and GND) is initially set to 10.000V ±5mV for the AD587KNZ version used here, without any external adjustment. In addition, temperature compensation inside the cell gives the basic voltage reference a very low temperature drift coefficient – typically ±10ppm/°C. Note that a slightly lower-spec version of the AD587 is also available, the AD587JNZ. This offers an initial (untrimmed) DC output voltage of 10.000V ±10mV, with a temperature drift coefficient of ±20ppm/°C. So you could use it as an ‘almost as good’ alternative if the KNZ version becomes unavailable. Although the ‘untrimmed’ initial accuracy of the AD587KNZ (10.0V ±0.05%) is good enough for calibrating most low-cost DMMs, the chip can also be easily trimmed to improve its accuracy by a factor of greater than 10 times, ie, to around ±0.002%. This is done by connecting its TRIM pin (pin 5) to a trimpot circuit, connected between the VOUT and GND terminals. This allows the gain of A1 to be adjusted to give an output anywhere within the range 9.900V to 10.300V, March 2014  45 +18V 9V BATTERY 1 12k K D1 1N4004 POWER A A LED1 BLUE K +9V START 5 6 K D2 1N4004 A 10k 8 NR 1 µF S1 9V BATTERY 2 2 VIN λ 14 VDD AUTORST CSEL B MRST CSEL A Q/Q SEL 22k 100nF 10k 3 2 1 RS IC2 4541B TRIM 6 + 5 2.2k VR1 1k (25T) 13 12 – 9 8 10.000V OUTPUT 6.8k 100nF 100Ω G CTC MODE VOUT GND 4 D OUT RTC IC1 AD587 KNZ S Q1 BUZ71 OR IRF1405 10 Vss 7 Q1 LED1 SC 20 1 4 PRECISION 10V REFERENCE MK2 K A G D D S Fig.2: the complete circuit diagram. IC1 is the precision 10V reference, while IC2 operates as a 90s timeout counter. When S1 is pressed, IC2 turns Mosfet Q1 on for 90s and connects IC1 and LED1 across the 18V supply. with no adverse effect on temperature stability. If this trim adjustment range seems a little wide, this has been done deliberately to provide the option of setting the output voltage to 10.240V. It can then be used as a reference source for binary DACs and ADCs (more about this later). The 400mV adjustment range does mean that in order to accurately set the output voltage, we have to use a 25-turn trimpot in series with two fixed resistors. And of course, in order to take advantage of this trimming feature, you really need access to an even higher precision voltage reference to compare it with. Either that, or access to a recently calibrated high-resolution DMM. Circuit details Refer now to Fig.2 for the complete circuit details. There’s not a lot to it – just the AD587KNZ precision voltage reference (IC1) plus some extra circuitry to allow the AD587KNZ to run from two 9V alkaline batteries to provide a truly portable reference. This additional circuitry is based around IC2, a programmable CMOS 46  Silicon Chip timer. It provides a 90-second timeout function and controls IC1’s operation via Q1, a BUZ71 (or IRF1405) Nchannel Mosfet. IC2 (4541B) is basically a binary counter with 16 stages. It can be configured as either an 8, 10, 13 or 16-stage counter by changing the logic levels to which its two ‘CSEL’ programming inputs (pins 12 & 13) are connected. In this circuit, both these inputs have been connected to +9V (ie, tied high), to configure the counter to use its full 16 stages. The 4541B also contains its own clock oscillator, the frequency of which is set by the RC timing components connected to pins 1, 2 & 3. In this case, the values specified give an overall timer period of around 85-90 seconds. IC2’s output at pin 8 drives Mosfet Q1’s gate via a 100Ω resistor. As a result, each time pushbutton switch S1 is pressed (and resets the counter), pin 8 of IC2 goes high and Q1 turns on and connects IC1 across the 18V supply for the duration of the 85-90s timing period. At the end of this period, pin 8 switches low and Q1 turns off to remove power from IC1 and conserve battery life. Pressing S1 again starts the timing period all over again, if further calibration checks are necessary. Power comes from the two 9V batteries, while D1 & D2 act as voltage clamps to provide reverse polarity protection if a battery is connected the wrong way around. LED1 and its associated 12kΩ current limiting resistor are connected across IC1’s supply pins, so the LED functions as a power-on indicator. Using a high-efficiency 3mm blue LED gives a very visible indication while adding less than 1.5mA to the total current drain. By the way, you may be wondering why we have used a BUZ71 or IRF1405 power Mosfet for Q1 when IC1 and LED1 only draw a maximum of 16mA or so, even with a 10mA external load (the maximum current the AD587 can provide). This is because the BUZ71 (or IRF1405) offers a much lower onresistance than smaller low-power Mosfets like the 2N7000. This provides a much lower voltage drop and allows us to achieve significantly longer life from the 9V batteries. The connections for IC1 itself are easy to follow. The 1µF capacitor connected between pin 8 (NR) and pin 4 siliconchip.com.au LED1 10k 22k 100nF D2 10k BINDING POSTS (MOUNTED ON LID) 0V OUT 6.8k 100Ω LINK 4004 – TRIM 2.2k IC2 4541B + (25T) K 100nF (BATTERY 1) 9V BATTERY (BATTERY 2) 9V BATTERY PWR VR1 1k S1 A +10V OUT 4 1 0 2 C (ON LID) 1 µF IC1 AD587 12k – 4004 + V 0 1 N OI SI C E R P E C NEREFER CD 14140140 D1 Q1 BUZ71 Fig.3: follow this layout diagram build the unit but note that switch S1 and the two binding post terminals are soldered to the PCB only after they have been mounted on the case lid (see text). Leave out trimpot VR1 and the 2.2kΩ and 6.8kΩ resistors if you don’t intend calibrating the unit. Note: the prototype PCB shown in the photo lacks the reverse-polarity protection diodes and the strain relief holes for the battery leads included in the final version. (GND) is there to provide additional low-pass filtering of any noise generated by the AD587’s buried zener. It works in conjunction with series resistor RS, which is shown in Fig.1. Trimpot VR1 and its two range setting resistors are for ‘trimming’ the output voltage of IC1 to the desired 10.000V or 10.240V. However, note that there’s no point in fitting these parts unless you have access to a very accurately-calibrated DMM, to compare it against while you’re doing the trimming adjustment. In fact, these parts must be left out if you have no way of performing the calibration, otherwise they will upset the accuracy. Conversely, if you are able to carry out calibration, the resistor values shown (2.2kΩ & 6.8kΩ) will give a trimming range centred on 10.000V. Alternatively, if you want the trimming range to be centred on 10.240V, change the 2.2kΩ ‘upper’ resistor to 8.2kΩ and change the 6.8kΩ ‘lower’ resistor to 1.0kΩ. In both cases trimpot VR1 should have a value of 1kΩ as shown, and should be of the 25-turn cermet type. Construction Building the Precision 10V Refer- The PCB is secured to the case lid on two M3 x 15mm spacers at one end before soldering the switch and binding post terminals. ence Mk.2 is easy. All parts except for the binding post output terminals, switch S1 and the two 9V alkaline batteries are mounted on a single PCB coded 04104141 and measuring 63 x 53mm. This board fits inside a diecast aluminium box measuring 111 x 60 x 30mm, which not only protects the assembly but also provides shielding. Fig.3 shows the parts layout on the PCB. Note that although trimpot VR1 and its series resistors are shown here, these parts are optional and should only be installed if you can calibrate Table 1: Resistor Colour Codes   o o o o o o o siliconchip.com.au No.   1   1   2   1   1   1 Value 22kΩ 12kΩ 10kΩ 6.8kΩ 2.2kΩ 100Ω 4-Band Code (1%) red red orange brown brown red orange brown brown black orange brown blue grey red brown red red red brown brown black brown brown the device (as mentioned earlier). Begin the assembly by installing the wire link, then fit the five fixed resistors on the lefthand side of the PCB, plus the two series resistors for trimpot VR1 if it’s being used. That done, fit the three multilayer ceramic capacitors, making sure that the 1µF  Table 2: Capacitor Codes Value µF Value IEC Code EIA Code 1µF   1µF   1u0   105 100nF 0.1µF 100n   104 5-Band Code (1%) red red black red brown brown red black red brown brown black black red brown blue grey black brown brown red red black brown brown brown black black black brown March 2014  47 Parts List 1 diecast aluminium case, 111 x 60 x 30mm (Jaycar HB-5062 or similar) 1 PCB, code 04104141, 63 x 53mm 1 front-panel label 1 SPST panel-mount momentary pushbutton switch (S1) 1 14-pin DIL IC socket (optional) 1 red binding post terminal 1 black binding post terminal 2 M3 x 15mm tapped spacers 5 M3 x 6mm machine screws 1 M3 hex nut 1 M3 shakeproof washer 2 9V battery clip leads 2 9V alkaline batteries 1 1kΩ cermet trimpot, 25-turn vertical (VR1) 1 100mm length double-sided tape Semiconductors 1 AD587KNZ or AD587JNZ 10V voltage reference (IC1) 1 4541B programmable CMOS timer (IC2) 1 BUZ71 or IRF1405 Mosfet (Q1) 2 1N4004 diodes (D1, D2) 1 3mm high-intensity blue LED (LED1) Capacitors 1 1µF multilayer ceramic 2 100nF multilayer ceramic Resistors (0.25W, 1%) 1 22kΩ 1 12kΩ 2 10kΩ 1 6.8kΩ (or 1kΩ for 10.240V output) 1 2.2kΩ (or 8.2kΩ for 10.240V output) 1 100Ω capacitor goes in at top right. Now for the two ICs. IC1 must be soldered directly into the board, to ensure reliability (and avoid possible contact resistance). IC2, on the other hand, can either be soldered directly to the PCB or can be installed via a 14-pin DIL socket. Make sure that both ICs are correctly orientated. Trimpot VR1 is next on the list, followed by Mosfet Q1. Note that Q1’s leads must be bent down through 90° about 5mm from its body before mounting it in place. Push it all the 48  Silicon Chip A 11.5 D 23 C B 9.5 A C L 9.5 26.5 23 D 28 19.5 16 A HOLES A: 3.0mm DIAMETER HOLE B: 3.5mm DIAMETER HOLE C: 12.5mm DIAMETER HOLES D: 9.0mm DIAMETER (ALL DIMENSIONS IN MILLIMETRES) Fig.4: this diagram shows the drilling template for the front panel. It can either be copied or downloaded from the SILICON CHIP website. way down so that its metal tab sits flush against the PCB and secure it using an M3 x 6mm machine screw, nut and shakeproof washer. Do the screw up firmly, then solder the Mosfets leads to their respective pads (note: don’t solder the leads first, otherwise the PCB tracks will crack as the mounting screw is tightened down). LED1 can now be installed, making sure its longer anode (A) lead is orientated as shown. It should be mounted about 7mm proud of the PCB (use a cardboard spacer). Solder just one lead and don’t trim the leads at this stage, as you may have to adjust its height later, after the PCB assembly has been mounted on the rear of the lid. Next, pass the four battery snap leads through the strain-relief holes and solder them to the PCB. That done, cover these connections with silicone to prevent the leads from breaking. Be sure to connect the red wire from each battery snap to the pad marked ‘+’. Your PCB assembly will now be finished and can be placed aside while you prepare the case – or strictly, the case lid since there are no holes to be drilled in the case itself. Drilling the case lid Fig.4 shows the drilling template for the case lid. You have to drill/ream seven holes in all – for the output terminals, switch S1, power LED and PCB mounting, plus a screwdriver access hole for trimpot VR1 (if necessary). Fig.4 shows the location and size of each of these holes. You can either follow this diagram to mark out the lid for drilling or you can copy it, cut it to size and attach it directly to the lid (using double-sided tape) for use as a drilling template. The drilling template is also available for download from our website (free for subscribers). Use a small pilot drill to start the holes, then remove the template and carefully drill and ream them to size. Deburr each hole with an oversize drill or in the case of the three larger holes, a small rat-tail file. Now for the front panel artwork. This artwork can be obtained either by photocopying Fig.5 onto an adhesivebacked label or it can be downloaded as a PDF file from the SILICON CHIP website (again, free for subscribers) and printed out. It can then be covered with a self-adhesive transparent film to protect it from finger marks. Alternatively, it can be photocopied onto plain paper, hot-laminated into a clear protective sleeve and then attached to the lid using double-side tape or silicone adhesive. The various holes can then be cut out using a sharp hobby knife. Pushbutton switch S1 can now be mounted on the lid, taking care to orientate it so that its two connection lugs are aligned along the long axis. This is necessary so they will later fit through their holes in the centre of the PCB. That done, attach the two output terminals (binding posts) to the lid, making sure that the red terminal goes to the ‘+’ position and the black terminal to the ‘-’ position. siliconchip.com.au Specifications •  Output voltage: 10.000V DC (10.240V optional – see text) •  Basic accuracy: ±0.05% (±5mV) without adjustment, ±0.002% after trim adjustment •  Long term drift: <15ppm per 1000 hours, mostly in first year of operation •  Temperature stability: <7mV change between 0°C and +70°C •  Maximum output current: 10mA •  Noise on output: <4µV peak-to-peak (0.1Hz – 10Hz); <180µV peak-to-peak (DC – 1MHz) •  Load regulation: less than ±100µV/mA for loads up to 10mA •  Power supply: 2 x 9V alkaline batteries; quiescent current drain (when operating) <6.5mA •  Auto-off time: 90 seconds; standby current 10nA Helping to put you in Control Voltage to 4-20 mA Converter Converts any DC voltage range from below 0.1 V to above 30 VDC to 4 to 20 mA. 2 trimpots and a switch allow you to easily configure it. 8 to 30 VDC powered with DIN rail mount enclosure. SKU:KTA-289 Price:$75+GST Power HD Giant Servo The Power HD 1235MG servo is all about torque. This 1/4-scale servo can deliver an incredible 560 oz-in of torque at 7.4 V or 490 oz-in at 6 V, and it features an allmetal gear train, digital control electronics, and two ball bearings on the output shaft. SKU:MOT-320 Price:$79.95+GST Arduino Yun The Arduino Yun is packed with features, comes with an ATmega32U4 microcontroller (the same as the Leonardo) and a Linux system based on the Atheros AR9331 chipset. Additionally, there are built-in Ethernet and WiFi capabilities, enabling it to communicate with networks out of the box. SKU:POL-2472 Price:$99.95+GST 12 VDC Relay Card On DIN Rail Eight-way each relay card on DIN rail mount. Relay is triggered if the low input is pulled below 0.8 VDC or if the high input is pulled above ~2.4 VDC. 12 VDC powered. SKU:RLD-128 Price:$109.95+GST DIN Rail Power Supply 120 W Slim High Efficiency DIN Rail Power Supply takes 88 to 264 VAC / 124 to 370 VDC input and gives 24 VDC out at up to 5 A. Power in and out are connected via screw terminals. A trimpot allows the output voltage to be adjusted approximately ±10%. SKU:PSM-251 Price:$129.60+GST Programmable Bar Graph Display Universal linear input, DIN rail mount programmable tri-colour LED bar graph unit. Programmable 3 digit display with 1 x SPDT relay output. 12 to 24 VDC/AC powered. SKU:CMC-030 Price:$259+GST This is the view inside the completed unit. The two 9V batteries are held together and to the bottom of the case using double-sided adhesive tape. Tighten the mounting nuts of the terminals as firmly as possible, so that they’re held securely in place. Final assembly As shown in the photos, the PCB mounts on the back of the lid and is supported by two M3 x 15mm tapped siliconchip.com.au spacers at one end and by the two output terminal connections at the other. The first step is to fit the two M3 x 15mm spacers to the ‘battery end’ of the PCB. That done, the PCB can be fitted in place, making sure that (1) both switch lugs pass through their matching holes; (2) LED1 passes up Site-Log Progammable Datalogger LPVB-1 is an 7-channel, stand alone programmable voltage data logger, which records up to 4Mb of data and stores it in non-volatile flash memory for later retrieval. DIN rail mount and housed in an alumium enclosure. Battery life up to 10 years. SKU:MED-001 Price:$549.00+GST For OEM/Wholesale prices Contact Ocean Controls Ph: (03) 9782 5882 oceancontrols.com.au March 2014  49 Device Availability Analog Devices make 18 different versions of the AD587, many of them in small outline (SOIC) SMD plastic or CERDIP packages. By contrast, the AD587KNZ and AD587JNZ both come in 8-pin PDIP packages and are quite reasonably priced. Both are currently available in Australia from suppliers such as element14 and RS Components. For example element14 (au.element14.com) has the AD587KNZ (order code 2143134) available for $13.57 plus GST, while the lower-spec AD587JNZ (order code 9605169) costs $9.57 plus GST. SILICON CHIP PRECISION 10V DC REFERENCE Similarly, RS Components (australia.rs-online.com) sells the AD587KNZ (order code 523-7415) for $9.38 plus GST, while the AD587JNZ (order code 412-579) is actually slightly more at $9.58 plus GST. POWER You shouldn’t have any trouble getting the 4541B programmable timer, either. For example, element14 has it (order code 1106124) for less than $1.00. POWER ON through its corresponding hole in the lid; and (3) the binding post spigots pass down through their matching holes in the PCB. The PCB can then be fastened in position using two more M3 x 6mm machine screws which pass through the lid and into the spacers. Once it’s in place, the switch lugs and binding post spigots can be soldered to their respective PCB pads. If necessary, the solder connection on the LED lead can then be melted and the LED adjusted so that it just protrudes through its front-panel mounting hole. The remaining LED lead can then be soldered and the first lead then redone with some fresh solder. Finally, the battery snap leads can be fitted to a pair of new 9V alkaline batteries, after which the batteries can be held together using a strip of doublesided adhesive tape between them. Two more strips of double-sided tape are then used to secure the batteries to the bottom of the case, after which the lid/PCB assembly can be fitted and the lid fastened down using the four countersunk M4 screws supplied. That’s it – your Precision 10V DC 1 Reference Mk.2 is complete. Now for the smoke test. Using it There are no adjustments to be made to the unit, unless (as previously stated) you have access to a highprecision, recently-calibrated DMM to calibrate it against. If you’re not calibrating the unit, you will be relying on the ±5mV or better precision provided by the AD587KNZ chip itself. In that case, check that trimpot VR1 and/or its two associated resistors have been left out of circuit, otherwise the accuracy of the unit will be compromised. Using the Precision 10V Reference is simple – just press S1 to turn the the unit on for about 90s. As soon as you press S1, LED1 should light to show that the unit is operating and providing 10.000V ±5mV at its output terminals, ready for calibrating your DMM or whatever. If you haven’t finished making measurements when LED1 turns off (ie, when the unit unit powers down), it’s simply a matter of pressing S1 again to power it up for another 90s. Rigid PCBs (up to 32 layers), Rigid-Flexi, Flexible & Metal Core 3 PCB Assembly (TH, SMT, micro BGA, QFN) – 10.000V TRIM + Fig.5: this full-size front panel artwork can be laminated and attached using silicone adhesive or double-sided tape. Incidentally, you’ll find that when you first connect the battery snap leads to the batteries, LED1 will turn on to show that the unit is operating. This is normal and is simply due to the way that the 4541B timer chip works. Finally, if you wish to calibrate the unit, make sure VR1 and its associated resistors have been installed. It’s then just a matter of monitoring the output on a 6.5-digit (or better) bench DMM and adjusting VR1 to get a reading as close as possible to 10.00000V (or SC 10.24000V if you prefer). ualiEco Circuits Pty Ltd. 2 Component Procurement Laser Cut SMT Stencil 4 Functional Testing IC Programming 100% Genuine Parts 1300-BUY PCB (1300 289 722) pcb<at>qualiecocircuits.com.au www.qualiecocircuits.com.au 100% Refund Cheapest Price 100% Replacement Guarantee* 1 Year Warranty* 24x7 Support 50  Silicon Chip We will refund 100%, if you are not entirely satisfied with our quality or service* *Conditions Apply siliconchip.com.au