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Low-cost, Accurate Voltage/Current/ Resistance Reference This small module is based on a lithium coin cell, a voltage reference IC, a precision resistor and little else. It provides a reference voltage of 2.5V±1mV (±0.04%), a resistance of 1kΩ±1Ω (±0.1%) and a current of 2.5mA±3.5µA (±0.14%). It can be used for checking or calibrating multimeters or anywhere that an accurate and stable voltage is required. By Nicholas Vinen How accurate are your multi­ meters? This accurate Voltage/ Current/Resistance Reference is ideal for checking and calibrating multimeters on a regular basis. T HIS SMALL module can be kept with your multimeter or other test instrument and used to periodically check its calibration. With occasional use, the battery will last for its shelf life which is normally at least 10 years for a fresh cell. It can sink or source up to 10mA so the accuracy of the reference voltage is not affected by bias currents and a divider can be connected across the outputs to provide lower reference voltages, as long as its impedance is at least 250Ω. For example, this would allow it to be used in combination with our Lab-standard 16-Bit Digital Potentiometer from the July 2010 issue to give an adjustable reference voltage from 0V to 2.5V in 38µV steps. It could also be hooked up to a microcontroller to be used as an analog-to-digital converter (ADC) reference voltage, for accurate voltage measurements by the micro. This project effectively supersedes the Precision 10V Reference published in the March 2014 issue (and the one from May 2009 too). While this one is not adjustable and its output voltage is lower, its basic accuracy is better, it’s much smaller and cheaper to build, uses a much smaller (and cheaper) battery and the previous projects did not offer the resistance or current references. Circuit description The full circuit is shown in Fig.1 and there isn’t much to it. IC1 is the Maxim voltage refer40  Silicon Chip siliconchip.com.au IC1 MAX6071 (1.25V,1.8V,2.048V,2.5V) 4 VIN IOUT OUTS 5 1k 0.1% 2.2k OUTF 6 4.7 µF 6.3V ON LED1 OUT+ BANDGAP VOLTAGE REFERENCE 4.7 µF 6.3V A GNDF 1 3 λ OUT– GNDS 2 EN K ON SWITCH S1 D1 1N4148 (OPTIONAL, SEE TEXT) D G K BATTERY1 3V Q1 IRLML6344 100Ω 4.7 µF 6.3V S 1N4148 10M CATHODE BAND A A K MAX6 0 71 20 1 5 VOLTAGE/CURRENT/RESISTANCE REFERENCE Fig.1: the circuit is based on a MAX­6071 2.5V precision voltage regulator. Mosfet Q1 switches power to the circuit for 15-20s when ever pushbutton switch S1 is pressed. ence which contains a band-gap circuit and precision op amp with trimmed resistive divider. The band-gap circuit measures the voltage across a couple of PN junctions and incorporates temperature compensation so that its output is stable (typically just 1.5ppm change per degree Celsius). The band-gap reference produces 1.25V and the internal op amp and resistors provide a suitable gain to give the specified output. In this case, we’re using a 2.5V reference, although other values are available and can be substituted. We’re using 4.7µF input bypassing and output filtering capacitors for a stable output voltage. LED1 and its series current-limiting resistor are connected across the reference’s supply so that the LED lights while ever the reference is powered. Mosfet Q1, together with pushbutton S1 and the RC network, switches power to the reference for a limited time, so that the cell won’t be accidentally discharged. When S1 is pressed, a third 4.7µF capacitor charges from the 3V battery supply via a 100Ω current-limiting resistor. This capacitor is connected between Q1’s gate and source terminals so when it charges up, Q1 switches on siliconchip.com.au 6 5 D 3V VERSION G K A IRLML6344 SC  LED 1 S 4 2 3 Features & Specifications: 2.5V Version Reference voltage: 2.5V±1mV, 0-10mA sink/source Reference current: 2.5mA±1.4µA, 1kΩ source impedance Reference resistance: 1kΩ±1Ω, 1/8W Power supply: 3V lithium button cell Operating current: ~600µA Standby current: <1µA Cell life: typically >10 years with intermittent use Other features: auto-off (20s), power indicator LED, compact size and connects the reference ground to battery ground, thus switching it on. A 10MΩ resistor across this 4.7µF capacitor discharges it over the course of about 15-20 seconds and once its voltage drops low enough, Q1 switches off and current flow from the battery ceases. Thus, S1 is pressed before the reference is used and provides power for long enough for a measurement to be taken. Total current draw is around 0.6mA when the reference is powered (150µA for IC1 and 450µA for LED1) and Q1’s leakage current when off is less than 1µA. The output reference voltage is available between the OUT+ and OUT– pads on the PCB. A 0.1% 1kΩ precision resistor is connected between OUT+ and IOUT and so resistance calibration can be performed between these two terminals. Together, the voltage reference and precision resistor provide an accurate 2.5mA current between the IOUT and OUT– terminals. The separate calibration article in this issue describes how measurement shunt resistance can affect this current. Note that if all you want is a voltage reference, you can leave the 0.1% resistor out of circuit. Some button cell holders (including the type Jaycar stocks) will not apply power to the circuit if the cell is inserted upside-down. However, some do but we can’t use a series diode for reverse polarity protection as we normally would, since IC1 requires a minimum of 2.8V to operate and even a Schottky diode would reduce the 3V from the cell by too much. Thus, an optional 1N4148 diode (D1) can be reverse-connected across the holder to provide protection in case the cell is accidentally inserted backwards. The internal resistance for a CR2032 August 2015  41 IC1 MAX6071 (3V,3.3V,4.096V,5V) 4 VIN IOUT OUTS 5 1k 0.1% 2.2k OUTF 6 4.7 µF 6.3V 4.7 µF 6.3V A ON LED1 OUT+ BANDGAP VOLTAGE REFERENCE GNDF 1 3 λ OUT– GNDS 2 EN K ON SWITCH S1 D Q1 IRLML6344 100Ω G BATTERY1 6V 4.7 µF 6.3V 3 S 2 LED CATHODE BAND D2 BAV99 10M 1 BAV99 3 K 1 A MAX6 0 71 IRLML6344 SC  20 1 5 VOLTAGE/CURRENT/RESISTANCE REFERENCE Semiconductors 1 IRLML6344 N-channel Mosfet, SOT-23 package (Q1) 1 1N4148 small signal diode (D1) Capacitors (SMD 3216 [1206] or 2012 [0805]) 3 4.7µF 6.3V X5R/X7R ceramic Resistors (1% SMD 3216 [1206] or 2012 [0805]) 1 10MΩ 1 2.2kΩ 1 1kΩ 0.1% 2012/0805 (eg, element14 1506077) 1 100Ω Additional parts for versions up to 2.5V output 1 20mm button cell holder (Jaycar PH9238, Altronics S5056) 1 CR2032 3V lithium cell 1 MAX6071AAUT25+T 2.5V reference IC* (IC1) 1 high-brightness red, green or yellow LED, SMD 3216 (1206) or 2012 (0805) package (LED1) (eg, element14 2290347) 42  Silicon Chip 6 5 D 6V VERSION G S 1 4 2 3 1 1N4148 small signal diode (D1) Fig.2: this alternative circuit is used for output voltages of 3V or more. It’s powered by a 2-cell (6V) battery and diode D2 is included to reduce the supply voltage to 5.5V. * OR 1 MAX6071AAUT12+T for 1.25V output 1 MAX6071AAUT18+T for 1.8V output 1 MAX6071AAUT21+T for 2.048V output cell is typically 10Ω so if your holder does allow a cell to make contact upside-down, D1 should survive long enough for you to realise your mistake and protect IC1 from damage. Parts List 1 double-sided PCB, code 04108151, 44.5 x 23mm 1 tactile pushbutton with short actuator (Jaycar SP0600, Altronics S1120) 1 50mm length 20mm-diameter clear heatshrink tubing 2 Additional parts for versions over 2.5V output 1 dual 20mm button cell holder (element14 3029827) plus 2 x CR2032 3V lithium cells OR 1 20mm button cell holder (Jaycar PH9238, Altronics S5056) plus 2 x CR2016 3V lithium cells 1 MAX6071AAUT50+T 5V output reference IC** (IC1) 1 high-brightness blue LED, SMD 3216 (1206) or 2012 (0805) package (LED1) (eg, element14 2217982) 1 BAT54S or BAT54C dual SMD Schottky diode, SOT-23 package (D2) ** OR 1 MAX6071AAUT30+T for 3V output 1 MAX6071AAUT33+T for 3.3V output 1 MAX6071AAUT41+T for 4.096V output Different output voltages IC1 can be changed to a 1.25V, 1.8V or 2.048V type with no other changes to the circuit. This is simply a matter of using an IC with a different part number (see the parts list). We have chosen 2.5V as the “default” option since this is the highest reference voltage obtainable using a single lithium cell. However, 1.8V is also a good choice as many low-cost DMMs have a 2V range and thus this will be ideal for calibrating them. The 2.5V option works well for meters with a 4V range, which is quite common for more expensive multimeters. You can also get an output of 3V, 3.3V, 4.096V or 5V but this will require a 2-cell battery to provide a sufficiently high input supply voltage. You have two options: either use a standard button cell holder and two slim cells (CR2016, ~100mAh) or use a doublestack cell holder and two of the more common CR2032 cells (~200mAh). There are two advantages to using siliconchip.com.au Construction Most of the parts are SMDs and all but one have widely-spaced connections, making them easy to solder. The only slightly tricky one is IC1 but it really isn’t that hard. It’s best to solder the SMDs first, starting with IC1, before finishing with the through-hole parts. Refer to the appropriate overlay diagram – Fig.3 for outputs of up to 2.5V and Fig.4 for higher voltages. First, it’s a good idea to clean the PCB by swabbing it with a little alcohol (eg, methylated spirits) and a lint-free cloth. Also, applying flux to the SMD pads before soldering will make the job easier. Melt a little solder onto one of IC1’s six pads, then place the IC alongside and inspect it under magnification. There will be a small dot laser etched on top. This is the pin 1 marker and it goes towards the dot in the lower-right corner of the PCB. Orientate IC1 as such, then heat the solder you added earlier and slide the chip into place using angled tweezers. If it appears that IC1 is correctly placed, gently press down on the chip using the tip of the tweezers while heating the solder pad to ensure that it is sitting properly on the PCB. Then check under magnification that all six leads are centred over their pads. Once it’s in place, solder the leads on the opposite side (don’t worry about bridging them), then go back and solder the three on the other side, including the one you tacked down earlier. Add some more flux, then clean up the joints using some solder wick. This will remove any bridges and should also ensure that a proper fillet has formed for each pin. Remove any flux residue using alcohol or a siliconchip.com.au 04108151 4 µ7 4 µ7 4.096V 2.500V 2.048V 1.250V D2 Q1 OUT– 4 µ7 STACKED BUTTON CELL HOLDER IC1 1.8V 4 µ7 4.096V 2.500V 2.048V 1.250V 3V VERSION (OPTIONAL DIODE D1 UNDERNEATH) 6V VERSION Fig.3: follow this PCB parts layout diagram to build the versions with outputs up to 2.5V. Fig.4: this is the layout for the 3V to 5V versions. It includes diode D2 and a 2-cell holder. These two photos show an assembled 2.5V version at left and a 5V version at right. The white screen-printed squares on the PCB let you mark the selected output voltage. It’s a good idea to cover the completed assembly in clear heatshrink tubing. Diode D1 in the 3V-powered version is optional. It can either be soldered across the battery holder on the underside of the PCB as shown at left (cathode to positive) or it can be left out as shown at right (see text). proper flux solvent and then inspect with magnification to ensure all leads have been soldered properly. You can then move on to Q1 and, if you are building the 6V-powered version, diode D2. These are easier to solder as their leads are much further apart. As before, tack one lead down first, then check that the device is flat against the PCB and that its leads are properly lined up with the pads before soldering the remaining pins and refreshing the first one. Be careful when fitting D2 as two of the pads are quite close together and easy to accidentally bridge. If you are not fitting D2 then these two pads should be shorted, either with a solder bridge or a very short length of wire (eg, made from a component lead off-cut). You can now fit the resistors and capacitors in a similar manner, as shown in Fig.3 or Fig.4. The resistors will have their values marked on top (eg, 1001 = 1kΩ, 222 = 2.2kΩ), while the capacitors will be unmarked. The last SMD is LED1 but you will have to check its orientation first. Set a IOUT OUT+ 4 µ7 04108151 S1 3V 5V 3.3V 10M + OUT+ IC1 1.8V IOUT 2.2k BUTTON CELL HOLDER LED1 A 100Ω 4 µ7 + 2.2k Q1 S1 3V 5V 3.3V 10M 1k WIRE LINK 1k LED1 A 100Ω CR2016: (1) you can get the holder and cells from a local store (eg, Jaycar) and (2) the resulting unit is a little more compact. Unless you will be using the unit frequently, the reduced cell capacity probably won’t matter. Regardless, when using two cells, diode D2 will need to be fitted. That’s because IC1’s maximum recommended operating voltage is 5.5V and D2’s forward voltage will reduce the ~6V from two fresh cells to be very close to 5.5V. The alternative circuit is shown in Fig.2. With D2 in circuit, there’s no need to fit D1 as D2 will block reverse current. Otherwise, the circuit remains the same. DMM to diode test mode and connect the probes to either end. If it lights up, the red probe will be on the anode and this goes in the corner of the board. Try to avoid heating it up too much as this can damage the LED. If it doesn’t light up in either orientation, your DMM may not put out enough voltage in which case you’ll have to use a small battery with a current-limiting resistor to determine the anode. Once LED1 has been fitted, solder the tactile pushbutton and cell holder in place. In both cases, push them down hard to make sure they are flat on the PCB before soldering their pins. The cell holder will have three plastic posts which go through matching holes in the board. You may have to push fairly hard to get these to go in. Optional diode D1 Finally, if building the 3V-powered version, you can flip the board over and solder the 1N4148 diode in place as shown on the above photo. Alternative, you can leave this out if you’re confident that you will always install the cell with the correct polarity. We’re August 2015  43 OUT– Using This Board With An Arduino not sure whether IC1 would survive a reversed cell; it might, due to the cell’s internal resistance limiting current but we haven’t been game to test this. Finishing it up Before placing the unit in its protective heatshrink sleeve, check that it’s working properly. First you need to insert the cell (or cells). Check the polarity markers on the holder and cell(s) and then slide them into place. Next, press S1 and verify that LED1 lights up, then goes out about 20s later. Note that if you touch the back of S1, your skin resistance can be enough to cause the unit to turn on briefly (this will be prevented once the heatshrink is in place). If LED1 does not turn on, it may have been fitted backwards or there could be a soldering problem. Press S1 and measure the voltage across LED1; if it is 2V or more, then LED1 is suspect, otherwise voltage is not getting to it for some reason. Assuming LED1 lights up, measure the voltage between OUT+ and OUT– and verify that it’s within specifications. If it seems low, press S1 again to ensure Q1 is fully on. Now is also a good time to use a marker pen to indicate which output voltage has been selected by marking one of the rectangles provided on the PCB silkscreen. If you’ve fitted the 1kΩ resistor you can now check its resistance (between 44  Silicon Chip WIRE LINK Q1 ENABLE FROM MICRO/ ARDUINO OUTPUT 04108151 3.3V/5V FROM MICRO/ ARDUINO 3V 5V 3.3V VOLTAGE/ RESISTANCE/ CURRENT REFERENCE TO AREF 4 µ7 GND FROM MICRO/ ARDUINO IC1 4 µ7 Fig.5: here’s how to interface the unit to an Arduino for accurate ADC measurements. Note that you need to cut one of the PCB tracks. 101 If you’re going to use this board with an Arduino, you can omit some of the parts. You certainly won’t need the cell holder or pushbutton switch as power will come from the Arduino board itself. You could also leave off Mosfet Q1 and short it out if you don’t need the micro to be able to switch the reference voltage on and off. For now though, we’re assuming this is useful, so Fig.5 shows how you can wire it up. The reference IC runs off 5V from the Arduino, which means you can’t use the 5V reference but any of the others should be OK. The “enable” line can be driven from one of the micro’s outputs to turn the reference voltage on and off if required, or tied to the 5V rail to leave it permanently on. Note the top layer track cut. This is important for maximum accuracy because without it, some of the supply current for the 1.8V TO AGND 4.096V 2.500V 2.048V 1.250V CUT TRACK (TOP SIDE) ‘AREF’ VERSION FOR A MICRO OR ARDUINO reference could flow via the analog ground connection and cause a voltage drop across it, which would reduce the voltage seen by the micro’s AREF pin. When writing software for the micro, keep in mind that you will probably need to tell the ADC to use the AREF input as its voltage reference, rather than its AVDD supply rail voltage. Its full scale reading (eg, 1023 for a 10-bit ADC) will then indicate a voltage equal to (or just slightly less than) the new reference OUT+ and IOUT) and verify the expected current by connecting a DMM set to measure milliamps between IOUT and OUT-. Note that the reading may be a little lower than expected; see the article on multimeter calibration in this issue for an explanation. Now it’s just a matter of sliding the clear heatshrink tubing over the unit and shrinking it down. Don’t cover the test terminals right at the end of the board, although it’s a good idea to insulate everything else. You can cut off any excess after shrinking. Note that if using the double-stack CR2032 cell holder, the tubing will be a tight fit but we managed to get it onto our prototype unit OK. You’re now ready to check and/or calibrate your multimeter(s) – see the accompanying article for details on doing this. Other uses This voltage reference may also be useful to allow very accurate voltage measurements to be made by microcontrollers, including those on Arduino boards. The ADC in a microcontroller needs some sort of reference voltage. This is usually either its supply voltage (5V or 3.3V) or an internally generated reference. However, the internal reference is usually pretty inaccurate (±0.1V is typical) so in most cases you’re better off using the supply voltage instead. voltage, rather than the 5V reading it would have indicated previously. This means that you may need to re-scale the results to suit the new ADC reference voltage. Note that, if using the enable feature, the AREF pin will be pulled near the positive supply input when the reference is disabled. If the micro is running off 3.3V, it’s likely it will not tolerate 5V on this pin, so be sure to either run the reference off the 3.3V supply or leave it permanently enabled. This also has the advantage that any voltage up to the supply voltage can be measured using the ADC. However, you are then at the mercy of the accuracy of the regulator providing this supply. It may have a stated error of less than 1%; for example, the MCP1700 low-dropout linear voltage regulator has a typical tolerance of ±0.4%. However it isn’t uncommon for a linear regulator to have a much larger output voltage error such as ±2% or even ±5%. You also have to consider noise which may be injected into this rail from other devices drawing power in bursts, which can add an extra layer of uncertainty to ADC measurements. It’s much better to use an accurate voltage reference, normally fed into a dedicated pin on the micro (labelled something like “AREF”). This will be free of noise and has the potential to have a much better defined voltage. Note though that if you expect to make accurate measurements using an ADC fed with such a reference voltage, you will also need to make sure that any voltage dividers feeding ADC inputs use resistors with accurate values or that you have the ability to trim them. You will also need to keep the source impedance for the ADC inputs low, ie, don’t use high values in the divider. If in doubt, check the microconSC troller’s data-sheet. siliconchip.com.au