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Review by Nicholas Vinen Keysight 34470A 7.5-digit bench multimeter review This 7.5-digit benchtop multimeter supersedes the venerable 34401A which dates from the Hewlett-Packard era. In addition to its high measurement precision, it has wide measurement ranges, a 4.3-inch colour LCD, standard USB and ethernet ports, high-speed data logging modes, histogram/chart plotting modes and more. A S YOU WOULD expect for a 7.5-digit multimeter, the 34470A has impressive accuracy specifications; its basic DC voltage accuracy is specified as 16 ppm or an error of just 0.0016%. We won’t list all the accuracy figures here as they would take too much space. If you’re interested you can look up the specification sheet at www.keysight.com/de/pd-2520154pn-34470A/digital-multimeter-7-digitperformance-truevolt-dmm The 34401A is a Cat.II unit with the following modes: DC and AC volts, DC and AC current, 2/4-wire resistance, frequency, capacitance, diode test, continuity and temperature. The resolution at its most sensitive settings is 1µV (DC/AC volts), 1nA (DC/AC amps), 50  Silicon Chip 0.1mΩ (resistance), 0.01mHz (frequency), 0.1pF (capacitance), 0.1mV (continuity), 0.001° (temperature). Maximum readings are 1000/700V (DC/AC volts), 10A (DC/AC amps), 1GΩ (resistance), 300kHz (frequency), 100µF (capacitance) and 5V (diode test). For the temperature mode, RTD, PTC, thermistor and thermocouple probes are supported. It has front and rear panel inputs and the active inputs are switched by a latching pushbutton. As with most high-precision multi­ meters, there is an accuracy/speed trade-off. This unit is capable of up to 50,000 measurements per second when it’s used in a data-logging or PCconnected mode but when operated at that rate, it only reads four digits. To get the full 7.5 digits of precision, it needs to average its readings over 10 power line cycles, so with our 50Hz mains that means five readings per second. Of course, that’s more than adequate when you’re using it as a DMM. In fact, in any mode, you can select the sampling rate as either a multiple of power line cycles or in milliseconds and the unit will then adjust the number of digits displayed to reflect how accurate the readings are under that condition. You also get the choice to override the number of digits displayed; eg, to eliminate last digit “bobble” when you simply don’t need that much precision. Having said that, in 7.5-digit mode, the last digit is usually pretty siliconchip.com.au stable with only a little noise evident. This unit features Keysight’s “TrueVolt” technology which uses highspeed digital sampling to compute true RMS values. This allows for accurate measurements with a crest factor of up to 10, compared with a typical figure of only three for devices which do the RMS conversion in the analog domain. They also claim significantly lower input bias current (25pA) and noise (~50nA) than their competitors, for accurate readings on the lowest ranges. For voltage readings on the 100mV, 1V and 10V ranges, the input impedance can be increased from 10MΩ to 10GΩ which will result in much more accurate readings when the source impedance is high. Also, when sensing small currents, it will withstand up to 3A; some multimeters limit you to 1A or 500mA under such conditions and it’s easy to accidentally blow the fuse. Advanced features That’s all pretty standard, albeit impressive stuff. What we’re more interested in though are the extra features that they’ve built into the unit, along with some simple niceties that make your life easier when using it. As an example of the latter, in most modes the default is for the single reading to dominate the display (see lead photo). But you can also select a second reading to be displayed in a smaller font below the main reading (the main reading will shrink slightly; see Fig.1). For example, in DC volts mode, the second reading can either be AC volts (ie, ripple), minimum/ maximum/peak-to-peak readings or the “pre-math” reading (eg, if you have nulled out an offset, this reading includes the offset). Other modes offer similar options. The Display menu allows you to enable a bargraph display which appears below the numeric reading(s) in any mode (see Fig.2). This is very convenient for quickly being able to see how a value is changing over time and its response time is pretty fast; up to 10 updates per second. You can also enable a statistics display which shows the number of samples collected, minimum, average, maximum, span (ie, peak-to-peak) and standard deviation. In fact, the display is large enough that you can enable the second reading, bargraph and statistics all at the same time and they are all legible as long as you siliconchip.com.au aren’t too far away from the unit. Or you can move the measurement into the top lefthand corner of the screen and the centre is then dominated by a trend chart (Fig.3) or histogram (Fig.4). The trend chart mode is like having an oscilloscope with a very long timebase, although unlike a scope, the readings can be AC voltage, current, resistance etc. The timebase can be set to give a full-screen span of 1, 5, 10, 30 or 60 minutes. You can also temporarily switch to a display which shows all readings taken so far. The vertical scale can be set manually or automatically determined, based on the range of readings so far. Data is shared between trend and histogram mode, so you can switch back and forth between them without losing data. And in histogram mode you can either have automatic or manual binning (ie, the extents of the horizontal axis). There are also options for cumulative mode and for cursors. Importantly, you can save the captured data at any point, either into internal storage or to a USB flash drive plugged into the front panel. Fig.1: a voltage reading from our 2.5V reference project (presented elsewhere in this issue) in full 7.5-digit precision mode. A second reading, in this case AC voltage, is displayed below although this appears to be somewhat exaggerated as the noise voltage is so much less than the DC reading. Additional modes Fig.2: a mains voltage reading with frequency shown as the second measurement, along with the bar­ graph display. The bargraph makes it easy to see rapidly changing measurements. Then there are the “math” modes which include measurement scaling. These allow readings to be shown in dB, dBm, % or Mx-B with adjustable reference values. There’s also the usual nulling offset adjustment plus the option for a smoothing filter with a response time of 10, 50 or 100 readings. One very simple but clever feature that we think will really come in handy is “probe hold”. This can be activated in any mode and it waits for the reading to become stable, then stores it in a list of up to eight readings. If you then probe a different part of the circuit and get a different reading, once it stabilises it too will be added to the list. This means you don’t have to constantly swap back and forth between probing the circuit and making notes. Unlike a handheld DMM, bench multimeters typically have various triggering options in addition to the normal “auto” triggering where sampling is continuous. This one supports external triggering as well as level-based (like a scope), single triggering (manual) or auto with a pre-determined sampling rate. It can be configured to delay sampling from the trigger event and can also be set The lower-cost model 34461A is a 6.5-digit version of the unit reviewed here. It has less memory and a slower update rate than the 34470A but has most of the latter’s features. Refer to Keysight’s website for further details. August 2015  51 equally to other bench multimeters. By the way, we should point out that this unit takes a few seconds to “boot up” and calibrate; something that we’re now pretty much used to with DSOs but not so with multimeters. And a further minor issue: when you go to save a screen grab or data series, the default name is always the same so you have to manually change it, rather than the software picking a new sequence number for you each time. The rear panel of Keysight’s 34470A bench multimeter carries a wide range of input and output sockets. Note that the GPIB interface is an optional extra. to produce multiple samples per trigger event. Connectivity The front panel USB port allows screen grabs, logged data or settings to be saved to a flash drive. Logged data is saved to a CSV (comma separated variable) file which can also include the date and time that logging started. Unfortunately, the mode and capture rate are not included so you need to note that separately if you want to Fig.3: a trend plot of the AC mains voltage in our location over a 5-minute period. This shows ~30s variations in voltage which is not apparent look­ing at the waveform on a scope. It’s prob­ably due to some nearby industrial equipment cycling on and off. Fig.4: a histogram of the same AC mains voltage data shown in Fig.3. The number of samples is shown along with the percentage represented by each “bin”. The bins can be chosen automatically based on sampled values or else the number and span can be entered manually. 52  Silicon Chip plot the data as a time series. The unit can also be controlled by a PC and data transferred using the rear panel USB socket or it can be done over an Ethernet network. Either way, it’s done using the “BenchVue” software which is available as a free download for Windows, Android or iOS. Note though that some of the advanced features such as histograms and unrestricted data logging require the purchase of the “Pro” version. The 34470A has a backwards compatibility mode which allows software written for the 34401A to work without (or with few) modifications. The 34401A was the standard instrument for so long that many contracts and specifications will be written with the 34401A in mind, so this is an important feature to give such users an upgrade path; especially since 34401A production will end in 2016, 24 years after its launch. Minor issues It’s hard to fault the basic operation of this unit, ie, reading voltage, current and resistance. However, some of the other functions seem like a bit of an afterthought. For example, only being able to read signal frequencies up to 300kHz is rather limiting; many DMMs will go up to 10MHz or 20MHz. Similarly, the maximum capacitance reading of 100µF is a little low, although having said that, it will read very low values (<1pF) which is quite useful. And while the diode test mode is great for checking LEDs, its 5V limit means it isn’t much use for checking low-voltage zener diodes. A few DMMs have test voltages of around 20V which does allow this. Of course, these are all relatively minor issues and will apply more or less Options & accessories Our review unit was supplied with some options installed. These include the capability of 50,000 readings per second (5000/s standard), two million readings memory (50,000 standard) and an optional GPIB interface. While the review unit is a 7.5-digit type, three similar 6.5-digit models are also available: the 34460A, 34461A and 34465A. These differ in their basic accuracy, sampling rates, memory sizes, measurement ranges and capabilities. The 34465A is essentially identical to the 34470A except for being a 6.5-digit type with slightly worse accuracy figures, while the others lack some features (refer to the Keysight website for more details). Conclusion The 34470A is a worthy replacement for the 34401A. It brings greater accuracy and precision, a much better display and many useful new features. We particularly like its data logging and display modes, especially the fact that you can observe the data while capturing it and then off-load it to a PC for analysis without needing to connect the two directly. The display and its flexible configuration are particularly nice to work with and the user interface is pretty easy to figure out. In essence, this type of unit brings the capabilities of the bench-top multimeter more in line with that of a modern DSO with screen-grab, data capture and display and convenient filtering and statistical analysis features. The 34470A is available from Trio Test & Measurement (www.triotest. com.au) for $3420+GST. The 34465A is $1651+GST while the less-capable 34461A and 34460A are $1298+GST and $1118+GST respectively. For more information or to place an order, call Trio on 1300 853 407 or e-mail sales<at> SC triotest.com.au siliconchip.com.au