Keysight Technologies How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software Application Note
02 Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note When you want to capture state changes in a device under test (DUT), you need to simultaneously measure different signals as fast as possible. This presents several problems: How do you determine the sample rate of all the instruments? How do you achieve precise sample timing? How do you get the results into readable form? BenchVue software from Keysight Technologies, Inc. provides a way to address these problems enabling you to be productive quickly without having to write a computer program. Choosing the Right Instruments Once you know how fast the signal to capture is changing, you can choose the right instrument for the job. An oscilloscope can capture a voltage waveform at virtually any frequency, but with only 8-bits of resolution, this is good for only about 2½ digits. If you need finer voltage resolution, a scope cannot be used. If you need to measure current, you will need to use a sense resistor in series with one lead, and use the scope to measure the voltage across it, or use a differential measurement using two scope channels. You could also use a current probe, but they can be expensive and hard to find. A data acquisition (DAQ) instrument, such as the Keysight 34970A or 34972A, can also be used since it has a digital multimeter (DMM) with a choice of plug-in switch cards, which allows you to take different types of readings on each channel. It can also be reconfigured quickly and easily. However, the top speed including channel switching is only 60-240 channels per second depending on the switch card. If the signal you are trying to digitize is faster, a DAQ device won t work. Digital multimeters are useful and relatively inexpensive instruments. One DMM can be set up to measure voltage, another one current, and they can be set up to take readings at exactly the same time. Measurement accuracy of up to 6½ digits is common, and DMMs can read AC and DC voltage, current, resistance, frequency and more. As floating instruments, they can do this without having to have one lead at ground potential. They can sample very quickly without having to be reconfigured, called digitizing, but the resolution is reduced accordingly and may be as low as 3½ digits.
03 Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note Let s take a simple example. You want to measure both voltage and current of a signal driving a load. Figure 1 shows how you would typically set this up. Your DUT generates a stimulus signal that drives a load. One DMM is inserted in the path to measure current, and the other is connected across the load to measure voltage. Let s suppose that the DUT is generating a repetitive exponential rise with a 125 ms period. DMM1: voltmeter Device under test DMM2: ammeter Load Measurement Tip For number of power line cycles (NPLCs) of 1, 10 and 100, the sample time is actually based on internally measured power line frequency. In countries with 60 Hz power, one power line cycle is 16.67 ms, and in countries with 50 Hz power, one power line cycle is 20 ms. These apertures are used to cancel out line-related noise. When higher speeds (and resulting lower accuracy) are required, a PLC less than 1 is specified. For these, aperture time, though specified in NPLC, is actually an approximation, not a precise time. Figure 1. Measuring both voltage and current of a signal driving a load. Measurement Speed The fastest measurement period ( aperture ) of the 34461A DMM is 0.02 power line cycles (PLC), which is 400 µs, however the DMM limits the sample rate to a much longer time. In fact it is about 1 ms, or 1000 readings per second. Some other Keysight DMMs can measure much faster. The 34465A, for example, has an optional aperture time as low as.001 PLC, or 20 µs, with no additional overhead, which allows a top reading speed of 50,000 readings per second, which is equivalent to a 50 KHz sampling rate. Without the digitizer option, the 34465A can take up to 5,000 readings per second. To avoid having the interface bus slow down the reading rate, measurements are often stored in the DMM s memory, then read back to the PC later. Some DMMs can take readings and pass them to the interface at the same time without affecting the sample rate. There are other measurement options that can slow down readings: autorange, autozero, and in Ohms mode, offset compensation. All of these cause extra readings to be taken to improve the accuracy of the measurement. When digitizing a signal, these are not generally needed and should be turned off.
04 Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note Precise Timing Triggering DMMs start taking measurements when triggered. A trigger can be generated internally, externally, or via a software command over the chosen interface (USB, LAN or GPIB). The software command is called a bus trigger, and there will be processing delays depending on when the command is received by the instrument and how long it takes its internal processor to decode and act on the command. Internal (also called immediate) triggers let the instrument free-run at its own speed, so two free-running DMMs could be off by as much as one measurement cycle after triggered. The only way to achieve truly precise timing is to use an external trigger, usually in the form of a TTL-compatible pulse applied to a BNC input connector on the back panel (typically labeled Ext Trig ). Once set up, a pulse with a width of at least 1 µs starts the reading cycle. The trigger system lets you specify the rising or falling edge of this signal. Multiple instruments can be started by connecting all external trigger inputs together and tying them to a common trigger source, such as a function generator set to output pulses. The DMMs can be set up to take more than one sample per trigger if desired. VM Complete When a DMM finishes a reading, it outputs a pulse to a BNC connector on the back panel called voltmeter complete (typically labeled VM Comp ). If no external trigger source is available, the VM Comp signal from one instrument can be used as an external trigger for any others; this will result in a timing delay for the other instruments of exactly one measurement cycle. In many cases, this may be sufficient to satisfy the definition of at the same time.
05 Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note Test Setup Let s see how to set this measurement up in BenchVue. Accessing the digitizer functionality in BenchVue requires the Pro license. Also, this is an advanced measurement, so some front panel interaction is also needed. For example, when you select External Trigger, BenchVue enables the DMM to capture the specified sample count at its maximum rate once a single trigger arrives. So we need one and only one pulse from the function generator. If you use two identical DMMs, such as two 34461As, the sample times at this maximum rate will be virtually identical between the instruments. But if one DMM, for example, a 34465A, which has a specified maximum standard reading rate that is five times faster than the 34461A, the readings will not occur simultaneously. Therefore, if you want to pace the readings from an external source to keep them in sync, the DMMs cannot be started from BenchVue. Also, the Scope App has no way of setting Normal trigger mode, which is essential if you want to capture the waveforms as they happen. But there is a way to work around these limitations. First let s connect the cables. See Figure 2. A function generator is set up to externally trigger both DMMs. Although any interface can be used, USB 2.0 was chosen. (USB 3.0 connections, marked SS next to the USB connector, have been shown to be unreliable for instrument I/O depending on motherboard, O/S, USB driver and cabling.) A scope is connected to capture the various signals just for purposes of this application note to show how the trigger information is captured. Oscilloscope: monitors signals Function generator: external trigger source DMM1: voltmeter PC connections: USB Device under test DMM2: ammeter Load For DMMs, enable USB SCPI & file access Figure 2. Instrument connections. A function generator generates external triggers to the two DMMs, one set up to measure voltage and the other to measure current. The scope is only used to monitor signals to show how the timing works.
06 Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note Now let s start BenchVue 2 and launch the oscilloscope, function generator, and two DMMs. See Figure 3. First select the 4-pane view. Figure 3a. Selecting 4-pane view. Figure 3b. All four instruments launched in BenchVue. The position of instruments can be changed as desired by dragging and dropping the instrument icons along the instrument bar at the bottom of the screen. Figure 3c. To see all the setup information, we can click the maximize icon within any app. Set up the two DMMs first, one for voltage and one for current. To maximize reading speed, turn off Auto Zero, select a fixed range, set the Trigger Delay to 0, and pick the fastest aperture of.02 NPLC. Enter 1000 for the Sample Count, and select External Trigger. See Figure 4. Figure 4. DMM setups. Both DMMs are set up as digitizers with 1000 samples, fastest aperture (.02 NPLC), fixed range and Auto Zero off.
07 Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note Next select the Function Generator App and set it to generate one TTL pulse after a 2 second delay. See Figure 5. Figure 5a. Function generator setup for use as an external trigger to DMMs. This setup outputs one TTL-compatible pulse after a 2 second delay when started. Figure 5b. Manual trigger mode generates a bus trigger to this instrument when either BenchVue s Start button is clicked or the instrument s front panel Trigger button is pushed.
08 Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note Synchronizing the readings to the signal It may be advantageous to start the measurements in response to a signal from the DUT. To do this, you could set the function generator up with its own external trigger. This could be as simple as turning on a 5 V power supply, or perhaps you have access to a timing signal that occurs when you need to take measurements. In this example, it was started manually from BenchVue. Start the measurement cycle A 2 second trigger delay is chosen so that the function generator does not start at the same time as the DMMs or the oscilloscope. This gives the DMMs and the scope plenty of time to initialize. When Start All is clicked, both DMMs enter the Wait for Trigger state and the scope goes to SINGLE mode. After the 2 second trigger delay, the function generator outputs the trigger pulse. Once the readings are done, BenchVue reads in the data and displays the results graphically. Figure 6 shows the DMM readings, and Figure 7 the triggering results. Figure 6. DMM readings returned to BenchVue. Figure 7a. Triggering results, showing that one external trigger started the operation. The two DMMs, because they are the same type of instrument, measured two parameters at the same time. Figure 7b. The data can now be exported to Excel by selecting Export All at the top right of the screen.
09 Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note However, the x-axis (time) values consist of calculated times based on starting and ending timestamps,, not the actual 900 µs sample interval that was achieved. There is no way to know this exact timing without measuring it and then manually placing the correct time intervals into the Excel file. Let s now run the same experiment using one 34461A and one 34465A. As shown in Figure 8, the slower DMM samples at 900 µs intervals, while the faster one samples five times as fast. The faster DMM will therefore finish gathering 1000 readings in 1/5 the time of the slower one. But notice that the representation of the waveform is much more accurate using the faster digitization. So, in this case, two 34465A DMMs would give us a much more accurate picture of what s going on. Figure 8. Two DMMs sampling at different rates. The signal being digitized is shown in yellow. The 34461A s VM Comp output is shown in green, the 34465A s in blue. A single external trigger is shown in pink. Conclusion This application note has described how to measure two signals at the same time using BenchVue. By extension, you can see that many more signals could also be measured and captured, all without having to write any software. In fact, although only four panes are visible at any one time in BenchVue s main window, they can be popped out (moved into their own separate windows), allowing many instruments to be controlled at once. BenchVue streamlines the process of using instruments, making you more productive. BenchVue Software Download BenchVue software today for free. If you re interested in the Pro apps, which extend datalogging time beyond the 1 hour available in the free version and add the digitizing capabilities described in this application note, contact your sales representative or order online through an authorized Keysight distributor. www.keysight.com/find/benchvue
10 Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note From Hewlett-Packard through Agilent to Keysight For more than 75 years, we ve been helping you unlock measurement insights. Our unique combination of hardware, software and people can help you reach your next breakthrough. Unlocking measurement insights since 1939. For more information on Keysight Technologies products, applications or services, please contact your local Keysight office. The complete list is available at: www.keysight.com/find/contactus Americas Canada (877) 894 4414 Brazil 55 11 3351 7010 Mexico 001 800 254 2440 United States (800) 829 4444 1939 THE FUTURE Asia Pacific Australia 1 800 629 485 China 800 810 0189 Hong Kong 800 938 693 India 1 800 11 2626 Japan 0120 (421) 345 Korea 080 769 0800 Malaysia 1 800 888 848 Singapore 1 800 375 8100 Taiwan 0800 047 866 Other AP Countries (65) 6375 8100 Europe & Middle East Austria 0800 001122 Belgium 0800 58580 Finland 0800 523252 France 0805 980333 Germany 0800 6270999 Ireland 1800 832700 Israel 1 809 343051 Italy 800 599100 Luxembourg +32 800 58580 Netherlands 0800 0233200 Russia 8800 5009286 Spain 800 000154 Sweden 0200 882255 Switzerland 0800 805353 Opt. 1 (DE) Opt. 2 (FR) Opt. 3 (IT) United Kingdom 0800 0260637 For other unlisted countries: www.keysight.com/find/contactus (BP-07-24-15) www.keysight.com/go/quality Keysight Technologies, Inc. DEKRA Certified ISO 9001:2008 Quality Management System This information is subject to change without notice. Keysight Technologies, 2015 Published in USA, July 30, 2015 5992-0555EN www.keysight.com