Line Up Those Measurements with Synchronized Clocks

Measurements on two or more channels need not occur within a picosecond of each other for you to consider them synchronized. I prefer to think of synchronized measurements as measurements on two or more signals, where the samples occur close enough in time that you don’t miss anything significant. Just like “real time” means “fast enough,” I think of “synchronized” as “close enough in time” relative to a sample clock.

In some applications, you can use software to generate clocks and triggers. Suppose you’re measuring the temperature at two points inside an enclosure. Temperature changes slowly, so you might sample at 10 samples/s. Even if you have a separate 10-Hz ADC clock for each channel, measurements will occur within 50 ms of each other. That’s probably good enough, so you might be able to rely on a software loop to time the measurements in this application. According to Alex Ivchenko, R&D engineering manager at United Electronic Industries (Watertown, MA), Windows can skew your measurements by tens of milliseconds.

In most applications, though, you’ll need to use hardware to synchronize your measurements. Suppose you’re performing a shock or vibration test on the enclosure. A 10-Hz sample rate is far too slow. You might have to measure shock at several points on the enclosure at the moment of impact. Now, you need to sample accelerometer outputs at perhaps 200 ksamples/s. Forget about using software loops to time your measurements. For precise, repeatable sampling times, you must use hardware to generate clock and trigger signals.

Although software loops provide a degree of timing control, for accuracy below about 50 ms, you’ll have to rely on hardware to synchronize your measurements. Unfortunately, most general-purpose buses such as PCI, CompactPCI, ISA, and VME lack signals designed to synchronize operations.

Many data-acquisition cards for the PCI bus and the ISA bus circumvent this problem by providing external buses that let you synchronize clocks and triggers across multiple cards. The PXI (PCI eXtensions for Instrumentation) bus and VXI (VME eXtensions for Instrumentation) cards have an instrumentation bus that supplies dedicated clock and trigger lines that can control instrument cards. You can use these clock and trigger lines to synchronize not only measurements but also analog and digital stimulus signals.

External Clocks

In addition to onboard ADC clocks, most data-acquisition cards have external clock and trigger inputs on their main I/O connectors. You can connect an external clock source to the clock inputs of two or more cards, which gets them to synchronously digitize their input signals. You can also connect a common trigger signal to the cards’ external trigger inputs. When the cards receive the trigger signal, they simultaneously digitize their analog inputs.

Figure 1. A D flip-flop triggers two cards running at different clock rates. (Courtesy of Gage-Applied.)

The application starts with a 15-MHz external clock and uses a divider circuit to produce the 100-kHz clock. A manual pushbutton acts as the trigger for both cards. The acquisition starts on the first rising edge of the 100-kHz clock after the pushbutton is pressed. In this case, the skew produced by the propagation delay in the divider is insignificant.

You may not need an external clock or trigger source to synchronize measurements across two or more cards. You can sometimes use one card’s ADC clock as a master clock for other cards. Many ISA and PCI cards have an external bus that sends ADC clock and trigger signals from a master card to slave cards.¹ Figure 2 shows two PCI boards connected through a ribbon cable. (Some cards use rigid boards instead of flexible cables.) The cable carries ADC clock and trigger signals from a master card to a slave card.

After you install your ISA or PCI cards and connect their external buses together, you must use the manufacturer’s software to configure the cards as a master and slave(s). To program your cards, send commands to their driver. The programming commands tell the master to make its clock and trigger signals available to the external bus. You also command the slave board to initiate an acquisition based on the bus trigger input and clock from the master.

Figure 2. Many data-acquisition cards use a local bus to transfer clocks and triggers between cards. (Courtesy of National Instruments.)

For some applications, a data-acquisition card with a multiplexer won’t suffice. Instead, you’ll need a card with sample-and-hold (S/H) circuits or dedicated-channel ADCs. (See “Scan, Hold, or Dedicate an ADC?” below). In automotive crash testing, for example, you need to make synchronized measurements, often from several accelerometers. A multiplexed card won’t work; you need either an S/H card or one with dedicated ADCs.

Using a master/slave arrangement gets the samples synchronized—almost. There’s a propagation delay between the time the master card samples and the time the master’s sample clock reaches the slave card or cards. The slaves will be in sync with each other, but they will sample slightly later than the master. To compensate, a master card might store the measurements of the master in memory for one or two ADC clock cycles. Some manufacturers fix the delay time based on their card’s performance. Others let you program this delay through a card’s driver.

Instrumentation Buses

External buses, S/H circuits, and dedicated ADCs can synchronize measurements on ISA and PCI cards. PXI cards and VXI cards, though, extend their PCI and VME buses, respectively, by adding trigger and clock lines to their bus backplane. These additional lines were originally added to the VME bus, which became the VXI bus. The same signals extended the PCI bus (in the CompactPCI form factor) to form the PXI bus. Figure 3 shows a “star trigger” line and an eight-line trigger bus.

Figure 3. The PXI and VXI bus add a trigger bus and a star trigger to the CompactPCI bus and the VME bus for synchronizing measurements across cards. (Courtesy of National Instruments.)

You can set up your PXI and VXI cards to make any of the eight lines on the trigger bus as a trigger output, trigger input, clock output, or clock input. The PXI and VXI buses also include a 10-MHz reference clock that you can a divide or multiply and use it to clock an ADC.

The star trigger connects from a trigger master card (which you must place in slot 2 in a PXI system) to all other I/O slots (3 to 8). All slots have equal-length PCB traces from slot 2, so the star-trigger signal arrives at all slots at the same time. That’s important for high-speed digitizing. Electrons travel at 1 ft/ns. With a card sampling at 2 GHz, a foot of wire or PCB trace will cause a skew of 2 samples.

How can you use the instrumentation “extensions” in PXI and VXI cards to synchronize measurements? In one example, an ultrasonic materials test uses PXI digitizers. An ultrasonic (100 kHz–200 kHz) pulse hits the DUT, and digitizers measure the sound through five ultrasonic microphones mounted on the DUT. The captured signals indicate how the DUT distorts the signal. This application uses three two-channel PXI digitizers sampling at 100 Msamples/s. The card in slot 2 generates the 100-MHz master clock from the 10-MHz reference clock and places it on the star-trigger bus. The master digitizer card detects the pulse and sends a signal over the trigger bus to the slave cards. The trigger pulse tells the slave cards to sample at 100 Msamples/s starting on the next 10-MHz clock signal.

The applications I’ve described give you an idea of how to synchronize ADCs. When you synchronize measurements, don’t accidentally defeat the synchronizing by running different wire lengths between your data-acquisition system and signal sources. Remember the 1 ft/ns rule for traveling electrons. So at high digitizing rates such as 100 Msamples/s, differing wire lengths can add skew to your measurements. Keep all signal wires the same length so you’ll get equal delays. T&MW

FOOTNOTE

1. Manufacturers of data-acquisition cards use proprietary external buses with different names. Data Translation (Marlboro, MA) uses the name DT-Connect while National Instruments (Austin, TX) calls its bus Real-Time System Integration (RTSI). See www.ni.com/support/daq/program/rtsi for information about RTSI.

You can contact Martin Rowe at mrowe@cahners.com.