Connect signals to oscilloscopes, minimize errors

-April 20, 2017

When you look at the waveform of a signal on an oscilloscope, you want to be sure that the signal is a good representation of what's happening on the board or on a signal line. Many things can affect a test signal's integrity as it travels from a test point to the oscilloscope. Let's look at a couple of those things: probe choice and test signal connections.

First, you must ensure signal integrity by using the right probe. When the highest frequency that most engineers had to measure was maybe 200 MHz or 300 MHz, a passive probe with a BNC connector worked just fine. Passive probes with high-impedance resistor dividers are inexpensive, rugged, and can be used to measure signals up to 600V. Unfortunately, the capacitive loading they impose on a circuit is relatively high. This limits the frequency range for which they can be used; most are rated at 500 MHz or less.

If the signals you need to measure are higher than 500 MHz, consider active probes. They also have a high input impedance, but the input capacitance is significantly lower. This combination preserves the signal integrity of the test signal. Active probes are available with bandwidths of up to 2.5 GHz.

Of course, you don't get this bandwidth for free. Active probes are much more expensive than passive ones. A 2 GHz probe can cost more than $2,000. Also, the voltages you can measure with active probes are much lower than what you can measure with passive probes. Active probes typically have a dynamic range of ±8 V, versus hundreds of volts for passive ones.

At high frequencies, making the connection to the test signal is also critical. Most active probes come with a whole kit of accessories for making a solid connection. For example, in many cases, a two-inch ground lead will have too much inductance and distort a measurement. Some probes have pogo-pin ground leads (Figure 1) that have much less inductance and help preserve signal integrity. To use this type of ground lead, though, there must be a ground connection near the test point.

Figure 1 This printed circuit board adapter lets you connect directly to a test signal.
(Source: Tektronix)

Another type of accessory to consider is an adapter that mounts right on the circuit board (Figure 2). You plug the probe directly into the adapter, making a direct connection to ground. Though this is not practical in all cases, it might be the best approach when you need to make critical amplitude and timing measurements.

Figure 2 This pogo-pin ground lead helps preserve signal integrity by reducing the length of a ground lead, thereby reducing its inductance. (Source: Tektronix)

References

  1. Eight Hints for Better Scope Probing, Application Note, Keysight Technologies
  2. ABCs of Probes, Tektronix (registration required)

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