Eight hints for better scope probing.

Probing is critical to making quality oscilloscope measurements, and often the probe is the first link in the oscilloscope measurement chain. If probe performance is not adequate for your application, you will see distorted or misleading signals on your oscilloscope.

Selecting the right probe is the first step toward making reliable measurements. How you use the probe also affects your ability to make accurate measurements and obtain useful measurement results.

Here are eight useful hints for selecting the right probe and making your scope probing better. These tips will help you avoid most common probing pitfalls.

I. Passive or Active Probe?

For general-purpose, mid- to low-frequency <600-MHz measurements, passive high-impedance resistor divider probes are good choices. These rugged and inexpensive tools offer a wide dynamic range of >300 V and high input resistance to match a scope's input impedance.

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However, they impose heavier capacitive loading and provide lower band-widths than low-impedance passive probes or active probes. All in all, high-impedance passive probes are a great choice for general-purpose debugging and troubleshooting on most analog or digital circuits.

For high-frequency applications of >600 MHz that demand precision across a broad frequency range, active probes are the way to go. They cost more than passive probes, and their input voltage is limited. But because of their significantly lower capacitive loading, they give you more accurate insight into fast signals.

Figure 1 is a screen shot from a 1-GHz scope measuring a signal that has a 1-ns rise time. On the left, an Agilent 1165A, 600-MHz passive probe was used to measure this signal. On the right, an Agilent 1156A, 1.5-GHz single-ended active probe measured the same signal.

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The blue trace shows the signal before it was probed and is the same in both cases. The yellow trace represents the signal after it was probed, which is the same as the input to the probe. The green trace illustrates the measured signal or the output of the probe.

A passive probe loads the signal down with its input inductance and capacitance (yellow trace) and does have an effect on the DUT. The probed signal's rise time becomes 1.9 ns instead of the expected 1 ns, partly due to the probe's input impedance but also because of its limited 600-MHz bandwidth in measuring a 350-MHz signal (0.35/1 ns = 350 MHz).

The inductive and capacitive effects of the passive probe also cause overshoot and ripple on the probe output (green trace). The 1.85-ns rise time of the measured signal with the passive probe actually is faster than the probe's input due to these capacitive and inductive effects. Some designers are not concerned about this amount of measurement error. For others, this amount of measurement error is unacceptable.

The signal is virtually unaffected when we attach the 1.5-GHz active probe to the DUT. The signal's characteristics after being probed (yellow trace) are nearly identical to its unprobed characteristics (blue trace). In addition, the rise time of the signal is unaffected by the probe.

Also, the active probe's output (green trace) matches the probed signal (yellow trace) and measures the expected 1-ns rise time. Using the active probe's 1.5 GHz bandwidth makes this possible.

2. Compensate Probe Before Use

Most probes are designed to match the inputs of specific oscilloscope models. However, there are slight variations from scope to scope and even between different input channels in the same scope.

Make sure you check the probe compensation when you first connect a probe to an oscilloscope input because it may have been adjusted previously to match a different input. To deal with this, most passive probes have built-in compensation RC divider networks. Probe compensation is the process of adjusting the RC divider so the probe maintains its attenuation ratio over the probe's rated bandwidth.

If your scope can automatically compensate for the performance of probes, it makes sense to use that feature. Otherwise, use manual compensation to adjust the probe's variable capacitance.

Most scopes have a square wave reference signal available on the front panel to use for compensating the probe. You can attach the probe tip to the probe compensation terminal and connect the probe to an input of the scope. Viewing the square wave reference signal, make the proper adjustments on the probe using a small screwdriver so that the square waves on the scope screen look square.

You can have either overshoot or undershoot on the square wave when the low-frequency adjustment is not properly made, as shown in Figure 2. This will result in high-frequency inaccuracies in your measurements. It's very important to make sure this compensation capacitor is correctly adjusted.

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3. Probe Loading Check With Two Probes

Before probing a circuit, connect your probe tip to a point on your circuit and then connect your second probe to the same point. Ideally, you should see no change on your signal. If you see a change, it is caused by the probe loading.

In an ideal world, a scope probe would be a nonintrusive wire having infinite input resistance, zero capacitance, and inductance attached to the circuit of interest, and it would provide an exact replica of the signal being measured. But in the real world, the probe becomes part of the measurement, and it introduces loading to the circuit.

To check the probe loading effect, first connect one probe to the circuit under test or a known step signal and the other end to the scope's input. Watch the trace on the scope screen, save the trace, and recall it on the screen so that the trace remains on the screen for a comparison. Then, using another probe of the same kind, connect to the same point and see how the original trace changes because of the double probing.

You may need to make adjustments to your probing or consider using a probe with lower loading to make a better measurement. For instance, in this example, shortening the ground lead did the trick. In Figure 3, the circuit ground is probed with a long 18-cm ground lead. In Figure 4, the same signal ground is probed with a short spring-loaded ground lead. The ringing on the probed signal (green trace) went away with the shorter ground lead.

4. Low-Current Measurement Tips

In recent years, engineers working on mobile phones and other battery-powered devices have demanded higher-sensitivity current measurements to help them ensure the current consumption of their devices is within acceptable limits. Using a clamp-on current probe with an oscilloscope is an easy way to make a current measurement that does not necessitate breaking the circuit. But this process gets tricky as the current levels fall into the low milliampere range or below.

As the current level decreases, the oscilloscope's inherent noise becomes a real issue. All oscilloscopes exhibit one undesirable characteristic--vertical noise. When you are measuring low-level signals, measurement system noise may degrade your actual signal measurement accuracy.

Since oscilloscopes are broadband measurement instruments, the higher the bandwidth of the oscilloscope, the higher the vertical noise will be. You need to carefully evaluate the oscilloscope's noise characteristics before you make measurements.

The baseline noise floor of a typical 500-MHz bandwidth oscilloscope measured at its most sensitive V/div setting is approximately 2 mVpk-pk. In making low-level measurements, it is important to note that the acquisition memory on the oscilloscope can affect the noise floor.

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On the other hand, a modern AC/DC current probe such as Agilent's N2783A 100-MHz current probe can measure 5 mA of AC or DC current with approximately 3% accuracy. The current probe is designed to output 0.1 V per 1-A current input. In other words, the oscilloscope's inherent 2-mVpk-pk noise can be a significant source of error if you are measuring less than 20 mA of current.

So, how do you minimize the oscilloscope's inherent noise? With modern digital oscilloscopes, there are a number of possible approaches:

Bandwidth Limit Filter

Most digital oscilloscopes offer bandwidth limit filters that can improve vertical resolution by filtering out unwanted noise from input waveforms and decreasing the noise bandwidth. Bandwidth limit filters are implemented with either hardware or software. Most bandwidth limit filters can be enabled or disabled at your discretion.

High-Resolution Acquisition Mode

Most digital oscilloscopes offer 8 bits of vertical resolution in normal acquisition mode. High-resolution mode on some oscilloscopes provides much higher vertical resolution, typically up to 12 to 16 bits, which reduces vertical noise.

Typically, high-resolution mode has a large effect at slow time/div settings where the number of on-screen data points captured is large. Since high-resolution mode acquisition averages adjacent data points from one trigger, it reduces the sample rates and bandwidth of the oscilloscope.

Averaging Mode

When the signal is periodic or DC, you can use averaging mode to reduce the oscilloscope's vertical noise. Averaging mode takes multiple acquisitions of a periodic waveform and creates a running average to reduce random noise.