Tuesday, January 3, 2012

Passive or Active Scope Probe?

In this post we will look at what type of scope probe to use, passive or active, and discuss the trade offs of each. For general-purpose mid-to-low-frequency (less than 600-MHz) measurements, passive high-impedance resistor divider probes are good choices. These rugged and inexpensive tools offer wide dynamic range (greater than 300 V) and high input resistance to match a scope’s input impedance. These are the probes that often come with the scope when you purchase it. However, they begin to impose heavier capacitive loading as the frequency of the signal being measured goes up. The input capacitance of the probe and scope combine to create an impedance to between the signal being measured and ground. As the frequency of the signal goes up the impedance created by the capacitance drops. If the impedance drops to low it can effect your signal being measured, this is known as capacitive loading. For instance a capacitance of 10 pF presents only 100 Ohms of impedance to a 150 MHz signal so it is important to know the input capacitance of the passive probe and the scope you are using. Low-impedance (z0) passive probes (talk more about in last section) and active probes (talk more about next) offer higher bandwidths than high-impedance passive 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 (greater than 600 MHz) that demand precision across a broad frequency range, active probes are the way to go. They cost more than passive probe and their input voltage is limited, but because of their significantly lower capacitive loading, they give you more accurate insight into fast signals.

In the two figures below we see screen shots from a 1-GHz scope measuring a signal that has a 1-ns rise time. In the first figure an Agilent 1165A 600-MHz passive probe was used to measure this signal. In the second figure an Agilent 1156A 1.5-GHz single ended active probe was used to measure the same signal. The blue trace shows the signal before it was probed and is the same in both cases. The yellow trace shows the signal after it was probed, which is the same as the input to the probe (showing the loading effects of the probe). The green trace shows the measured signal, or the output of the probe.
Passive probe: blue -- signal before probed, yellow -- signal after probed, green -- output of probe
Active probe: blue -- signal before probed, yellow -- signal after probed, green -- output of probe
A passive probe loads the signal down with its input inductance and capacitance (yellow trace). You probably expect that your oscilloscope probe will not affect your signals in your device under test (DUT). However, in this case the passive probe 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 due to 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 ripping effects in the probe output (green trace). The 1.85-ns rise time of the measured signal with the passive probe is actually 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.

We can see that the signal is virtually unaffected when we attach an active probe such as Agilent’s 1156A 1.5-GHz active probe to the DUT. The signal’s characteristics after being probed (yellow trace) are nearly identical to its un-probed characteristics (blue trace). In addition, the rise time of the signal is unaffected by the probe being maintained at 1 ns. Also, the active probe’s output (green trace) matches the probed signal (yellow trace) and measures the expected 1-ns rise time. Using the 1156A active probe's 1.5 GHz bandwidth (or 1-GHz system bandwidth when the probe is used with 1-GHz oscilloscope) makes this possible. Below is a table comparing high Z passive probes to active probes.

The above table I got from an older publication so the one mistake on the active probe side of the table is bandwidth "up to 13GHz." Agilent currently offers active probes up to 30 GHz. Also one other thing to note is there are low impedance passive probes known as "Resistive divider passive probe" that can have bandwidths up to 6 GHz. They are typically much lower cost than active probes. They must be used with a 50 Ohm input scope, have a lower amplitude capabilities than other passive probes, and do not work well with high impedance signals.

If you have any insights or useful comments to add please use the "Comments" section below.


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