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Know Your Probe Input Impedance Curves to Reduce Probe Loading

Blog Post created by BoonCampbell Employee on Apr 30, 2018

Did you know that when you probe your DUT, the probe becomes part of the circuit? All probes have a loading effect on your circuit to some extent. These effects can manifest into overshoot, ringing, slow rise/fall times, propagation delays and DC offset problems. In addition, the loading impacts vary as you probe from DC voltages through high frequency ranges. Over this large frequency span, your probe impedance can vary greatly.

 

RCRC vs RC Impedance Characteristics

The capacitive and inductive components in a probe are what causes loading on your device. The traditional model off a probe’s looks more like the read trace in figure 1. However, newly developed high-end and high frequency active probes two knee or crossover points (RCRC) and provide different loading responses. Understanding the probe’s input impedance characteristics over frequency enables you to make the best probe selection for the circuits you are testing.

 

Let’s look at some probe input impedance vs frequency curves in Figure 1 below to understand the impact to your measurements. A probe’s input impedance is shown on the vertical axis and frequency is shown on the horizontal axis. Both RC and RCRC probe curves are shown.


The red trace is a typical RC probe response over frequency. Note that from DC to around 10 MHz, the RC probe holds steady at a 50 K? of differential impedance. Higher than 10 MHz, the RC probe’s capacitive reactance comes into play at 210 fF, and the probe impedance continues to decrease as the frequency increases. This is what is called an RC input impedance profile of most conventional probes on the market.

Lower impedance will have accumulative loading impacts on the circuit you are probing.

 

The blue trace is an RCRC probe’s response. Notice from DC to 10 Hz the inductance is at 100 K? and then falls to 1K? from 10 Hz to 10 KHz. The 1 K? inductance will load your circuit more than the RC probe’s 50 K? in this frequency band, but past 10 MHz, the RC loading will be much worse because the RC probes impedance decreases rapidly driven by the capacitance of the probe. The RCRC holds this 1 K? impedance from 10 KHz to around 1 GHz. Past that, the capacitive reactance at 32 fF starts to come into play, reducing the 1 K? impedance further. So, you can see at higher frequencies, above several hundred MHz, the RCRC probe proves to be the better choice because it will decrease loading effects at higher frequencies.

 

input impedance vs frequency of probes
Figure 1: Input Impedance vs Frequency of modern high-performance probes

Red = RC probe example
Blue = RCRC probe example
Pink = RCRC probe example

 

The pink trace is another RCRC probe’s response for additional comparisons. Note that from DC to around 100 MHz the impedance is 100 K?. But from 100 KHz to 10 MHz the probe’s 110 pF capacitive reduces the inductance to 450 ?s. This change in impedance results in a significant amount of additional loading relative to the initial 100 K?s at lower frequencies. And then at 100 MHz and above, the probe’s 65 fF capacitance reduces the impedance further.

 

To summarize the curves in Figure 1, your probe selection for the lowest circuit loading should be:

  • RC probes Higher input impedance for lower loading at mid band (kHz to GHz)
  • RCRC probes – Higher input impedance for lower loading at higher bands (>GHz)

 

Applications tips for each probe type

Use an RCRC probe for:

  • Accurate high frequency content above GHz due to low loading
  • High speed signals with low source impedance, such as a 50 ohm transmission line
  • Reproducing wave shapes with fast edge speeds

 

Use an RC probe for:

  • Mid-band frequencies due to low loading
  • Buses that transition to a “high Z” state such as DDR and MIPI signals
  • Signal sources with high impedance
  • Signals with long time constants

 

Conclusion

A common misconception is that a higher priced, higher bandwidth probe can more effectively measure signals across all bandwidths. However, this is not the case. The best probe for your application will be dependent on what frequencies you are working with. Always factor in the probe loading effects on your measurement.

 

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