Many people think that differential probes are made to only probe differential signals. Did you know you can also probe single-ended signals with your differential probe? Today we’ll learn how to maximize the usage of your differential probe and get the best signal fidelity possible. You will need to examine the performance and usability trade-offs between your differential and active probes to make this decision. The simplified models of each probe are shown in Figure 1 below and will be referenced throughout this blog.
Figure 1. Simplified models of single-ended and differential probes.
Because the physical geometries of the two probes are similar, the values of the capacitors and inductors will also be similar. The ground connection of the single-ended active probe (lg) is sensitive to the length the ground accessory used in the measurement. Since you are not measuring relative to ground with a differential probe (you are measuring relative to another voltage), you are not making an additional LC circuit with a ground lead, so you see much less inductance with a differential probe.
Note that the differential probe tips connecting to the DUT (device under test) have a tip resistor on both tips (Zp and Zm). Whereas the single-ended probe has a tip resistor (Zs) on the signal connection and none the ground connection. These tip resistors damp the resonance caused by the probe input inductors and capacitors (ls, lg, and cs in the single-ended probe and lp, lm, cp, and cm in the differential probe).
There is a lack of damping on single-ended probe ground connections. Because of this, a voltage can develop between your probe’s ground and your DUT’s ground. This happens at higher frequencies and reduces the input to your single-ended probes amplifier input, thereby reducing your probe’s output. This disadvantage of your single-ended probe can be minimized by keeping your ground lead length as short as possible.
Common Mode Rejection
Unwanted common mode signals can also affect the signal you see on screen. They can be caused by electromagnetic interference, cross talk, and other noise characteristics on or around your DUT. Your probe needs to be able to reject common mode noise in order to give you the most accurate signal. The amount a probe can reject is represented by a ratio: common mode rejection ratio CMRR. But this rejection is only relative to the signals that appear simultaneously and in-phase on both probes’ inputs. You can see in Figure 2 below that the single-ended probe (green line) has less rejection over the frequency band measured than the differential probe (dotted red line). Around 1.5 GHz, the difference is close to 20 dB. This is significant since the common mode (noise) will be amplified by the single-ended probe, causing considerably more noise on its output relative to the differential probe.
Figure 2. CMRR of the single-ended vs. the differential probe.
In Figure 3 and Figure 4 below you can see:
- The input signal to the oscilloscope is show in yellow on all the following figures (un-probed and fed directly into Channel 1 of the oscilloscope). We will call this signal Vin.
- Figure 1 below shows the single-ended probe measurement of Vin on Channel 2 of the oscilloscope (blue trace).
- Figure 2 below shows the differential probe measurement of Vin on Channel 2 of the oscilloscope (red trace). Note: one side of the differential probe is connected to Vin and the other is connected to the same ground as the single-ended probe.
Figure 3. Single-ended measurement. Figure 4. Differential measurement.
Notice that the two probe measurements look like they carry the same amount of noise relative to the un-probed signal in yellow. But let’s look at little closer and change the volts per division on the oscilloscope from 200 mV/div (millivolts per division) to 20 mV/div. You can now see in Figure 5 below the single-ended measured signal in blue, and in Figure 6 you can see the differential measured signal in red. Notice that the single-ended measurement in blue has much more noise than the differential measurement in red due to less common mode correction by the single-ended probe.
Figure 5. Single-ended measurement. Figure 6. Differential measurement.
Differential probes can make the same types of measurements that single-ended probes can perform. However, due to the common mode rejection on both inputs of the differential probe, the differential measurements can have significantly less noise. The common mode rejection in the differential probe reduces the amount of noise that goes into its amplifier, resulting in much less noise displayed on the oscilloscope. This allows you to see a better representation of your DUT’s signals and not be misled by random probe noise.
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