#Performance probe serial
Here in the example (see Figure 2), the 100:1 resistor divider probe with 5,000 ? input resistance and the active probe with 1 M? input resistance are measuring a 5 V I2C serial bus with a pull-up of 10 k?.įigure 2. To measure the amplitude of a signal with relatively high source impedance accurately, it is important to use a probe with high input impedance. Many open collector or open drain outputs of ICs require the use of an external pull-up or pull-down resistor to keep the digital output in a defined logic state. When you use this probe, however, you should be very careful with the resistive loading effect because it may alter the measured amplitude of the signal as well as the bias point. The total input impedance at DC or low frequency range is only 500 ? (10:1) or 5,000 ? (100:1) when the probe is terminated into the 50 ? input of the scope.įor many designers, this probe is often selected as a low-cost alternative to a higher priced active probe. The low-impedance resistor divider probe provides either 500 ? or 5 k? input resistance to give 10:1 or 100:1 attenuation with the 50 ? input of the scope. The probe tip typically contains a resistor, either 450 ? or 4,950 ?. At the cost of resistive loading, this probe offers a deceivingly very low input capacitance (~2 pF or less) and high bandwidth (>1.5 GHz). One type of passive probe is a low-impedance resistor divider probe, also known as a 50 ? passive probe or Z0 passive probe. Since the effective ground plane between the signal connections in differential probes is more ideal than most of the ground connections in single-ended probes, differential probes can make better and more repeatable measurements on single-ended signals than single-ended probes can. Also, differential probes provide better signal integrity due to very low impedance grounding and higher input impedance. This provides a significantly high common mode rejection (CMRR) performance as compared to a single-ended active probe or passive probe. Also, a high-voltage differential probe is a tool of choice when it comes to measuring high-voltage floating signals commonly found in power supplies or motor drives.Ī differential probe uses a differential amplifier to subtract two input signals, resulting in one differential signal for measurement by one channel of the scope. The best way to make a measurement on small differential signals is to use a differential active probe. Since these small voltage levels are susceptible to noise and electromagnetic interference, designers are frequently choosing to use differential signals. To reduce power consumptions, today’s designs are using smaller voltage signals. However, it is impossible to totally eliminate the loading effect of a probe, regardless of whether it’s an active or a passive one. In general, active probes provide less loading effects at high bandwidth ranges than passive probes. Therefore, the less loading there is, the fewer adverse effects a probe has on the signal, or the less that it distorts or changes the target signal. This causes the scope to make a different measurement, and the deviation depends on how much the probe loads the circuit. The issue with probe loading is that when you attach a probe to the target system in order to make a measurement, the probe becomes a part of the circuit, and it introduces loading to the circuit. The primary consideration should be probe loading. However, there are other important characteristics of the probes that need to be considered, especially when you’re measuring high-speed signals.
In fact, they are usually the probe of choice when users need high bandwidth, high signal fidelity performance.Ĭlearly, the primary benefit of an active probe over a passive one is higher bandwidth. Although active probes are more expensive than passive ones, they offer a superior level of performance that may be essential in certain circumstances. They offer a wide dynamic range and bandwidth as high as >500 MHz when connected to 1 M? input of the oscilloscope. Compared to active probes, passive probes are more rugged and less expensive. When you buy a low- to mid-range oscilloscope, it usually comes standard with a high-impedance passive probe per oscilloscope channel.
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