Notice that the impedance of higher performance probe is much higher in the frequency range from 1 MHz to 1 GHz. Such a probe is shown by the blue trace in Figure 2. A higher performance probe (blue trace) offers higher impedance as input frequency increases.įortunately, there are higher performance probes on the market with higher impedance characteristics that are able to maintain high input impedance at high frequencies while providing high signal fidelity. Stray inductance in the probe’s inputs can also cause the input impedance to increase slightly.įigure 2. Ultimately at very high frequency, even small features of the probe become capacitive and the impedance drops again. This probe has an impedance of roughly 500Ω for signal frequencies below 2 GHz. The probe’s impedance starts out high, but drops as the signal’s frequency increases. The purple trace in Figure 2 shows the impedance of a high bandwidth probe vs. However, adding capacitance to the front-end can result in a lower input impedance at high frequencies. Extending the frequency and voltage range of a probe requires a front-end network of resistors and capacitors. The most desirable probes like that shown in the lower right of Figure 1 have high frequency performance as well as high dynamic range. As the frequency of the signal being measured increases, the parasitic elements that make up the probe tip and probe amplifier can look like capacitors such as that shown in the middle of Figure 1. Typical impedance specifications at low frequencies range from 40 kΩ to 1 MΩ. At these frequencies, the impedance of typical probes looks like a large resistor as shown in the top left of Figure 1.
There are many probes available with high input impedance at low frequencies (DC-100Hz). With the low-power circuit running in a high-impedance state, the impedance of the probe needs to be as high as possible. Otherwise, the probe can load down the circuit or cause unwanted distortion of data signals. In general, the oscilloscope probe needs to present a higher impedance input than the characteristic impedance of the circuit to prevent loading. LPDDR2 and LPDDR3 both use this method that allows the bus to float to the tri-state level when the bus is not being actively driven. This state may be accomplished using a large, weak pull-up resistor to the standby level. Others return to a standby voltage level when the bus is un-driven. For example, the MIPI D-PHY bus operates in an unterminated state when it is communicating in its low power, low data rate state. Some systems even run without termination to conserve power. When low-power components change state to a low-power mode they will often be in a high-impedance state.
These shorter interconnects in mobile devices consume less power than longer interconnects found in large computer systems. A third technique to reduce power consumption is to use shorter interconnects with reduced capacitance.
The lower-power states may include a standby mode or they may involve the system running at a reduced data rate. Another technique is to switch the circuits’ operating mode between a high-speed state and low-power states. One simple method is to reduce the power supply voltages and the peak-to-peak voltage swing of high-speed signals. Low-power devices use a variety of techniques to reduce power consumption. While connecting to the low-power device under test might be possible with a coaxial cable, many systems require an oscilloscope probe to acquire the signals. The most common tool for these tests is the oscilloscope. Typical tests performed include voltage and timing measurements, including jitter and noise measurements. In addition, testing a design ensures its reliability in use and across multiple manufacturing lots. Testing the low-power components for compliance with industry standards is often required to ensure interoperability between components or systems. Since many low-power devices are also high-speed systems, there is a need to test and verify their performance. Low-power circuits combine high speed and low-power consumption in a single component, presenting a number of probing and testing challenges.
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