Active Pole-Zero Cancellation Oscilloscope Probe
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Solution Overview
Problem
Oscilloscope probes face limitations in maintaining high impedance and bandwidth due to parasitic capacitance and finite slew rate of amplifiers, leading to loading effects and reduced dynamic range, especially when dealing with high impedance devices under test.
Innovation Solution
The active pole-zero cancellation probe design incorporates a tip resistor and capacitor in parallel, along with a feedback resistor and capacitor, and a terminating attenuator to offset parasitic capacitance effects, allowing for increased amplitude handling while minimizing low-frequency noise gain through AC-coupled terminating attenuators and switchable attenuation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the tip resistor value is increased to maintain high impedance and minimize loading effects on the DUT, then the loading effect on the DUT is reduced, but the bandwidth is limited due to parasitic capacitance
Solution Approach 1:
The patent changes the electrical parameters of the probe by introducing active pole-zero cancellation circuitry that dynamically adjusts the impedance characteristics. The tip resistor value can be effectively modified through the cancellation effect, allowing high impedance at low frequencies while maintaining bandwidth at high frequencies through the active circuit compensation.
Solution Approach 2:
The patent introduces an intermediary active circuit between the tip resistor and ground that provides pole-zero cancellation. This intermediary circuit includes operational amplifiers and capacitors that generate a cancellation signal to offset the bandwidth-limiting effect of parasitic capacitance, thereby resolving the contradiction between high impedance and bandwidth.
2Ease of operation
If the probe cable length is increased to provide convenient probing distance, then the ease of operation is improved, but the signal loss increases significantly at high frequencies
Solution Approach 1:
The patent employs feedback mechanisms in the active pole-zero cancellation circuit that continuously monitor and compensate for signal degradation along the cable. The feedback loop detects high-frequency signal loss and applies corrective amplification and phase adjustment to maintain signal integrity over long cable lengths.
3Reliability
If the amplifier slew rate is increased to handle larger signal amplitudes, then the dynamic range is improved, but the low-frequency noise gain increases
Solution Approach 1:
The patent implements dynamic attenuation that automatically adjusts the gain at different frequencies. At low frequencies, the attenuation is higher to suppress noise gain, while at high frequencies, the attenuation is reduced to preserve signal amplitude. This dynamic adjustment allows the system to handle large signal amplitudes without excessive low-frequency noise amplification.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design enhances the dynamic range and frequency response flatness of oscilloscope probes, reducing loading effects and noise gain, thereby providing a more accurate representation of electrical signals across a broader bandwidth.
Implementation Method 1
The active pole-zero cancellation probe of FIG. 1B was developed to address limits of the passive resistive divider oscilloscope probe in FIG. 1A. In the active pole-zero cancellation probe of FIG. 1B, the active pole-zero cancellation probe includes the tip resistor 101, the cable 102, the terminating resistor 103, a tip capacitor 104, a feedback resistor 105, a feedback capacitor 106, and an amplifier 107. The term 'pole-zero' cancellation is a reference to poles and zeros which cancel each other out in the frequency response.
Implementation Method 2
An oscilloscope probe is used to transfer an electrical signal from a device under test (DUT) to an input of an oscilloscope which measures the electrical signal.
Data Source
AI summary
An oscilloscope probe includes a tip network, a low-loss signal cable, and a terminating assembly. The tip network is connected to the signal cable and is configured to electrically connect to a device under test via a tip network node. The terminating assembly includes an amplifier, a feedback network and a terminating attenuator. The amplifier has an inverting input, a non-inverting input connected to ground, and an amplifier output configured to connect to an oscilloscope input. The feedback network is connected between the inverting input and the amplifier output. The terminating attenuator includes a first loop circuit and a second loop circuit. The first loop circuit is provided between the signal cable and the inverting input of the amplifier. The second loop circuit is provided between the signal cable, and ground. Resistance of terminating resistors in the loop circuits are selected to match characteristic impedance of the signal cable.