Probe System Compensation Network Common-Mode Rejection
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Solution Overview
Problem
Existing probe systems struggle with maintaining consistent common-mode rejection across a wide frequency range and are sensitive to manufacturing tolerances, leading to conversion of common-mode voltages into differential voltages.
Innovation Solution
The placement of a compensation network on the output side of the differential amplifier or in its feedback path, where the difference in input currents is formed before being fed to the compensation network, significantly improves common-mode rejection and reduces sensitivity to common-mode noise, while also introducing a pole in the input network to enhance high-frequency input impedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the compensation network is arranged on the input side of the differential amplifier, then the circuit structure is simpler, but the common-mode rejection is poor and sensitivity to common-mode noise is high
Solution Approach 1:
The patent inverts the conventional arrangement by placing the compensation network on the output side of the differential amplifier instead of the input side. This inversion allows the differential amplifier to first reject common-mode signals through its differential action, and then the compensation network fine-tunes the frequency response, achieving superior common-mode rejection while maintaining circuit simplicity.
Solution Approach 2:
The differential amplifier performs preliminary common-mode rejection before the signal reaches the compensation network. By suppressing common-mode signals at the amplifier stage, the compensation network only needs to handle the differential signal, reducing its burden and improving overall performance.
2Measurement precision
If two independent compensation networks are used on the input side, then each input current can be compensated individually, but manufacturing tolerances cause common-mode voltages to be converted into differential voltages
Solution Approach 1:
The patent merges the compensation function into a single network on the output side that processes the already-differenced signal. This eliminates the need for two independent compensation networks and their associated tolerance mismatches, as the single compensation network only needs to match the differential amplifier's characteristics.
Solution Approach 2:
The patent converts the potential harm of tolerance mismatches in dual compensation networks into a benefit by using the differential amplifier's inherent common-mode rejection to eliminate common-mode signals before compensation, making the system less sensitive to component tolerances.
3Reliability
If the compensation network is placed on the output side of the differential amplifier, then common-mode rejection is significantly improved, but the circuit configuration becomes more complex
Solution Approach 1:
The output-side compensation network serves multiple functions: it compensates for the differential amplifier's frequency response, maintains the improved common-mode rejection achieved by the amplifier, and can be designed as a simple RC network that integrates easily into the existing circuit, thus not significantly increasing overall complexity.
4Device complexity
If only one zero point is provided in the input network, then the circuit is simpler, but the input impedance at high frequencies is insufficient
Solution Approach 1:
The patent introduces a series resistance in the input network that creates an additional zero point specifically targeted at high frequencies. This local modification to the input network structure provides the necessary high-frequency input impedance without fundamentally changing the overall circuit architecture.
Data Source
Figure 1~2
Figure 3~4
AI summary
A probe system for sensing a differential input signal has a first input network (ENp), to which a first component (Vinp) of the differential input signal is applied in order to produce a first intermediate signal (Ip), and a second input network (ENn), to which a second component (Vinn) of the differential input signal is applied in order to produce a second intermediate signal (In). In the direction of signal flow downstream of the input networks, a differential amplifier (A1) is arranged for the purpose of amplifying the difference between the intermediate signals (Ip, In). At least one compensating network (KNpn) is used to compensate for the influence of the input networks (ENp; ENn) and is arranged on the output side of the differential amplifier (A1) or in a feedback path which connects an output to an input of the differential amplifier.