PoE Gain Circuit Offset Compensation
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Solution Overview
Problem
Conventional gain circuits for Power over Ethernet (PoE) suffer from poor precision in voltage gain due to manufacturing process variations, leading to significant errors and inability to compensate for input offset voltages, especially when current from powered devices is 7.5 mA.
Innovation Solution
A gain circuit with an operational amplifier, resistors, and a voltage adjusting circuit that sets the negative terminal voltage greater than the maximum offset voltage, using a capacitor to store and deduct offset voltage, and a clock signal to control connections, ensuring the negative terminal voltage is not limited by grounding, thereby maintaining precision and compensating for input offset.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional gain circuit with operational amplifier is used, then the circuit can amplify voltage, but manufacturing process variations cause device mismatches that result in input offset voltage and poor gain precision
Solution Approach 1:
The patent applies preliminary action by storing the input offset voltage in a capacitor before the amplification process. The circuit captures and stores the offset voltage generated by device mismatches in advance, then subtracts it from the amplified signal to compensate for the precision error. This proactive approach prevents the offset voltage from degrading the final measurement precision.
Solution Approach 2:
The patent implements feedback by measuring the input offset voltage and feeding it back to the output stage for compensation. The circuit measures the offset voltage generated by mismatched transistors, processes this information through the capacitor storage and switching mechanism, and subtracts the measured offset from the final output signal, thereby improving voltage gain precision through closed-loop compensation.
2Ease of manufacture
If the negative terminal of operational amplifier is grounded, then the circuit structure is simple, but the input offset voltage cannot be compensated when current is 7.5 mA
Solution Approach 1:
The patent introduces an intermediary mechanism between the grounded negative terminal and the offset compensation function. A capacitor is connected to the negative terminal, acting as an intermediary element that can store the input offset voltage. The switching mechanism then enables this capacitor to contribute its stored voltage to the negative terminal, effectively providing offset compensation while maintaining the simple grounded structure during normal operation.
Solution Approach 2:
The patent applies dynamics by making the negative terminal voltage dynamic rather than fixed at ground potential. A switching mechanism controlled by a clock signal dynamically connects the capacitor to the negative terminal at specific times, allowing the terminal voltage to vary between ground potential and a compensated level. This dynamic adjustment enables offset compensation only when needed, maintaining simplicity otherwise.
3Ease of manufacture
If device dimensions are made identical to ensure matching, then manufacturing is easier, but process variations still cause offset voltage that cannot be compensated
Solution Approach 1:
The patent converts the harmful input offset voltage into a beneficial compensation signal. Instead of trying to eliminate the offset voltage through more precise manufacturing, the circuit deliberately measures the offset voltage that arises from device mismatches and uses it to compensate for itself. The capacitor stores this offset voltage, and the switching mechanism applies it back to cancel the original error, transforming the harmful effect into a corrective action.
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
The solution achieves precise voltage gain with reduced errors, maintaining accuracy even when input offset voltage ranges from -3.75 mV to -10 mV, and supports stable voltage supply to power sourcing equipment with multiple ports.
Implementation Method 1
a capacitor to store and deduct offset voltage
Implementation Method 2
The gain circuit 10 includes the operational amplifier OPA, and the first resistor R1 and the second resistor R2 connected in series to an output terminal VO of the operational amplifier OPA and a ground GND
Implementation Method 3
The return signal on terminal PortN is inputted through the power transistor 51, and converted into a voltage Vrs by the resistor Rsense
Data Source
AI summary
A gain circuit for power sourcing equipment of Power over Ethernet is disclosed and provides a guard band for the input offset voltages of its operational amplifier. The guard band may be represented by a voltage value and may be set to be greater than the maximum input offset voltage of the input terminals of the operational amplifier. In some embodiment, the guard band allows the voltage of the negative input terminal of the operational amplifier to be greater than 0 V without sacrificing the gain precision.