Sigma-Delta Input Current Cancellation for High Common-Mode Sensing
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Solution Overview
Problem
Sensing differential voltages between nodes with high common-mode voltage is challenging due to the difficulty in isolating the high common-mode voltage from the measurement circuit, leading to errors and increased circuit size, power consumption, and reduced operating speed.
Innovation Solution
An integrated circuit with a voltage sensing circuit and an input current cancellation circuit that uses a sigma-delta modulator to generate a modulated digital bit stream for compensating charge transfer, directly reusing the modulator's digital output to correct for offset and gain factors, thereby canceling signal-dependent input currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the sampling rate is increased to improve measurement speed, then operating speed is improved, but residual current and associated measurement errors increase
Solution Approach 1:
The patent applies preliminary anti-action by injecting a compensating charge through the second capacitor that预先 counteracts the residual current error before it affects the measurement. The compensation circuit generates an equal and opposite charge transfer that cancels the erroneous current flow through the RC filter, thereby eliminating measurement errors even at high sampling rates where such errors would normally accumulate
Solution Approach 2:
The patent implements feedback by using the digital output from the sigma-delta modulator to control the compensation circuit. The modulator continuously monitors the differential input voltage and feeds this information back to the charge injection circuit, which adjusts the compensating charge in real-time to maintain accurate measurements despite variations in sampling rate and input conditions
2Measurement precision
If isolation barrier components are added to block high common-mode voltage, then measurement accuracy is improved, but circuit size and complexity increase
Solution Approach 1:
The patent merges the isolation barrier function with the input sampling capacitors themselves. The first capacitor and its associated switches serve dual purposes: they act as the isolation barrier that blocks high common-mode voltage from reaching the low-voltage circuitry, and simultaneously function as the input sampling capacitors that capture the differential voltage signal. This eliminates the need for separate isolation components
Solution Approach 2:
The first capacitor exhibits multi-functionality by serving as both the high-voltage isolation barrier and the input sampling capacitor. This universal component performs multiple critical functions within the circuit, reducing overall component count and simplifying the circuit architecture while maintaining measurement accuracy through effective common-mode voltage blocking
3Measurement precision
If RC filter time constant is increased to reduce noise, then measurement accuracy is improved, but residual current errors increase due to longer current flow duration
Solution Approach 1:
The patent applies preliminary anti-action by injecting a compensating charge that counteracts the residual current error before it can cause significant voltage drop across the RC filter. The compensation occurs during the sampling phase itself, preventing the error from propagating through the filtering stage and affecting the final measurement, thereby allowing longer filter time constants without proportional increases in error
Data Source
AI summary
An analog-to-digital converter circuit usable for measuring a voltage having a large common-mode voltage includes two input voltage nodes, a voltage sensing circuit (that includes a sigma-delta modulator) that senses a voltage between the nodes, a digital filter that outputs a multi-bit digital value, and an input current cancellation circuit. The input current cancellation circuit supplies/draws cancellation currents to/from the nodes to compensate for currents drawn from/supplied to the nodes by the voltage sensing circuit. The input current cancellation circuit includes a digitally-programmable digital processing circuit and a current canceling circuit. In one example, the digital processing circuit includes a sigma-delta modulator that transforms a single-bit digital signal output by the voltage sensing circuit into a single-bit digital signal that drives the current canceling circuit. The transfer function of the current compensation loop is programmable and adjustable by loading digital trim values into the circuit.