ADC Input Current Cancellation for High Common-Mode Voltage Sensing
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Solution Overview
Problem
Sensing differential voltages between nodes with high common-mode voltages is challenging due to the need to isolate the high common-mode voltage and maximize the input impedance to minimize measurement errors.
Innovation Solution
An integrated circuit with a voltage sensing circuit and an input current cancellation circuit that uses a sigma-delta converter to produce a digital bit stream, which controls a charge injection circuit to cancel the signal-dependent differential input current, thereby reducing measurement errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the sampling rate is increased to improve measurement speed, then the sampling period decreases, but the RC filter time constant becomes significantly longer compared to the sampling period, causing residual current errors to increase
Solution Approach 1:
The patent applies preliminary anti-action by injecting a compensating charge through the charge injection circuit that anticipates and counteracts the residual current error before it affects the measurement. The compensation charge is calculated based on the known RC filter characteristics and sampling parameters, and is injected in advance to cancel out the expected error, thereby maintaining measurement accuracy even at high sampling rates where the RC filter time constant would normally cause significant errors.
2Measurement precision
If the input impedance is increased to minimize measurement errors, then the interaction with source impedance is reduced, but the ability to drive the RC filter and maintain proper charging becomes compromised
Solution Approach 1:
The patent uses the charge injection circuit as an intermediary element that mediates between the high impedance voltage sensing circuit and the RC filter. The charge injection circuit injects compensation charges that effectively drive the RC filter without requiring the voltage sensing circuit itself to have low output impedance. This intermediary approach allows the sensing circuit to maintain high input impedance for accurate measurement while still properly charging the RC filter through the compensating charge injection mechanism.
3Object-affected harmful factors
If the RC filter time constant is increased to reduce high-frequency noise, then filtering performance improves, but residual current flow increases causing larger IR drops and measurement errors
Solution Approach 1:
The patent extracts the error compensation function from the main voltage sensing path and implements it separately through the charge injection circuit. By taking out the residual current error as a separate compensatable quantity, the system can use a large RC filter time constant for effective noise filtering while independently compensating for the resulting IR drops through the charge injection mechanism. This separation allows both goals—noise filtering and measurement accuracy—to be achieved simultaneously.
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 effectively cancels the signal-dependent differential input current, improving the accuracy of voltage sensing by minimizing errors associated with residual currents and source impedance interactions.
Implementation Method 1
A charge injection circuit of an input current cancellation circuit of the integrated circuit injects a compensating charge transfer in response to a modulated single-bit digital bit stream signal
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.