Differential Sensor ADC Circuit With Switched Current Balancing
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Solution Overview
Problem
Existing sensor systems with resistive sensing elements in a bridge configuration face challenges in generating low noise and accurate reference voltage levels, leading to reduced performance due to high frequency noise conversion into the digital domain and high linearity requirements for the ADC.
Innovation Solution
A sensor circuit with first and second sensor paths connected between supply lines, utilizing a differential analogue to digital converter with control logic to selectively switch current from current sources between the paths, improving power supply rejection ratio and requiring only two sensing elements, which are isolated from the supply line.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If reference voltage levels are generated indirectly from the supply voltage using voltage dividers, then the ADC can operate with a simple circuit configuration, but the reference voltage levels become noisy and inaccurate, and a significant fraction of power is consumed for their generation
Solution Approach 1:
The patent extracts the sensing elements from the supply voltage line by introducing a virtual ground node that isolates the differential sensing circuit from the single-ended supply. This separation removes the noisy reference voltage generation from the supply domain, allowing accurate differential measurements without complex reference voltage circuits.
Solution Approach 2:
The virtual ground node acts as an intermediary between the single-ended supply voltage and the differential sensing elements. It provides a stable reference point for the differential pair without requiring direct connection to the supply, thereby eliminating the need for noisy voltage divider circuits while maintaining circuit simplicity.
2Power
If high frequency noise is present at the supply voltage, then the ADC can process a large full signal swing, but the noise is converted into the digital domain causing aliasing and requiring high linearity requirements
Solution Approach 1:
The patent converts the harmful high-frequency noise from the supply voltage into a beneficial filtering opportunity. By using the differential configuration with virtual ground, the circuit naturally rejects common-mode noise through differential signaling, and the low-pass filtering inherent in the ADC input stage attenuates high-frequency components before they can cause aliasing.
Solution Approach 2:
The virtual ground node creates an equipotential reference point that balances the differential sensing circuit. This equipotential reference allows the ADC to handle large signal swings symmetrically while maintaining common-mode rejection, thereby processing large voltage ranges without amplifying supply noise into the digital domain.
3Measurement precision
If four sensing elements are used in a full bridge configuration, then the sensor system provides balanced differential output, but the circuit occupies more area and requires more components
Solution Approach 1:
The patent segments the full bridge configuration into a simplified differential pair structure. Instead of using four sensing elements in a complete bridge, it employs two sensing elements connected to a virtual ground, achieving the essential differential measurement function with fewer components and reduced area while maintaining measurement precision.
Data Source
AI summary
A sensor circuit incorporates an analog to digital converter for providing a digital signal derived from sensing elements connected in a bridge configuration. The sensor circuit comprises first and second paths comprising respective first and second sensing elements connected between first and second supply lines; an analog to digital converter having a differential input connected to receive a differential voltage signal (Vinp−Vinn) between the first and second sensing elements and an output for providing a digital output signal (Dout) representing a difference between the first and second sensing elements, the analog to digital converter comprising: current sources connected between the first and second supply lines, each current source being switchably connected to either the first or second sensing elements; and control logic configured to selectively switch current from each of the current sources to either the first path or the second path in dependence on the differential voltage signal.