Hourglass ADC Front-End for Wide-Range Current Measurement
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Solution Overview
Problem
Current measurement technologies face challenges in accurately measuring small electrical currents from sensors, particularly in biosensing applications where currents are often in the range of sub-picoamps to microamps, requiring wide dynamic range, high linearity, and low bandwidth.
Innovation Solution
The development of an analog-to-digital converter circuit, referred to as the 'hourglass ADC,' which includes an hourglass switch and a first-order predictor to prevent feedback capacitor saturation, allowing for improved linearity and dynamic range by splitting the input current into fine and coarse components, and using a linear digital-to-analog converter to reduce the input range, thereby enhancing the circuit's ability to handle small currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional transimpedance amplifier is used to measure small currents, then the measurement capability is limited, but the circuit complexity remains low
Solution Approach 1:
The input current is segmented into two components: a coarse current component that is subtracted by a digital-to-analog converter, and a fine current component that is measured by the transimpedance amplifier. This segmentation allows the TIA to operate in its optimal linear range while maintaining the ability to measure both large and small currents, thereby improving measurement precision without excessive circuit complexity
Solution Approach 2:
The coarse current component is predicted and subtracted before the fine current measurement takes place. The predictor estimates the input current and the DAC generates the corresponding coarse current to be subtracted from the total input current, preparing the signal in advance for accurate measurement by the TIA
2Measurement precision
If the feedback capacitor integrates current over time, then the dynamic range is reduced, but the measurement accuracy is improved
Solution Approach 1:
The circuit dynamically switches between integrating the fine current component and subtracting the predicted coarse current component. The hourglass switch alternates between connecting the TIA output to the feedback capacitor and connecting it to the DAC input, allowing the system to adapt to different current magnitudes and maintain both accuracy and dynamic range
Solution Approach 2:
The circuit uses feedback from the TIA output to control the hourglass switch and the DAC. When the TIA output exceeds a reference voltage, the comparator triggers the hourglass switch to change state, and the DAC adjusts the coarse current subtraction accordingly, creating a feedback mechanism that maintains operation within the optimal dynamic range
3Adaptability or versatility
If the feedback capacitor saturates, then the measurement range is limited, but the power consumption is low
Solution Approach 1:
The predictor and DAC perform preliminary anti-action by estimating and subtracting the coarse current component before it can cause the feedback capacitor to saturate. This prevents saturation by counteracting the large current component in advance, allowing the TIA to maintain its measurement range without requiring additional power-intensive components
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 hourglass ADC achieves a dynamic range of 160 dB, a Schreier figure of merit of 197 dB, and 7 ppm integral non-linearity error, effectively handling currents from 100 femtoamps to 10 microamps with improved linearity and reduced power consumption.
Implementation Method 1
a transimpedance amplifier including a feedback capacitor, wherein the transimpedance amplifier has a non-inverting input and an inverting input
Implementation Method 2
integrating, by a transimpedance amplifier including a feedback capacitor, a fine input current
Data Source
AI summary
In one aspect, an analog-to-digital converter circuit includes a transimpedance amplifier including a feedback capacitor electrically connected between an inverting or a non-inverting input of the transimpedance amplifier and an output of the transimpedance amplifier. The circuit includes an hourglass switch electrically connected on a first side to a first input and a second input, and electrically connected on a second side to the non-inverting input and the inverting input. A fine input current to the transimpedance amplifier is received at the first and second inputs. In a first mode, the hourglass switch electrically connects the first input to the non-inverting input and the second input to the inverting input, and in a second mode, the hourglass switch electrically connects the second input to the non-inverting input and the first input to the inverting input.