Continuous-Time DC-Blocking TIA for Compact AC/DC Signal Separation
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Solution Overview
Problem
Existing integrated circuits face challenges in efficiently separating the AC and DC components of sensor signals like PPG signals, requiring substantial chip area for capacitors and discrete components, which increases costs and device size.
Innovation Solution
Implementing a transimpedance amplifier with a floating bulk field effect transistor (FET) as a resistive element and a 25 pF integrating capacitor to cancel the DC component in the analog domain, using operational phases to isolate the AC component, thereby reducing the need for discrete components and minimizing chip area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If discrete capacitors and resistors are used to filter AC component from DC component, then filtering performance is improved, but chip area and device size increase
Solution Approach 1:
The patent extracts the DC component from the sensor signal using a dedicated DC measurement path with switching circuitry. The DC component is measured separately during a DC measurement phase when the LED is off, then subtracted from the total signal during the AC measurement phase. This extraction approach eliminates the need for large discrete capacitors and resistors that would otherwise be required for filtering, thereby reducing chip area while maintaining filtering performance.
Solution Approach 2:
The patent employs periodic switching between DC measurement mode and AC measurement mode. During DC measurement phase, the switching element connects the integrator to measure the DC component. During AC measurement phase, the switching element connects the TIA to measure the AC component. This periodic action allows the same hardware components to serve dual purposes, eliminating the need for separate discrete filtering components and reducing overall chip area.
2Measurement precision
If discrete components are used for DC blocking, then signal separation accuracy is improved, but manufacturing cost and device complexity increase
Solution Approach 1:
The patent merges the DC blocking function with the TIA and integrator circuits by using switching elements to dynamically reconfigure the circuit topology. The same operational amplifiers and capacitors serve both as DC blocking elements and as signal processing components. This merging eliminates the need for separate discrete DC blocking components, reducing device complexity and manufacturing cost while maintaining signal separation accuracy through precise switching control.
Solution Approach 2:
The patent introduces dynamic switching control to change the circuit configuration based on measurement phase. The switching elements dynamically connect or disconnect circuit paths to enable DC measurement mode or AC measurement mode. This dynamic reconfiguration allows the circuit to adapt its topology for different measurement requirements, eliminating the need for static discrete DC blocking components and reducing overall device complexity.
3Measurement precision
If large capacitors are used for low frequency AC component measurement, then measurement accuracy is improved, but chip area and material requirements increase
Solution Approach 1:
The patent uses periodic switching to separate DC and AC measurement into different time phases. During AC measurement phase, the switching element connects the TIA to process only the AC component at frequencies around 1 Hz. During DC measurement phase, the integrator measures the DC component. This temporal separation allows the use of smaller integrated capacitors instead of large discrete capacitors, as the capacitors only need to handle AC signals during their active phase, reducing chip area and material requirements while maintaining measurement accuracy.
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
Enables efficient separation of AC and DC components within integrated circuits, reducing material and size requirements while maintaining signal accuracy for pulse rate and SpO2 measurements.
Implementation Method 1
a capacitor coupled between the op amp inverting input of the operational amplifier and the op amp output of the operational amplifier
Implementation Method 2
a first field effect transistor (FET) coupled between the op amp inverting input of the operational amplifier and the TIA output of the TIA
Data Source
AI summary
An example of an apparatus includes a transimpedance amplifier (TIA) and an integrator circuit that includes an operational amplifier, a capacitor coupled between the operational amplifier's inverting input and output, a first FET coupled between the inverting input of the op amp and the output of the TIA, a resistive element connected to a lower supply rail, and a second FET coupled between the inverting input of the TIA and the resistive element and having a second gate connected to the output of the operational amp.