Chopper-Stabilized Sensor Readout Circuit for High-Impedance Biopotentials
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Solution Overview
Problem
Delta-sigma modulators used in sensor readout circuitry for biopotential signals face challenges in achieving high input impedance while minimizing noise, especially with small electrodes, as they can introduce flicker noise and require additional filtering, which complicates the design and increases the size of the circuitry.
Innovation Solution
The proposed sensor readout circuitry incorporates a transconductance amplifier with a pair of input transistors, a source degeneration resistor, and a current mirror, along with chopper stabilization and a flipped voltage follower, to achieve high input impedance and reduce noise by filtering out flicker noise without folding quantization noise into the signal band, allowing for a compact design.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a transconductance amplifier is added to achieve high input impedance, then input impedance is improved, but noise performance deteriorates due to flicker noise
Solution Approach 1:
The patent applies periodic chopping action at frequency fc to modulate the input signal and noise to different frequency bands. The chopper circuit periodically switches the input signal, moving the low-frequency flicker noise to higher frequencies where it can be filtered out, while the desired signal band is preserved through synchronous demodulation.
Solution Approach 2:
The patent converts the harmful flicker noise generated by the transconductance amplifier into a beneficial filtering opportunity. By chopping the signal at frequency fc and designing the loop filter to pass the chopped signal while attenuating the chopped noise frequencies, the circuit transforms the amplifier's intrinsic noise into a filterable artifact, effectively removing it from the output.
2Object-generated harmful factors
If chopper stabilization is used to filter flicker noise, then noise performance is improved, but quantization noise may be folded into signal band
Solution Approach 1:
The patent carefully selects and adjusts the chopping frequency fc to be higher than the maximum signal frequency but lower than the Nyquist frequency. This parameter selection ensures that when the signal is chopped and processed through the delta-sigma modulator, the quantization noise does not fold into the signal band, while still effectively moving flicker noise out of the passband.
Solution Approach 2:
The patent employs feedback through the delta-sigma modulator's inherent feedback loop, where the quantized output is fed back through a digital-to-analog converter and subtracted from the input signal. This feedback mechanism, combined with the chopping frequency selection, ensures that quantization noise is pushed to frequencies outside the signal band rather than folding into it.
3Object-generated harmful factors
If additional anti-aliasing filter is added to reduce noise, then noise performance is improved, but device complexity and size increase
Solution Approach 1:
The patent merges the anti-aliasing filter function with the existing loop filter in the delta-sigma modulator. The loop filter, which is already required for the modulator's operation, is designed to simultaneously perform noise shaping and anti-aliasing functions, eliminating the need for a separate anti-aliasing filter stage and reducing overall circuit complexity.
Solution Approach 2:
The patent makes the loop filter multi-functional by designing it to perform both its traditional noise-shaping role and the additional anti-aliasing function. This universal filter design allows a single circuit element to accomplish multiple tasks, reducing the total number of components and simplifying the overall readout circuit architecture.
4Volume of moving object
If circuitry size is reduced for implantable devices, then invasiveness is reduced, but noise filtering capability may be compromised
Solution Approach 1:
The patent uses periodic chopping action to achieve noise filtering without requiring large, complex filter circuits. The time-domain modulation approach allows effective noise rejection through frequency separation rather than through large analog filter components, enabling compact implementation suitable for implantable devices.
Solution Approach 2:
The patent replaces traditional analog filtering mechanisms with a digital-based noise rejection approach using chopping and digital signal processing. This substitution allows for more compact circuit implementation since digital processing requires fewer physical components than equivalent analog filter circuits, reducing the overall device size while maintaining noise filtering capability.
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
This solution enables high input impedance necessary for small electrodes, reduces noise performance, and maintains signal quality, making it suitable for neural probes and other biopotential signal sensors, while also eliminating the need for additional anti-aliasing filters.
Implementation Method 1
the transconductance amplifier comprises a first chopper between an input terminal for receiving the analog sensor input signal and an input transistor of the pair of input transistors for upmodulating the analog sensor input signal; whereby chopper stabilization may be used for filtering out the flicker noise
Implementation Method 2
the feedback signal from a digital-to-analog converter (DAC) of the delta-sigma modulator is received across the source degeneration resistor such that subtraction between the analog sensor input signal and the feedback signal is performed in the transconductance amplifier
Implementation Method 3
a current mirror, and current sources, wherein the transconductance amplifier and the current mirror incorporate a flipped voltage follower, wherein a feedback signal from a digital-to-analog converter (DAC) of the delta-sigma modulator is received across the source degeneration resistor such that subtraction between the analog sensor input signal and the feedback signal is performed in the transconductance amplifier and mirrored by the current mirror
Data Source
Figure 1~2
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AI summary
A sensor readout circuitry comprises: a delta-sigma modulator (102); an input stage (110) for receiving an analog sensor input signal, wherein a transconductance amplifier (120) and a current mirror (160) incorporate a flipped voltage follower (128, 130), wherein a feedback signal from a digital-to-analog converter (108) of the delta-sigma modulator (102) is received such that subtraction between the analog sensor input signal and the feedback signal is performed in the transconductance amplifier (120) and mirrored by the current mirror (160) to an output (170, 172) of the input stage (110); wherein the transconductance amplifier (120) comprises a first, second and third chopper (132, 134, 136), whereby a chopping loop between the first, second and third choppers (132, 134, 136) is formed including an input transistor (122) and current sources (180) and whereby the analog sensor input signal is in baseband at a node in which the feedback signal is received.