Neural Sensor Front-End With Impedance-Boosting Chopper Circuit
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Solution Overview
Problem
Existing micro devices for neural recording, such as brain dusts, face challenges in efficiently communicating neural signals due to high electrical impedance, ultra-compact dimensions, and limited power consumption, exacerbated by large dc offsets at the electrode-tissue interface.
Innovation Solution
A front-end device with a capacitive-coupled chopper circuit and an impedance boosting auxiliary path, incorporating a pre-charging buffer and a second gain element in the feedback path, to increase input impedance while maintaining gain adjustment independence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional instrumentation amplifier designs are used, then the device can be integrated into micro devices, but the input impedance is insufficient for biomedical applications
Solution Approach 1:
The amplifier circuit is segmented into multiple functional blocks: a capacitive-coupled chopper circuit for signal amplification, an auxiliary path for impedance boosting, and a feedback path for gain control. This segmentation allows each block to be optimized independently, achieving high input impedance without proportionally increasing overall circuit complexity.
Solution Approach 2:
An auxiliary path is introduced as an intermediary component between the input terminal and ground. This auxiliary path includes an impedance boosting circuit that actively manages the input impedance, mediating between the low-impedance chopper circuit and the high-impedance requirement of biomedical sensors, thereby achieving high input impedance without requiring a complete redesign of the amplifier core.
2Reliability
If the input capacitor is decreased to increase input impedance, then the input impedance improves, but the gain control becomes dependent on capacitor values
Solution Approach 1:
A feedback path is implemented that includes a second gain element and feedback capacitors. This feedback mechanism allows the gain to be controlled independently of the input capacitor value. The feedback path compensates for variations in input capacitance, ensuring that gain adjustment remains versatile and independent while maintaining high input impedance through the auxiliary path.
Solution Approach 2:
The circuit uses parameter changes in the feedback path to decouple gain control from input capacitor selection. By adjusting feedback capacitor values and gain element parameters independently, the system achieves both high input impedance (through small input capacitor) and independent gain adjustment (through feedback path parameter tuning).
3Reliability
If the input capacitor is decreased, then the input impedance increases, but the buffer charging time becomes insufficient
Solution Approach 1:
The auxiliary path performs preliminary action by pre-charging or pre-conditioning the input capacitor through the impedance boosting circuit. This preliminary action ensures that the buffer has sufficient effective charging time even when the input capacitor is small, because the auxiliary path prepares the voltage conditions in advance, allowing the main buffer to charge more efficiently.
4Volume of moving object
If micro device dimensions are reduced for implantation, then the device becomes more suitable for brain dust applications, but the available area for capacitors is reduced
Solution Approach 1:
The invention changes the parameter of input capacitor value to a small value, which directly reduces the area required for the input capacitor. Combined with the auxiliary impedance boosting path, this allows the device to maintain high input impedance while occupying minimal chip area, making it suitable for ultra-compact brain dust implementations.
Data Source
AI summary
A front-end device is arranged to amplify an electric signal from an associated sensor, e.g. for amplifying an electric signal from a neural activity sensor. The front-end device has an amplifier circuit connected between its input and output terminals (Vin, Vout), wherein the amplifier circuit comprises a capacitive-coupled chopper circuit comprising a first gain element and first, second and third chopper switches arranged for operating at a chopper frequency. Further, the amplifier circuit has A) an impedance boosting auxiliary path connected to the input terminal in parallel with a first chopper switch of the CCC, wherein the impedance boosting auxiliary path comprises a pre-charging buffer, and B) a second gain element connected in a feedback path of the CCC. Such front-end device has high input impedance, and the input impedance is uncorrelated with the gain. It is highly suited for implantable micro devices, e.g. brain dusts.