Capacitive Input Choppers for Beyond-Rail Differential Sensing
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Solution Overview
Problem
Instrumentation amplifiers and analog-to-digital converters face challenges in measuring small differential-mode voltages in the presence of large common-mode voltages, as existing solutions like transformers are costly and difficult to integrate, and capacitive-coupled input choppers with diode-protected gates require peak-to-peak clock voltages higher than the sum of diode and threshold voltages for reliable operation.
Innovation Solution
The use of capacitive-coupled input choppers with latches instead of diodes for gate protection, allowing the chopper transistors to be robustly switched with peak-to-peak clock voltages just above the threshold voltage, and floating n-wells for transistors to handle high common-mode voltages, along with additional protection mechanisms like mirrored capacitor discharge for negative surges, enables efficient operation beyond the supply rail without drawing common-mode supply current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If diode-protected gates are used in capacitive-coupled input choppers, then gate protection against voltage surges is improved, but the peak-to-peak clock voltage must be higher than the sum of diode and threshold voltages, increasing the voltage requirement
Solution Approach 1:
The patent changes the protection mechanism from diode-based to latch-based, which alters the voltage parameters required for operation. Latches protect the gates without requiring the peak-to-peak clock voltage to exceed diode forward voltage plus threshold voltage, thereby reducing the voltage requirement while maintaining gate protection reliability
Solution Approach 2:
The patent introduces latches as intermediary protective elements between the clock signal and the chopper transistor gates. These latches act as mediators that provide voltage surge protection without imposing the voltage drop constraints that diodes create, allowing more efficient use of the clock voltage
2Measurement precision
If transformers are used for differential-mode voltage measurement, then measurement capability beyond supply-rail is achieved, but cost and circuit board area increase significantly
Solution Approach 1:
The patent replaces the mechanical/physical transformer system with an electronic capacitive-coupled chopper circuit. This substitution eliminates the need for bulky transformer components and complex magnetic coupling mechanisms, achieving the same differential-mode voltage measurement capability through electronic switching and capacitive coupling, thereby reducing cost and circuit board area
Solution Approach 2:
The patent creates an electronic equivalent or 'copy' of the transformer's differential measurement function using capacitive coupling and chopper circuits. Instead of using the physical transformer to transfer and differentiate signals, the patent uses capacitive-coupled choppers to achieve signal differentiation electronically, maintaining measurement precision while simplifying the physical implementation
3Adaptability or versatility
If input choppers draw common-mode supply current, then operation beyond supply-rail voltage is enabled, but undesired influence on the system to be measured occurs
Solution Approach 1:
The patent extracts or removes the common-mode current draw from the input chopper circuit by using capacitive coupling. The capacitive coupling blocks DC common-mode current paths while still allowing the chopper to operate with common-mode voltages beyond the supply rail, thereby eliminating the harmful current draw while preserving the ability to measure high common-mode voltages
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 allows for reliable measurement of large differential-mode voltages beyond the supply rail without destroying the chopper transistors, reduces charge injection, and prevents common-mode current draw, resulting in low input offset and robust operation across a wide common-mode voltage range.
Implementation Method 1
The gates of the chopper transistors are coupled to clock signals by capacitors
Implementation Method 2
Each latch automatically clamps the voltage on the gate of a chopper transistor to a safe level
Implementation Method 3
chopper transistors with the sources of the chopper transistors connected together and to the inputs
Data Source
AI summary
A method of differential signal transfer from a differential input Vinp and Vinn having a common mode input voltage that can be higher than the power supply voltage by providing an input chopper having first through fourth chopper transistors, each having a source, a drain and a gate, the input chopper having Vinp and Vinn as a differential input, providing an output chopper, capacitively coupling a differential output Voutp and Voutn of the input chopper to a differential input of the output chopper, capacitively coupling a clock to the input chopper and coupling the clock to the output chopper, the clock having a first phase and a second phase opposite from the first phase, the first phase being coupled to the gates of the first and second transistors and the second phase being coupled to the gates of the third and fourth transistors, and providing protection of the gates of the first through fourth transistors from excessive voltages. Various embodiments are disclosed.