Low-Capacitance CMOS Switches for Wider PGA Bandwidth
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Solution Overview
Problem
Existing circuits such as Programmable Gain Amplifiers (PGAs) and Programmable Gain Instrumentation Amplifiers (PGIAs) face performance limitations due to excessive switch capacitance, which affects their AC performance and frequency bandwidth.
Innovation Solution
The implementation of low capacitance n-channel and p-channel analog switch circuits, as well as full CMOS transmission gates, using resistive decoupling and semiconductor regions separated by insulators to reduce parasitic capacitance, along with pole-zero cancellation techniques to improve frequency response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If switches are used in PGA or PGIA circuits to set gain, then the circuit can achieve programmable gain settings, but the switch capacitance limits the AC performance and frequency bandwidth
Solution Approach 1:
The patent extracts the problematic capacitance by separating the switch body from the signal path using an insulator. The switch control terminals remain electrically connected to control the pass FET, but the switch body capacitance is isolated from affecting the AC performance of the PGA/PGIA circuit, thereby resolving the contradiction between programmable gain capability and AC performance.
Solution Approach 2:
The patent introduces an insulator as an intermediary between the switch body and the signal path. This insulator acts as a mediator that allows the switch to control the pass FET while preventing the switch body capacitance from degrading the AC performance and frequency bandwidth of the circuit.
2Device complexity
If traditional switches are used, then the circuit structure is simple, but the parasitic capacitance degrades frequency response
Solution Approach 1:
The patent segments the switch into electrically isolated components: the control terminals remain connected to control the pass FET, while the switch body is separated by an insulator. This segmentation allows the control function to be maintained while the problematic body capacitance is isolated, improving frequency response without significantly increasing overall circuit complexity.
3Ease of operation
If switch body is connected to ground, then the switch operates normally, but the body capacitance couples to AC ground and limits bandwidth
Solution Approach 1:
The patent extracts the switch body from the AC ground connection by inserting an insulator between them. This allows the switch to operate normally through the pass FET control while removing the body capacitance coupling to AC ground that was limiting the bandwidth, thereby resolving the contradiction between normal operation and bandwidth performance.
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
These solutions effectively reduce parasitic capacitance and enhance frequency bandwidth and circuit performance by isolating switches from AC grounds and driving semiconductor regions to desired voltages, thereby improving the frequency response of PGA and PGIA circuits.
Implementation Method 1
A semiconductor region that is separated from a body region of a pass field-effect transistor (FET), such as by an insulator, can be coupled to or driven to a voltage similar to the input voltage or other desired voltage such as to help reduce parasitic capacitance of the switch or T-gate
Implementation Method 2
Resistive decoupling can be used to decouple or isolate the switch or T-gate from one or more AC ground nodes, such as one or more switch control signal inputs or supply voltages
Data Source
AI summary
A low capacitance n-channel analog switch circuit, a p-channel analog switch circuit, and a full CMOS transmission gate (T-gate) circuit are described. Resistive decoupling can be used to isolate the switch or T-gate from AC grounds. A semiconductor region that is separated from a body region of a pass field-effect transistor (FET), such as by an insulator, can be coupled to or driven to a voltage similar to the input voltage or other desired bias voltage (e.g., an operational amplifier output) to help reduce parasitic capacitance of the switch or T-gate. The switch or T-gate can help provide improved frequency bandwidth or frequency response. The switch can be useful in a programmable gain amplifier (PGA) or programmable gain instrumentation amplifier (PGIA) or other circuit in which excessive switch capacitance could degrade circuit performance.