Instrumentation Amplifier Common-Mode Shift for Rail-to-Rail Gain

Solution Overview

Problem

Conventional instrumentation amplifiers have a limited common mode input range, leading to rapid performance degradation when output voltages approach the supply rails, as they rely on NMOS and PMOS differential input pairs that are ineffective near the positive or negative supply voltages, resulting in voltage clamping and reduced gain.

Innovation Solution

The implementation of a common mode adjustment component that dynamically shifts the common mode output voltage using controlled current or voltage sources to prevent clamping, allowing for rail-to-rail input range and high amplification independent of the common mode input voltage, by employing feedback resistors or controlling voltage at input resistor nodes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If PMOS based differential input transistor pair is used, then low common mode input voltage range accuracy is improved, but high common mode input voltage range performance deteriorates

Engineering Contradiction:

Improvelow common mode input voltage range accuracyVSAvoidhigh common mode input voltage range performance

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The input stage is divided into two separate differential transistor pairs: one PMOS pair for low common mode voltage operation and one NMOS pair for high common mode voltage operation. Each pair operates in its optimal voltage range, eliminating the performance degradation that occurs when a single pair type is used across the entire range.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention changes the operating parameters by switching between PMOS and NMOS differential pairs based on the common mode input voltage level. This parameter-based selection allows each transistor pair to operate in its optimal region, achieving both low and high voltage range accuracy.

Inventive Principle:

Principle #35Parameter changes

2Measurement precision

If NMOS based differential transistor pair is used, then high common mode input voltage range accuracy is improved, but low common mode input voltage range performance deteriorates

Engineering Contradiction:

Improvehigh common mode input voltage range accuracyVSAvoidlow common mode input voltage range performance

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The input stage is divided into two separate differential transistor pairs: one PMOS pair for low common mode voltage operation and one NMOS pair for high common mode voltage operation. Each pair operates in its optimal voltage range, eliminating the performance degradation that occurs when a single pair type is used across the entire range.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention changes the operating parameters by switching between PMOS and NMOS differential pairs based on the common mode input voltage level. This parameter-based selection allows each transistor pair to operate in its optimal region, achieving both low and high voltage range accuracy.

Inventive Principle:

Principle #35Parameter changes

3Adaptability or versatility

If conventional amplifier design is used, then rail to rail input range is achieved, but gain and performance near supply rails deteriorates

Engineering Contradiction:

Improverail to rail input rangeVSAvoidgain near supply rails

Core Design Contradiction:

Adaptability or versatilityVSMeasurement precision

Solution Approach 1:

Different transistor pair types are assigned to different output voltage regions: PMOS pairs handle low output voltages near the negative rail while NMOS pairs handle high output voltages near the positive rail. This local optimization ensures high gain and performance in each region without the rapid degradation seen in conventional designs.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The invention dynamically changes the active transistor pair based on the output voltage level, switching between PMOS and NMOS configurations to maintain optimal gain and performance across the entire rail-to-rail output range.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS8823453B2Instrumentation amplifier with rail-to-rail input range

Publication Date: 2014.09.02 SEMTECH CORP

AI summary

A system and method for adjusting a common mode output voltage in an instrumentation amplifier is provided. In one aspect, the common mode output voltage is increased or decreased with respect to the common mode input voltage to enable high amplification of the signal input to the instrumentation amplifier. Moreover, the common mode output voltage can be driven to (or approximately to) a target voltage value such as, but not limited to, half the supply, even if the common mode input voltage is close to supply or ground rail voltage. Thus, a high amplification of the differential input voltage can be obtained and utilized for various applications requiring rail to rail input.