SBOS069A October   1997  – December 2024 INA122

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 Recommended Operating Conditions
    3. 5.3 Thermal Information
    4. 5.4 Electrical Characteristics
    5. 5.5 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Setting the Gain
      2. 6.3.2 Input Common-Mode Range
      3. 6.3.3 Input Protection
      4. 6.3.4 Output Current Range
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Offset Trimming
      2. 7.1.2 Input Bias Current Return Path
    2. 7.2 Typical Application
      1. 7.2.1 Resistive-Bridge Pressure Sensor
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
        3. 7.2.1.3 Application Curve
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Development Support
        1. 8.1.1.1 PSpice® for TI
        2. 8.1.1.2 TINA-TI (Free Software Download)
    2. 8.2 Receiving Notification of Documentation Updates
    3. 8.3 Support Resources
    4. 8.4 Trademarks
    5. 8.5 Electrostatic Discharge Caution
    6. 8.6 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

Package Options

Refer to the PDF data sheet for device specific package drawings

Mechanical Data (Package|Pins)
  • D|8
  • P|8
Thermal pad, mechanical data (Package|Pins)
Orderable Information

7.2.1.2 Detailed Design Procedure

This section provides basic calculations to design the instrumentation amplifier circuit with respect to the given design requirements.

One of the key considerations in resistive-bridge sensors is the common-mode voltage, VCM. If the bridge is balanced (no pressure, thus no voltage change), VCM(zero) is half of the bridge excitation (VEXT). In this example VCM (zero) is 2.5V. For the maximum pressure of 12psi, the bridge common-mode voltage, VCM(MAX), is calculated by:

Equation 2. V C M ( M A X ) =   V D I F F 2   +   V C M ( z e r o )

where

Equation 3. V D I F F = S M A X   ×   V E X T   ×   P M A X   =   2.5   m V V ×   p s i   ×   5 V   ×   12 p s i   =   150 m V

Thus, the maximum common-mode voltage applied results in Equation 4:

Equation 4. V C M ( M A X ) = 150   m V 2 +   2.5 V = 2.575 V

Similarly, Equation 5 calculates the minimum common-mode voltage.

Equation 5. V C M ( M I N ) = – 150   m V 2 +   2.5 V = 2.425 V

The next step is to calculate the gain required for the given maximum sensor output voltage span, VDIFF, in respect to the required VOUT, which is the full-scale range of the ADC.

Equation 6 calculates the gain value using the maximum input voltage and the required output voltage:

Equation 6. G = V O U T V D I F F ( M A X ) = 3.0   V 150   m V = 20 V / V

The INA122 has a gain range from 5V/V to 10000V/V. To set the gain to 20V/V, set RG to 13.3kΩ to provide the maximum output signal swing for the ADC.

Next, make sure that the INA122 can operate within this range by checking the Input Common-Mode Voltage vs Output Voltage curves listed in the Typical Characteristics section. The relevant figure is also in this section for convenience. Based on Figure 7-4, an output signal swing of 3V is supported for the input signal swing between 2.425V and 2.575V, thus allowing linear operation.

INA122 Input Common-Mode Voltage vs Output VoltageFigure 7-4 Input Common-Mode Voltage vs Output Voltage

An additional series resistor in the Wheatstone bridge string (R1) may or may not be required. This is decided based on the intended output voltage swing for a particular combination of supply voltage, reference voltage, and the selected gain for an input common-mode voltage range. R1 helps adjust the input common-mode voltage range, and thus can help accommodate the intended output voltage swing. In this particular example, R1 is not required and can be shorted to ground.