SBAS337E April   2005  – March 2018 DAC7811

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Block Diagram
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics: VDD = 5 V
    7. 6.7 Typical Characteristics: VDD = 2.7 V
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
    4. 7.4 Device Functional Modes
      1. 7.4.1 Serial Interface
      2. 7.4.2 Input Shift Register
      3. 7.4.3 SYNC Interrupt (Stand-Alone Mode)
      4. 7.4.4 Daisy-Chain
      5. 7.4.5 Control Bits C3 to C0
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Unipolar Operation Using DAC7811
      2. 8.1.2 Bipolar Operation Using the DAC7811
      3. 8.1.3 Stability Circuit
      4. 8.1.4 Amplifier Selection
      5. 8.1.5 Programmable Current Source Circuit
    2. 8.2 Typical Application
      1. 8.2.1 Single Supply Unipolar Multiplying DAC
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
        3. 8.2.1.3 Application Curve
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resource
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

8.1.5 Programmable Current Source Circuit

A DAC7811 can be integrated into the circuit in Figure 31 to implement an improved Howland current pump for precise voltage to current conversions. Bidirectional current flow and high voltage compliance are two features of the circuit. With a matched resistor network, the load current of the circuit is shown by Equation 3:

Equation 3. DAC7811 q_il_bas337.gif

The value of R3 in Equation 3 can be reduced to increase the output current drive of U3. U3 can drive ±20mA in both directions with voltage compliance limited up to 15V by the U3 voltage supply. Elimination of the circuit compensation capacitor C1 in the circuit is not suggested as a result of the change in the output impedance ZO, according to Equation 4:

Equation 4. DAC7811 q_zo_bas337.gif

As shown in Equation 4, with matched resistors, ZO is infinite and the circuit is optimum for use as a current source. However, if unmatched resistors are used, ZO is positive or negative with negative output impedance being a potential cause of oscillation. Therefore, by incorporating C1 into the circuit, possible oscillation problems are eliminated. The value of C1 can be determined for critical applications; for most applications, however, a value of several pF is suggested.

DAC7811 ai_bidirect_bas337.gifFigure 31. Programmable Bidirectional Current Source Circuit