herunterladen
Application Report
SNOA474A–January 2008–Revised April 2013
AN-1515 A Comprehensive Study of the Howland Current
Pump
.....................................................................................................................................................
ABSTRACT
It is well known to analog experts that you can use the positive and negative inputs of an operational
amplifier to make a high-impedance current source (current pump) using a conventional operational
amplifier (op amp). This basic circuit can put out both + and - output current (or zero current) into various
loads. The theory is simple. But the practical problems involved are not so simple or obvious. This
application note provides an indepth study of the Howland Current Pump.
Contents
1 A Comprehensive Study ................................................................................................... 2
2 Applications for the Howland Current Pump ............................................................................ 2
3 The “Improved Howland” Current Pump ................................................................................. 5
4 Dynamics ..................................................................................................................... 6
5 Choice of Amplifiers ........................................................................................................ 6
6 Special Applications ........................................................................................................ 7
7 The Howland Integrator .................................................................................................... 7
8 Multi-Range Current Pump ................................................................................................ 8
Appendix A Output Impedance as a Function of Trimming ................................................................. 9
Appendix B ....................................................................................................................... 11
Appendix C ....................................................................................................................... 12
Appendix D ....................................................................................................................... 13
Appendix E Output Capacitance of the "Improved" Howland Current Pump ............................................ 14
Appendix F Testing and Trimming of Z
OUT
................................................................................... 15
List of Figures
1 The Basic Howland Current Pump ....................................................................................... 2
2 Basic Howland Current Pump with Trim of Z
OUT
........................................................................ 2
3 Basic Howland Current Pump with Trim of Z
OUT
........................................................................ 4
4 Basic Howland Current Pump with Trim of Z
OUT
........................................................................ 4
5 The “Improved Howland” Current Pump................................................................................. 5
6 The “Improved Howland” with Trim for Z
OUT
............................................................................. 6
7 The Howland Integrator .................................................................................................... 7
8 The Positive or Differential Integrator (not a Howland Circuit)........................................................ 7
9 Multi-Range Current Pump ................................................................................................ 8
10 Basic Howland Current Pump with Precision Resistor Network .................................................... 10
11 Trimming Z
OUT
without a Trim-Pot ....................................................................................... 10
12 Transformer-Coupled Test for Z
OUT
...................................................................................... 15
13 Good Test for Z
OUT
Using Floating Scope .............................................................................. 16
All trademarks are the property of their respective owners.
1
SNOA474A–January 2008–Revised April 2013 AN-1515 A Comprehensive Study of the Howland Current Pump
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
