SBOA437A October   2020  – February 2023 INA1620 , INA592 , INA597 , OPA191 , OPA192 , OPA196 , OPA197 , OPA310 , OPA990

 

  1.   Abstract
  2.   Trademarks
  3. 1Discrete Improved Howland Current Pump – Design 1
  4. 2Discrete Improved Howland Current Pump With Buffer – Design 2
  5. 3Integrated Improved Howland Current Pump - INA592 and Buffer – Design 3
  6. 4Integrated Improved Howland Current Pump - INA592 and Settable Gain – Design 4
  7. 5Design Needs and Considerations
  8. 6Operational Amplifier Considerations
  9. 7References
  10. 8Revision History

2 Discrete Improved Howland Current Pump With Buffer – Design 2

Figure 2-1 Discrete Improved Howland Current Pump With Buffer

#GUID-9C0B6934-B7E7-4760-BE0F-E165360BB767 shows a similar configuration of an Improved Howland current pump that uses two op amps. The buffer has high input impedance, which introduces high output impedance into the current source. Note when the buffer is added, the circuit designer should no longer modify R4 by the value of Rs. Iload can now be calculated using #GUID-573B0A03-C43B-458A-ADCA-DFD01C422A3D provided below:

Equation 3. GUID-20200916-CA0I-GH6T-XP42-8CK2NWNCNWDB-low.png
Equation 4. GUID-20200916-CA0I-1NXX-TL25-ZVLVSGMP2J0V-low.png

Benefits: This configuration has the same benefits as the non-buffered configuration shown in GUID-E6D07E17-1EC8-4748-A33B-409C33FCC9B6.html#GUID-119B9B14-A9B3-417B-B40C-23ABEFAFD8F5; however, it has the added benefit of minimizing error by practically eliminating Ifeedback current due to the added buffer. The second op amp therefore results in the ability to choose lower value resistors for the feedback network. This allows the circuit designer to minimize thermal noise attributed to high value resistors and also minimizes any stability and bandwidth concerns in the circuit.

#GUID-997AC886-218F-4C16-B939-8734399AF37E shows the same 10-mA current source; the buffer practically eliminates Ifeedback current.

Figure 2-2 Ideal 10-mA Improved Howland Current Pump With Buffer

Disadvantages: A similar disadvantage to the one op amp design comes from the mismatched discrete resistors. The overall size of the circuit increases with the addition of a second op amp which can be a disadvantage for designs that are limited in space. Fortunately, many precision op amps are available in dual configurations, which hardly add to the size or cost of the circuit.