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

1 Discrete Improved Howland Current Pump – Design 1

Figure 1-1 Discrete Improved Howland Current Pump

#GUID-A2D32AA4-A90E-49BA-8E40-6708748252A6 shows the basic configuration of the Improved Howland current pump that uses one operational amplifier, five discrete resistors and a resistive load, Rload. The current through the load (Iload) can be calculated using #GUID-573B0A03-C43B-458A-ADCA-DFD01C422A3D.

Equation 1. GUID-20200916-CA0I-XZSD-V6HR-0M7GBWDRWMX0-low.png
Equation 2. GUID-20200916-CA0I-W0FF-JN2G-KQHZRVNLNNQN-low.png

In an ideal Improved Howland current pump; resistor R4 is sometimes set to equal R2 - Rs, which produces the expected current value by slightly altering the feedback in the positive loop. This design has limited practicality considering the standard resistor values to choose from, as well as their tolerances. More details on the functionality of the ideal Improved Howland current pump design can be found in the link provided in the GUID-2CBE2D35-D257-4892-8B5D-488525DB7832.html#GUID-06D6365E-3B22-4914-AE24-BC1D4DB6C2F2 section. #GUID-E80380A8-B890-4499-A613-009576A1BE7C shows an example of Design 1 and the results with a modified R4 resistor. The circuit is designed for a 10-mA output current with an input voltage difference of 5 V using ideal components.

Figure 1-2 Ideal 10-mA Improved Howland Current Pump

Benefits: One benefit of this configuration is the freedom to choose the gain value (G), and ultimately design for output headroom (the maximum output voltage swing or compliance range), by varying the Vshunt voltage. That is the case because all the resistors are discretely selected for the circuit. Another benefit is the ability to select an op amp which fits the specific design requirements of the application such as size, power, and supply voltage. One last benefit for this design is that only one op amp is required.

Disadvantages: One disadvantage of this configuration is an error that is caused by the Ifeedback current affecting the Iload current. An ideal current source has infinite output impedance; however, the finite output impedance of this configuration is determined by the two feedback resistors in series (R3+R4). This can lead to significant error in Iload that is more apparent when the design does not use the modified R4 resistor.

To minimize error caused by Ifeedback, choosing higher value resistors for the feedback paths increases the output impedance of the current source. This comes at the expense of more thermal noise due to larger resistor values. Possible bandwidth limitations and stability issues caused by large resistances and parasitic capacitances in the circuit also become more prevalent. To learn more about noise and stability, TI Precision Lab video links are found in the GUID-2CBE2D35-D257-4892-8B5D-488525DB7832.html#GUID-06D6365E-3B22-4914-AE24-BC1D4DB6C2F2 section.

Another disadvantage with this configuration comes from the discretely chosen resistors in the feedback network. Discrete builds with 0.1% tolerance resistors can have a worst-case CMRR value of around 60 dB, which can be too low for precision applications. More information on the importance of matching resistors is found in the link provided in the GUID-2CBE2D35-D257-4892-8B5D-488525DB7832.html#GUID-06D6365E-3B22-4914-AE24-BC1D4DB6C2F2 section. This resistor mismatch also creates gain error in the design, which contributes to the overall error. One final consideration for a discrete version of this configuration is the PC board space required considering external resistors are being used for the difference amplifier.