Analysis of Improved Howland Current Pump Configurations
Abstract
The Improved Howland current pump is a circuit that uses a difference amplifier to impose a voltage across a shunt resistor, creating a voltage-controlled current source capable of driving a wide range of load resistances. This versatility of this design can be useful in many applications that require a current source capable of bipolar (source or sink) operation. Part of the versatility is the ability to make small alterations to the design that improves the overall performance of the circuit. This article analyzes a few Improved Howland current pump configurations and provides recommendations on how to enhance performance.
A common goal of these designs is to create a high-output impedance current source that can source or sink approximately 25 mA of current while employing the Improved Howland current pump topology. Analysis is done on four different configurations and some benefits and disadvantages of each configuration is discussed. Depending on design requirements, one configuration can be more appropriate than another for a specific application. Take precautions when driving reactive loads in an Improved Howland current pump circuit. Additionally, some loads can cause the circuit to become unstable due to insufficient phase margin. Only resistive loads are discussed in this article.
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1 Discrete Improved Howland Current Pump – Design 1
#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.
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.
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.
2 Discrete Improved Howland Current Pump With Buffer – Design 2
#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:
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.
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.
