5 Design Needs and Considerations

The need for a simple voltage-controlled current source can be readily met by implementing the Improved Howland current pump topology and due to the versatility, there are many options to choose from as discussed previously. When designing for a specific application there are many parameters to consider. With all four configurations, non-ideal characteristics of any op amp and resistor are an inherent source of error in the design. One non-ideal characteristic of an op amp to consider is the offset voltage. The effect this non-ideal characteristic has on the final performance of the Improved Howland current pump circuit can be significant. Using precision op amps with very low offset voltage (< 100 µV) considerably reduces the error the circuit contributes. The offset voltage information is included in the electrical characteristics table of the op amp data sheet.

Another non-ideal characteristic of an op amp to consider is the output voltage swing limitations as the output current changes. When considering output headroom performance, refer to the typical values given in the Output Voltage Swing vs. Output Current graphs in the data sheet of the op amp. Doing so allows one to account for the output swing limitations. Similar considerations must be taken to make sure the common-mode input voltage range of the op amp is not violated. For designs 2 through 4, account for input and output swing limitations for both op amps. These input and output limitations contribute to the overall voltage compliance of the current source.

As mentioned throughout the article, each design has disadvantages and depending on the specific design goals, one design can be more desirable for an application than another design. Considering parameters such as the supply voltage of the amplifier, output impedance, thermal noise due to resistors, and the op amps, the amount of freedom to design for output headroom, and overall accuracy are a good start when narrowing down which design to use.

Use #GUID-626DCF40-8484-4151-A9D1-7097C39D81A4/GUID-F39199EE-832C-42CF-8A89-84594D5F4F10 as a starting place when choosing which design and the level of precision of op amps to implement. In some cases implementing a general purpose op amp, such as the OPA310, OPA2310, OPA990, or OPA2990, is enough for specific design goals compared to more precise op amps like the OPA192 or OPA2192.

When narrowing down which configuration to use, consider the error, cost, and size budgets of the system. Additionally, consider design specifications such as the current required through the load, output headroom, and voltage constraints for a specific design.