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Download this article in PDF format. (97 KB) Choose Resistors to Minimize Errors in Grounded-Load Current Source By David Guo Operational amplifiers are frequently used to make high-quality current sources in a variety of applications, such as industrial process control, scientific instrumentation, and medical equipment. Single Amplifier Current Sources , published in The improved Howland current source, shown in Figure 1, is very popular because it can drive a grounded load. The transistor, which allows relatively high currents, can be replaced by a MOSFET to achieve even higher currents. For low cost, low current applications, the transistor can be eliminated, as shown in Difference Amplifier Forms Heart of Precision Current Source , published in The accuracy of this current source is determined by the amplifier and the resistors. This article shows how to choose the external resistors to minimize errors.
Analysis of the improved Howland current source yields the transfer function:
In Equation 1, the load resistance influences the output current, but if we set R
Here, the output current is only a function of R
_{1} = R_{2} = R_{3} = R_{4}. Now, Equation 1 simplifies to:
If R
Here, the output current depends only on the resistance of R In some cases, the input signal may need to be attenuated. For example, with a 10-V input signal and R If R
Tip 3: Larger value for R In most cases, R
_{1} = R_{2} = R_{3} = R_{4} = 100 kΩ,_{5} = 100 Ω, and R_{L} = 500 Ω. With the input voltage set to 0.1 V, the output current should be 1 mA. Table 1 shows the output current error caused by different resistor tolerances. To obtain 0.5% current accuracy, choose 0.01% tolerance for R_{1}/R_{2}/R_{3}/R_{4}, 0.1% for R_{5}, and 5% for R_{L}. Resistors with 0.01% tolerance are expensive, so a better choice would be to use an integrated difference amplifier, such as the AD8276, which has better resistor matching and is more cost effective.
Analog Dialogue, Volume 45, Number 2, 2011.Loe, James M. Grounded-load current source uses one operational amplifier, Miller, Bill. Single Amplifier Current Sources , Moghimi, Reza. Ways to Optimize the Performance of a Difference Amplifier , AN-589. Zhao, Neil, Reem Malik, and Wenshuai Liao. Difference Amplifier Forms Heart of Precision Current Source ,
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