Low-Cost Current Source Using Unity-Gain Difference Amplifier

The article “Difference Amplifier Forms Heart of Precision Current Source” published in the September 2009 issue of Analog Dialogue describes how to implement a precision current source using the AD8276 unity-gain difference amplifier and the AD8603 micropower op amp. Figure 1 shows a simplified version of this circuit for low-cost, low-current applications.

Figure 1. Simple current source for low-cost, low-current applications.

The output current, I _O , is approximately equal to the differential input voltage, V _{IN +} – V _IN – , divided by R 1 , as derived below.

Therefore, this differential input voltage appears across R1.

Experimental setup

1. The AD5750EVB (AD5750 driver and AD5662 16-bit nanoDAC ^® ) provides the bipolar inputs to the AD8276.
2. Multimeter OI-857 measures input voltage, output voltage and resistance.
3. The nominal values ​​of R1 and R _{LOAD are 280 Ω and 1 kΩ, respectively, and the measured values ​​are 280.65 Ω and 997.11 Ω, respectively.}
4. The measured voltage divided by _LOAD gives the output current.

Figure 2. Ideal and actual output current vs. differential input voltage

Experimental Results
Figure 2 shows the relationship between output current and input voltage. The X-axis is the differential input voltage, ranging from –3.2 V to +3.2 V; the Y-axis is the output current. The four lines show the ideal current output and the actual output at –40°C, +25°C, and +85°C.

Figure 3 shows the output current error vs. input voltage. The three lines show the error at –40°C, +25°C, and +85°C, respectively.

Figure 3. Output current error vs. input voltage

The actual output current is limited by the short-circuit output current of the AD8276 shown in Figure 4. At –40°C, the short-circuit current is approximately 8 mA.

Figure 4. AD8276 short-circuit output current vs. temperature

Summary
By removing the external boost transistor and buffer and adding a resistor, the AD8276 can be used to construct a low cost, low current current source with a total error of less than about 1.5% over the –40°C to +85°C temperature range. The output current range is about –11 mA to +8 mA over the temperature range when operating from ±15 V supplies. A unipolar current source can be constructed when operating from a single +5 V supply.

About the Author
	David Guo [ david.guo@analog.com ] is a field applications engineer at ADI’s China Support Center in Beijing. After receiving his master’s degree in mechanical and electrical engineering from Beijing Institute of Technology, he worked at Changfeng Group for 2 years as a navigation terminal hardware engineer. He joined ADI in 2007.