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Volume 45 – May 2011

Low-Power, Unity-Gain Difference Amplifier Implements Low-Cost Current Source

In “Difference Amplifier Forms Heart of Precision Current Source,” published in Analog Dialogue in September 2009, the AD8276 unity-gain difference amplifier and AD8603 micropower op amp implement a precision current source. Figure 1 shows how the circuit can be simplified for use in low-cost, low-current applications.

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

The output current, IO, is approximately equal to the differential input voltage, VIN +VIN, divided by R1, as shown in the following derivation.

Thus, the differential input voltage appears across R1.

Experimental Setup

1. AD5750EVB (AD5750 driver and AD5662 16-bit nanoDAC®) provides a bipolar input to the AD8276.
2. OI-857 multimeter measures input voltage, output voltage, and resistance.
3. The nominal values of R1 and RLOAD are 280 Ω and 1 kΩ, respectively; the measured values are 280.65 Ω and 997.11 Ω, respectively.
4. The output current is calculated by dividing the measured voltage by RLOAD.

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

Experimental Results
Figure 2 shows the output current vs. the input voltage. The differential input voltage, which varies from –3.2 V to +3.2 V, is plotted on the X-axis; the output current is plotted on the Y-axis. The four lines show the ideal current and the real outputs at –40°C, +25°C, and +85°C.

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

Figure 3. Output current error vs. input voltage.

The real output current is limited by the short-circuit output current of the AD8276, as shown in Figure 4. Here, the short-circuit current is about 8 mA at –40°C.

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

Conclusion
By removing the external boost transistor and buffer and adding a single resistor, one can use the AD8276 to construct a low-cost, low-current source with a total error less than about 1.5% over the –40°C to +85°C temperature range. The output current range over temperature is about –11 mA to +8 mA when powered with a ±15-V supply. A unipolar source could be created with a single +5 V supply.

 Author David Guo [david.guo@analog.com] is a field applications engineer in ADI’s China Applications Support Team in Beijing. After earning a master’s degree in electronic and mechanism engineering from Beijing Institute of Technology, David spent two years as a navigation terminal hardware engineer at Changfeng Group. He joined ADI in 2007. (return to top)
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