Volume 45 – May 2011

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

By David Guo

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, I_O, is approximately equal to the differential input voltage, V_{IN +} – V_IN–, 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 R_LOAD 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 R_LOAD.

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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