Volume 45 – May 2011
article in PDF format. (926KB)
Low-Power, Unity-Gain Difference Amplifier Implements
Low-Cost Current Source
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.
1. Simple current source for low-cost and low-current applications.
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.
(AD5750 driver and AD5662 16-bit nanoDAC®) provides a bipolar input
to the AD8276.
OI-857 multimeter measures input voltage, output voltage, and resistance.
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.
The output current is calculated by dividing the measured voltage by RLOAD.
2. Ideal and real output current vs. differential input voltage.
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.
3 shows the output current error vs. the input voltage. The three lines
show the error at –40°C, +25°C, and +85°C.
3. Output current error vs. input voltage.
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.
4. AD8276 short-circuit output current vs. temperature.
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.
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.
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