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.
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.
- AD5750EVB (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.
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.
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.
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.