Voltage-to-current converter makes a flexible current reference
Edited by Bill Travis
Art Kay, Texas Instruments, Tucson, AZ -- EDN, September 18, 2003
The voltage-to-current converter in Figure 1 can both source and sink current. The circuit is more flexible than some traditional current references that require different topologies for current sourcing and sinking. Also, you can easily adjust the value of the current reference by simply adjusting the circuit's input voltage. Performing a simple nodal analysis generates the following equation:
You typically set R1 equal to R2. The output current is a function of R3 and the gain of the instrumentation amplifier. Note that capacitor C1 stabilizes the circuit. In this example, R3=10 MΩ, and the instrumentation amplifier's gain is unity. Varying the input, VIN, by ±10V yields a current-output range of ±1 µA. Performing a more detailed nodal analysis on the circuit in Figure 1 yields the following equation:
where VOS1 and VOS2 are the offset voltages of IC1 and IC2, respectively, IB2 is the input-bias current of IC2, and AIC2 is the gain of IC2.
This second equation is useful in understanding error sources and, consequently, can aid in selecting the components that are best suited to an application. For example, for a nanoampere current reference, you should consider the error that the instrumentation amplifier's bias current generates. The example in Figure 1 uses the INA121P FET-input instrumentation amplifier to minimize the input-bias current. A milliampere reference, on the other hand, would focus more on the input offset voltage of the instrumentation amplifier. In general, you can neglect the error that the offset voltage of the op amp generates if you use a precision, low-offset amplifier. However, resistor-mismatch and instrumentation-amplifier gain errors are inevitable, regardless of the application.
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There is no schematic for this article. The link to the PDF file is also broken!
Yaser Azizi - 2010-17-9 13:37:43 PDT
