# Precision programmable current source uses two ICs

This Design Idea mates a precision current source IC with precision difference amp chips to create a programmable current source or sink.

The resistor-programmable current source/sink in **Figure 1 **illustrates the basic topology, taking advantage of tightly matched on-chip resistor ratios instead of relying on absolute tolerances.

The *I _{IN}* current source gives rise to a reference voltage across

*R*at the op-amp inverting input. The same potential is found across the

_{f}*R*resistor, so that an output current of

_{f}/N*N*×

*I*will be generated.

_{IN}Despite a decreased compliance voltage due to the absence of direct connections to the op-amp inputs, the LT1991/5/6 is used as a single chip current divider. **Figure 2** shows an example configuration, with a REF200 as the input current reference. Due to the high value of the internal feedback resistor connected to the op-amp inverting input (450 kΩ), a parallel connection with the 450/9 kΩ resistor is necessary to avoid op-amp output saturation induced by the injected *I _{IN}* current. The negative feedback resistance is thus equal to 450/10 kΩ, or 45 kΩ.

Given the 450 kΩ positive feedback resistor, *N* for **Figure 2 **is 0.1, yielding an output sink current of 10 μA. Different values of the output current can be obtained using the other available internal resistors. If an output *source *current is needed, reverse the connections to the REF200 IC and connect it .

The dotted lines in **Figures 2 & 3 **illustrate how unused internal resistors can be paralleled to reduce voltage drops in the reference and load paths.

**Figure 3** shows a similar application based on the LT1995 chip and used to increase the output current, in this case, summing the currents from the two sections of the REF200 and multiplying by a factor of five.

For more detail: Precision programmable current source uses two ICs

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