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

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

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

*R*_{f}/N**will be generated.**

*N*×*I*_{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*current. The negative feedback resistance is thus equal to

_{IN}**450/10 kΩ,**or

**45 kΩ.**

Given the

**450 kΩ**positive feedback resistor,

**for**

*N***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.

The absence of an internal compensating capacitor in the

**LT1995**could induce instability. As experimentally verified, the circuit of

**Figure 3**will oscillate at a few megahertz if a

**33-100 pF**capacitor is not inserted as indicated.

**Read More: Precision programmable current source uses two ICs**