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.
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| Figure 1. | IREF can be increased or decreased to the load depending on N. |
The IIN current source gives rise to a reference voltage across RF at the op-amp inverting input. The same potential is found across the RF/N resistor, so that an output current of N × IIN will be generated.
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 IIN 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 to –VS.
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| Figure 2. | LT1996 used as a pin-strap-programmable current sink/ source, in this case, dividing the reference current by 10. |
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.
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| Figure 3. | LT1995 sinking five times the reference current. |
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.
Table 1 summarizes the connections for IOUT/IIN ratios between 2 and 7.
| Table 1. | Connections for IOUT/IIN ratios between 2 and 7 |
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Figure 4 shows an LT1991-based programmable current source where internal resistors are ratioed 1:3:9.
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| Figure 4. | An LT1991 is used to source/sink from 10-90 µA in 10 µA steps. |
The circuit can generate an output of 10 – 90 µA in steps of 10 µA. For currents up to 50 µA, the principle is the same as before. Above 50 µA, the second current reference in the REF200 is used to superimpose 100 µA onto the load to obtain the desired output current. Table 2 illustrates switch settings for the various current levels.
| Table 2. | Switch settings for the various current levels | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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