Instrumentation amp provides unipolar and bipolar outputs

David Rathgeber, Alles Corp, Toronto, ON, Canada -- EDN, 8/3/2000

Intelligent sensors and signal conditioners for plant-automation systems must produce outputs compatible with standard data-acquisition systems, such as PCs, programmable logic controllers, and remote terminals for supervisory and control systems. Standard analog inputs are 0 to 1V, ±10V, 0 to 1 mA, and 4 to 20 mA. The circuit in Figure 1 makes all these signals available, using only three packages in addition to the DAC. The voltage-output section requires a thin-film resistor package, Beckman Series 668/698 (which tracks with temperature), and two FET-input op amps. The current-output section requires the addition of a four-resistor package, Beckman Series 664/694, and two more op amps. A reference voltage of –1.000V yields voltage outputs of 0 to 1V and ±1V and either 0 to 1 mA with R_X=1 kW  and JP₂ and JP₄ connected or 4 to 20 mA with R_X=83.33W and JP₁ and JP₃ connected. A reference of –10.00V and appropriate values of R_X yield corresponding voltage and current outputs. All R₁ and R₂ values in Figure 1 are 100 kW±0.1%. You should select Q₁ and possibly use a heat sink, taking into consideration the supply voltage and output-current range.