The 1B21 is an isolated voltage-to-current converter that incorporates a unique circuit design utilizing transformer-based isolation and automated surface-mount manufacturing technology. It provides an unbeatable combination of versatility and performance in a compact plastic package. Designed for industrial applications， it is especially suited for harsh environments with extremely high common-mode interference. Functionally， the V/I converter consists of four basic sections： input conditioning， modulator， demodulator and current source （see Figure 1）。 The input is a resistor-programmable gain stage that accepts a 0 V to 1 V up to 0 V to 10 V voltage input. This maps into a 0 mA to 20 mA output or can be offset by 20% using the internal reference for 4 mA to 20 mA operation. The high level signal is modulated and passed across the barrier which provides complete input to output galvanic isolation of 1500 V rms continuous by the use of transformer-coupling techniques. Nonlinearity is an excellent ±0.05% maximum. Designed for multichannel applications， the 1B21 requires an external loop supply and can accept up to 30 V maximum. This provides a loop supply compliance of 27V， which is sufficient to drive a 1.35 kΩ load resistance. The 1B21 is fully specified over ?25°C to +85°C and operates over the industrial （?40°C to +85°C） temperature range.