The DAC161S997 (SPI) and DAC161P997 (single-wire interface) are 16-bit delta-sigma DACs. With an external NPN transistor, these devices can be used to construct a low-power, high-precision, industrial 4-20mA transmitter. The DAC, an internal op-amp, and several internal precision resistors are used to construct a voltage-to current (V-I) stage for the transmitter. For this device, a HART FSK signal can be capacitively coupled into an RC filter internal to the device. Figure 3-4 shows a block diagram of the DAC161S997 in a field transmitter application. The HART FSK signal is modulated onto the current loop from a connection to the C2 pin.

Some HART-enabled DACs are used in systems other than the 4-20mA loop. These systems incorporate programmable logic controllers (PLCs) or distributed control systems (DCS) where a specific output voltage or current is required.

The DACx750 are precision, fully integrated, 16-bit and 12-bit DACs and have a variety of programmable output current ranges for industrial process control applications. These devices use SPI for communications, and operate with either a single 10-V to 36-V supply, or dual supplies of up to ±18 V. Similarly, the DACx760 have the same function but add voltage outputs to the device. An input pin capacitively couples in a HART FSK signal to the DAC output. Figure 3-5 shows the block diagram of the DAC8760. The HART signal is modulated onto the current or voltage output through the HART-IN pin.

The single-channel DAC8771 and quad channel DAC8775 have both current and voltage outputs for industrial process control applications, but these analog outputs have bipolar ranges. Another added feature is an integrated buck-boost converter used for powering the DAC outputs. A block diagram for the DAC8775 is shown in Figure 3-3. The HART signal is modulated onto the current or voltage output through the HART-IN pin.