7.3 Feature Description

The DAC7811 is a single channel, current output, 12-bit digital-to-analog converter (DAC). The architecture, illustrated in Figure 25, is an R-2R ladder configuration with the three MSBs segmented. Each 2R leg of the ladder is either switched to I_OUT1 or the I_OUT2 terminal. The I_OUT1 terminal of the DAC is held at a virtual GND potential by the use of an external I/V converter op amp. The R-2R ladder is connected to an external reference input V_REF that determines the DAC full-scale current. The R-2R ladder presents a code independent load impedance to the external reference of 10kΩ  ±20%. The external reference voltage can vary over a range of –15V to +15V, thus providing bipolar I_OUT current operation. By using an external I/V converter and the DAC7811 R_FB resistor, output voltage ranges of –V_REF to V_REF can be generated.

Figure 25. Equivalent R-2R DAC Circuit

When using an external I/V converter and the DAC7811 R_FB resistor, the DAC output voltage is given by Equation 1:

Equation 1.

Each DAC code determines the 2R leg switch position to either GND or I_OUT. Because the DAC output impedance as seen looking into the I_OUT1 terminal changes versus code, the external I/V converter noise gain will also change. Because of this, the external I/V converter op amp must have a sufficiently low offset voltage such that the amplifier offset is not modulated by the DAC I_OUT1 terminal impedance change. External op amps with large offset voltages can produce INL errors in the transfer function of the DAC7811 due to offset modulation versus DAC code.

For best linearity performance of the DAC7811, a low offset voltage op amp (such as the OPA277) is recommended (see Figure 26). This circuit allows V_REF swinging from –10 V to 10 V.

Figure 26. Voltage Output Configuration