The DAC38RF82 (or DAC38RF89) uses a bandgap reference and control amplifier for biasing the full-scale output current. The DAC full scale output current is set by a combination of the fixed current through the external resistor RBIAS (connected to pin BIASJ) and current from course trim current sources:
The bias current IBIAS through resistor RBIAS is defined by the on-chip bandgap reference voltage VBG (nominally 0.9 V) and control amplifier. For normal operation, it is recommended that RBIAS is set to 3.6 kΩ for a fixed current through RBIAS of 250 µA. This current is scaled 128x internally, giving:
The course trim current sources are configured through SPI register field DACFS in register DACFS (8.5.72), as follows:
From the discussion above, the DAC full scale output current can be configured from 40 mA (DACFS[3:0] = 1111) down to 10 mA (DACFS[3:0] = 0000). In addition to the full scale signal current set by SPI register DACFS (8.5.72), an extra DC bias current is required to set the operating point of the output current sources(Table 45).
|DACFS (8.5.72)||Signal current (mA)||Total bias current (mA)(1)|
An external decoupling capacitor CEXT of 0.1 μF should be connected externally to terminal EXTIO for compensation. The full-scale output current can be adjusted from 40 mA down to 10 mA by varying resistor RBIAS From 3.6 kΩ to 14.4 kΩ.