DAC Basic Configurations 1 (Decoder Method)

Resistance Voltage Divider Type D/A Converter

In its simplest form, a D/A converter is referred to as a resistance string.

In the 3bit (resolution) D/A converter shown below, voltage is divided via resistors and selected at one node using switches.

However, although sufficiently high-speed operation can be achieved by reducing the resistance values and using a high-speed downstream buffer amp, operating speed is reduced at high resolutions due to parasitic capacitance of the switch.

Advantages include superior linearity and, in principle, guaranteed monotonicity.

The main disadvantage is exponentially increased circuit scale depending on the resolution.

8 resistors and a switch are needed for 3bit operation, 16 resistors and switch for 4bit, 1024 resistors and a switch for 10bit, etc.

Stage Resistor Voltage Divider Type D/A Converter

This type of resistor voltage divider D/A converter features a 2-stage configuration.

In the 1st stage (left) of the 6bit D/A converter shown below, we select both ends of one resistor between Vref and GND (the third resistor from the top on the left side).

In the 2nd stage (right) the voltage is further divided to obtain higher resolution.

A primary advantage over single-stage configurations is that by restricting circuit scalability the number of required resistors and switches even for a 6bit D/A converter can be limited to 16/18 units (in the case of a resistor voltage divider method, 64 units are required regardless).

Since 2 more amps are needed for each additional stage, considerations must be made when selecting the appropriate method based on the number of resistors/switches.

One disadvantage is that the problems associated with conventional D/A converters are exacerbated.

For example, regarding speed, there will be a delay due to the 2 amps.

And with respect to output voltage accuracy, there may be offset caused by the 2nd stage amps.