188.8.131.52 Design Requirements
The inherent architecture of the DAC9881, which consists of an R-2R architecture, enables great performance in regards to noise and accuracy, but at a cost of large glitch area. Glitch area, also known as glitch impulse area, is defined as the area associated with the overshoot or undershoot created by a code transition, and is generally quantified in Volt-seconds. Different code-to-code transitions produce different levels of glitch impulses. DACs with R-2R architectures produce large glitches during major-carry transitions.
There are two methods that can be used to reduce this glitch area:
- Add an external RC Filter to the output of the DAC.
- The low-pass filter helps attenuate high-frequency glitches that would normally propagate to the DAC output. Best practice is to use a small resistor value, as large resistance develops a large potential drop and reduces the voltage seen at the load. Capacitor values can be determined from the desired cutoff frequency of the low-pass filter, as well as settling time.
- Another technique is to employ a Sample and Hold (S&H) circuit following the DAC output.
- In its simplest form, the sample and hold circuit can be constructed from the following components: a capacitive element, output buffer, and switch. A schematic of the simplified S&H is shown in Figure 72.
Figure 72. Simplified Sample and Hold Circuit