SLASF96 April 2024 AFE20408
PRODUCTION DATA
Refer to the PDF data sheet for device specific package drawings
The externally applied output capacitors allow for noise filtering, and enable fast switching on the output channels of the device. Large capacitors can be connected to the output of the static channels: DACA0, DACA1, DACA2, DACA3 on group A, and DACB0, DACB1, DACB2, DACB3 on group B. Capacitors of lower values can be connected to the dynamic channels, OUTA0, OUTA2, OUTB0, and OUTB2. This capacitor arrangement means that the larger capacitors can quickly charge the smaller capacitors instead of relying on the DAC output buffers.
Figure 8-1 shows a simplified model of switch arrangement for the OUTA0 channel. The on-resistance of the switches are represented by RSW1 and RSW2. These resistors primarily serve to limit the settling time of VOUTA1 after a switching event, as the settling time is essentially an RC function.
For example, consider the case where DRVEN0 changes from a low-state to a high-state. The steady-state of VDACA0 is equal to VDACA1 before the switch event. After the DRVEN pin goes high, SW2 closes, connecting COUTA1 and CDACA0 to each other. As these capacitors are now in parallel, the voltages across each equalize to a new voltage. This voltage, described as VCDAC||COUT in the following equation, can be calculated by finding the charge stored in each capacitor. The total charge on the two capacitors in parallel is equal to the sum of the charge of each capacitor.
The time required for the two output to equalize, described as the Capacitive Settling Period, is calculated using the equation below. As DACA0 is lower potential than DACA1, VOUTA0 can be expressed as a charging function.
During the capacitive settling period, VDACA1 is expressed as a discharging RC function.
Connecting the capacitors together allows the output to change to VCDAC||COUT quickly, but after that period, the DAC output buffer continues to charge COUTA1 to the VDACA0 value. The settling time for that final transition depends on the RC function formed by the series resistance on the DAC output, the switch resistance, and the capacitive load on the DAC. In addition, the output current of the DAC is limited.
Figure 8-2 shows the switch response for the OUTA0 pin when switching from a static DAC channel to VSS, while Figure 8-3 shows the switch response of the OUTA0 signal when switching between static DAC outputs.