SLASFM1A June 2025 – December 2025 AFE10004-EP
PRODUCTION DATA
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: DAC1, DAC2, CLAMP1, and CLAMP2. Capacitors of lower values can be connected to the dynamic channels, DAC0, DAC3, OUT1, and OUT2. 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 OUT1 channel. The on-resistance of the switches are represented by RSW1 and RSW2.The on-resistance is specified for the channels in the Electrical Characteristics. The resistance primarily limits the settling time of VOUT1 after a switching event, as the settling time is essentially an RC function.
For example, consider the case where DRVEN1 changes from a low-state to a high-state. The steady-state of VOUT1 is equal to VCLAMP before the switch event. After the DRVEN1 goes high, SW2 closes, connecting COUT1 and CDAC1 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 in Figure 8-2, is calculated using the equation below. As CLAMP1 is lower potential than DAC1, VOUT1 can be expressed as a charging function.
During the capacitive settling period, VDAC1 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 COUT1 to the VDAC1 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 8-3 and Figure 8-4 show the switching transients in the application with 10µF capacitors on the static channels and 100nF capacitors on the dynamic channels. Figure 8-5 shows the small-signal settling time of the OUT signal switching to the DAC output.
Figure 8-3 CLAMP-to-DAC Switch Response
Figure 8-5 CLAMP-to-DAC Small Transient Switch Response
Figure 8-4 DAC-to-CLAMP Switch ResponseLarge capacitor values are potentially prohibitive in applications where small component size is required. This requirement results in capacitor selection where the capacitors on static channels are not orders-of-magnitude larger than the dynamic channels. For example, with 10nF and 1nF for the static and dynamic channels, respectively, the DAC settling capability dominates the charging time. Figure 8-6 shows the switch response for this case.
Figure 8-6 Low-Capacitance Switch Response