2 Output Features

The purpose of the OUT pin is to switch the PA gate between an off voltage, also known as the CLAMP, and an on voltage. The on voltage is provided by the even DAC in the OUT group. The CLAMP of the output pin can be set to the odd DAC voltage or the VSS supply. Using the odd DAC for the CLAMP allows for a more precise pinch-off voltage. Using the VSS for the CLAMP frees up the unused DAC to be used for additional gate biasing, allowing for up to 8 bias points on the AMC7908. The switches are toggled with software bits or hardware pins. Figure 2-1 shows how the OUTA0 switches are configured in the AMC7908. The OUT groups are shown in Table 2-1 .

The OUT pins feature fast on and off switching, which is desirable for certain applications such as TDD. The output switches toggle quickly through the use of capacitive charge sharing. To enable this, large capacitors must be externally placed on the DAC outputs and small capacitors on the OUT output. When the switch toggles between different outputs, the small capacitor on the output is quickly charged by the larger DAC capacitor, allowing for fast output switching.

Figure 2-1 Output Switch Configuration