SBOS778D April 2016 – April 2021 THS4551
The wideband FDA requires external resistors for correct signal-path operation. When configured for the desired input impedance and gain setting with these external resistors, the amplifier can be either on with the PD pin asserted to a voltage greater than (VS–) + 1.15 V, or turned off by asserting PD low (within 0.55 V of the negative supply). Disabling the amplifier shuts off the quiescent current and stops correct amplifier operation. The signal path is still present for the source signal through the external resistors, which provides poor signal isolation from the input to output in power-down mode.
Internal protection diodes remain present across the input pins in both operating and shutdown mode. Large input signals during disable can turn on the input differential protection diodes, thus producing a load current in the supply even in shutdown.
The VOCM control pin sets the output average voltage. Left open, VOCM defaults to an internal midsupply value. Driving this high-impedance input with a voltage reference within the valid range sets a target for the internal VCM error amplifier. If floated to obtain a default midsupply reference for VOCM, an external decoupling capacitor is recommended to be added on the VOCM pin to reduce the otherwise high output noise for the internal high-impedance bias (see Figure 7-45).