THS4521 datasheet(36/66 Pages) TI2

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8.3.2 Power-Down Control Pin (PD)

The THS4541 includes a power-down control pin, PD. Assert this pin high for correct amplifier operation. Do

not float the PD pin because there is no internal pullup or pulldown resistor on this pin to reduce disabled

power consumption. Assert this pin low (within 0.7 V of the negative supply) to put the THS4541 into a very low

quiescent state (approximately 2 µA). Switches in the default Vocm resistor string open to eliminate the fixed

bias current (25 µA) across the supply in this 200-kΩ voltage divider to midsupply.

8.3.2.1 Operating the Power Shutdown Feature

Assert this CMOS input pin to the desired voltage for operation. For applications that require the device to only

be powered on when the supplies are present, tie the PD pin to the positive supply voltage.

When the PD pin is somewhat below the positive supply pin, slightly more quiescent current is drawn; see Figure

6-56. For the minimum-on power, assert this pin to the positive supply.

The disable operation is referenced from the negative supply; normally, ground. For split-supply operation, with

the negative supply below ground, a disable control voltage below ground is required to turn the THS4541 off

when the negative supply exceeds –0.7 V.

For single-supply operation, a minimum of 1.7 V greater than the negative supply (ground, in this case) is

required to maintain operation. This minimum logic-high level allows for direct operation from 1.8-V supply logic.

8.3.3 Input Overdrive Operation

The THS4541 input stage architecture is intrinsically robust to input overdrives with the series input resistor

required by all applications. High input overdrives cause the outputs to limit into the maximum swings with the

remaining input current through the Rg resistors absorbed by internal, back-to-back protection diodes across the

two inputs. These diodes are normally off in application, and only turn on to absorb the currents that a large input

overdrive produces through the source impedance and or the series Rg elements required by all designs. Figure

6-12 and Figure 6-30 illustrate the exceptional output limiting and short recovery time for an input overdrive that

is attempting to drive the outputs to two times the available swing.

The internal input diodes can safely absorb up to ±15 mA in an overdrive condition. For designs that require

more current to be absorbed, consider adding an external protection diode such as the BAV99 device used in

the example ADC interface design of Figure 9-4.

8.4 Device Functional Modes

This 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–) + 1.7 V, or turned off by asserting PD low. 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.

The Vocm control pin sets the output average voltage. If 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.

8.4.1 Operation from Single-Ended Sources to Differential Outputs

One of the most useful features supported by the FDA device is an easy conversion from a single-ended input

to a differential output centered on a user-controlled, common-mode level. While the output side is relatively

straightforward, the device input pins move in a common-mode sense with the input signal. This common-mode

voltage at the input pins moving with the input signal acts to increase the apparent input impedance to be

greater than the Rg value. This input active impedance issue applies to both ac- and dc-coupled designs, and

requires somewhat more complex solutions for the resistors to account for this active impedance, as shown in

the following subsections.

THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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