THS4521 datasheet(25/51 Pages) TI

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9.3 Feature Description

In addition to the core differential I/O voltage feedback gain block, there are two 6-kΩ resistors internally across

the outputs to sense the average voltage at the outputs. These resistors feed the average voltage back into a

VCM error amplifier where the voltage is compared to either a default voltage divider across the supplies or an

externally set VOCM target voltage. When the amplifier is disabled, the default midsupply bias string is disabled

to save power.

To achieve the very-low noise at the low power provided by the THS4561, the input stage transistors are

relatively large, thus resulting in a higher differential input capacitance (2.4 pF in Section 9.2). When using the

16-pin WQFN package and the internal feedback traces to the input side of the package, include the nominal

trace impedance of 8.5 Ω in the design. These elements are not included in the TINA-TI™ model and must be

added externally to a design intending to use the RGT package.

9.4 Device Functional Modes

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+) – 0.5 V, or turned off by asserting PD low (1.8 V below the positive

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 power-down.

The VOCM control pin sets the output average voltage. The VOCM defaults to an internal midsupply value if left

open. 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 7-48).

9.4.1 Power-Down Mode

The power down ( PD) pin must be asserted to the desired voltage for proper power-down mode operation. A

physical internal pullup resistor is not provided on the PD pin so that if the pin is floated, the device defaults to an

ON state. Tie the PD pin to the positive supply voltage for applications that simply require the device to power on

when the supplies are present. For single-supply operation, a minimum of 0.5 V within the positive supply is

required for operation.

The disable operation is referenced from the positive supply. For an OFF state condition, the disable control pin

must be 1.8 V below the positive supply. The THS4561 has a unique power-down circuit that requires

overcoming the specified peak PD pulldown current when the PD voltage is pulled low. When this current

threshold is overcome, the PD current drops to a very small value. The benefit of the circuit is that the device

stays disabled without having to use an active pullup resistor that wastes crucial power to keep the amplifier

disabled in power-sensitive applications.

9.4.2 Single-Ended Source to Differential Output Mode

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. Although the output side is relatively

straightforward, the device input pins move in a common-mode manner with the input signal. The common-mode

voltage at the input pins, which moves with the input signal, increases the apparent input impedance to be

greater than the RG value. The input active impedance issue applies to both AC-coupled and DC-coupled

designs, and requires somewhat more complex solutions for the resistors to account for this active impedance,

as discussed in Section 10.1.2.

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THS4561

SBOS874C – AUGUST 2017 – REVISED DECEMBER 2020

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