THS4522IPW datasheet(34/71 Pages) TI2

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THS4521, THS4522, THS4524

SBOS458H – DECEMBER 2008 – REVISED JUNE 2015

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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. Left open, VOCM defaults to an internal midsupply value.

Driving this high-impedance input with a voltage reference within its 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.

8.4.1.1 AC-Coupled Signal Path Considerations for Single-Ended Input to Differential Output Conversion

When the signal path can be ac-coupled, the dc biasing for the THS452x family becomes a relatively simple task.

In all designs, start by defining the output common-mode voltage. The ac-coupling issue can be separated for the

input and output sides of an FDA design. The input can be ac-coupled and the output dc-coupled, or the output

can be ac-coupled and the input dc-coupled, or they can both be ac-coupled.

One situation where the output might be dc-coupled (for an ac-coupled input), is when driving directly into an

ADC where the VOCM control voltage uses the ADC common-mode reference to directly bias the FDA output

common-mode to the required ADC input common-mode. In any case, the design starts by setting the desired

VOCM.

When an ac-coupled path follows the output pins, the best linearity is achieved by operating VOCM at midsupply.

The VOCM voltage must be within the linear range for the common-mode loop, as specified in the headroom

specifications (approximately 0.91 V greater than the negative supply and 1.1 V less than the positive supply). If

the output path is also ac-coupled, simply letting the VOCM control pin float is usually preferred in order to get a

midsupply default VOCM bias with minimal elements. To limit noise, place a 0.1-µF decoupling capacitor on the

VOCM pin to ground.

After VOCM is defined, check the target output voltage swing to ensure that the VOCM plus the positive or negative

output swing on each side do not clip into the supplies. If the desired output differential swing is defined as VOPP,

divide by 4 to obtain the ±VP swing around VOCM at each of the two output pins (each pin operates 180° out of

phase with the other). Check that VOCM ±VP does not exceed the absolute supply rails for this rail-to-rail output

(RRO) device.

Going to the device input pins side, because both the source and balancing resistor on the non-signal input side

are dc-blocked (see Figure 74), no common-mode current flows from the output common-mode voltage, thus

setting the input common-mode equal to the output common-mode voltage.

This input headroom also sets a limit for higher VOCM voltages. Because the input VICM is the output VOCM for ac-

coupled sources, the 1.2-V minimum headroom for the input pins to the positive supply overrides the 1.1-V

headroom limit for the output VOCM. Also, the input signal moves this input VICM around the dc bias point, as

described in the section Resistor Design Equations for the Single-Ended to Differential Configuration of the FDA.

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THS4522IPW Datasheet(PDF) 34 Page - Texas Instruments

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