THS4561 Datasheet, PDF (21/35 Pages) Texas Instruments – Low-Power, High Supply Range, 70-MHz, Fully Differential Amplifier

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THS4561
SBOS874 – AUGUST 2017
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 (V_S+) – 0.5 V, or turned off by asserting PD low (1.5 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 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 V_CM
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 ).
9.4.1 Shutdown Mode
For proper shutdown mode operation, the power down (PD) pin must be asserted to the desired voltage. An
internal pullup resistor is not provided on the PD pin so that if the pin is floated, the device defaults to an on
state. For applications simply requiring the device to be powered on when the supplies are present, tie the PD
pin to the positive supply voltage.
The disable operation is referenced from the positive supply. For an off state condition, the disable control pin
must be 1.5 V below the positive supply.
For single-supply operation, a minimum of 0.5 V within the positive supply is required for operation. This logic
threshold range allows direct operation from a TBD-V supply logic when the THS4561 operates with a single
positive supply and ground.
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 R_G value. The 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 discussed in
the Setting Resistor Values Versus Gain section.
9.4.2.1 AC-Coupled Signal Path Considerations for Single-Ended Input to Differential Output
Conversions
When the signal path can be ac-coupled, the dc biasing for the THS4561 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 both can be ac-coupled. One situation where the output can be
dc-coupled (for an ac-coupled input), is when driving directly into an ADC where the V_OCM control voltage uses
the ADC common-mode reference to directly bias the FDA output common-mode voltage to the required ADC
input common-mode voltage. In any case, the design starts by setting the desired V_OCM. When an ac-coupled
path follows the output pins, the best linearity is achieved by operating VOCM at midsupply, which can be easily
delivered by floating the VOCM pin. The V_OCM voltage must be within the linear range for the common-mode
loop, as specified in the headroom specifications (approximately 0.6 V greater than the negative supply and
1.3 V less than the positive supply for the full –40°C to +125°C operation). If the output path is also ac-coupled,
simply letting the VOCM control pin float is usually preferred in order to obtain a midsupply default V_OCM bias with
minimal elements. To limit noise, place a 0.1-µF decoupling capacitor on the VOCM control pin to ground.
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