THS4509 datasheet(21/30 Pages) TI

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APPLICATION INFORMATION

APPLICATIONS

Differential Input to Differential Output Amplifier

Single-Ended

Input

THS 4509

Differential

Output

OUT+

OUT–

–

Input Common-Mode Voltage Range

OUT+

OUT–

S+

IN–

S–

IN+

THS4509

Differential

Input

Differential

Output

–

OUT

(1)

Single-Ended Input to Differential Output Ampli-

Setting the Output Common-Mode Voltage

THS4509

SLOS454 – JANUARY 2005

The following circuits show application information for

the THS4509. For simplicity, power supply decoupling

capacitors are not shown in these diagrams. Please

see the Subsection 1 section for recommendations.

For more detail on the use and operation of fully

differential op amps refer to

application

report

Fully-Differential Amplifiers (SLOA054) .

The THS4509 is a fully differential op amp, and can

be used to amplify differential input signals to differ-

Figure 81. Single-Ended Input to Differential

ential output signals. A basic block diagram of the

Output Amplifier

circuit is shown in Figure 80 (CM input not shown).

The gain of the circuit is set by RF divided by RG.

The input common-model voltage of a fully differential

op amp is the voltage at the '+' and '–' input pins of

the op amp.

It is important to not violate the input common-mode

voltage range (VICR) of the op amp. Assuming the op

amp is in linear operation the voltage across the input

pins is only a few millivolts at most. So finding the

voltage at one input pin will determine the input

common-mode voltage of the op amp.

Treating the negative input as a summing node, the

voltage is given by Equation 1:

Figure 80. Differential Input to Differential Ouput

Amplifier

To determine the VICR of the op amp, the voltage at

the negative input is evaluated at the extremes of

Depending on the source and load, input and output

VOUT+.

termination can be accomplished by adding RIT and

RO.

As the gain of the op amp increases, the input

common-mode voltage becomes closer and closer to

the input common-mode voltage of the source.

fier

The THS4509 can be used to amplify and convert

single-ended input signals to differential output sig-

The output common-mode voltage is set by the

nals. A basic block diagram of the circuit is shown in

voltage at the CM pin(s). The internal common-mode

Figure 81 (CM input not shown). The gain of the

control circuit maintains the output common-mode

circuit is again set by RF divided by RG.

voltage within 3-mV offset (typ) from the set voltage,

when set within 0.5 V of mid-supply, with less than

4mV differential offset voltage. If left unconnected,

the common-mode set point is set to mid-supply by

internal circuitry, which may be over-driven from an

external source. Figure 82 is representative of the

CM input. The internal CM circuit has about 700 MHz

of –3-dB bandwidth, which is required for best per-

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