THS4520 datasheet(21/37 Pages) NSC

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APPLICATION INFORMATION

APPLICATIONS

Differential Input to Differential Output Amplifier

Single-Ended

Input

THS4520

Differential

Output

OUT+

OUT–

–

Input Common-Mode Voltage Range

OUT+

OUT–

S+

IN–

S–

IN+

THS4520

Differential

Input

Differential

Output

–

OUT

(1)

Single-Ended

Input

Differential

Output

Setting the Output Common-Mode Voltage

THS4520

SLOS503B – SEPTEMBER 2006 – REVISED JULY 2007

The following circuits show application information

for the THS4520. For simplicity, power supply

decoupling capacitors are not shown in these

diagrams. For more detail on the use and operation

of fully differential op amps see application report

Fully-Differential Amplifiers (SLOA054) .

The THS4520 is a fully differential op amp, and can

be used to amplify differential input signals to

differential output signals. A basic block diagram of

the circuit is shown in Figure 59 (CM input not

Figure 60. Single-Ended Input to Differential

shown). The gain of the circuit is set by RF divided by

Output Amplifier

RG.

The

input

common-model

voltage

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 differential voltage

across the input pins is only a few millivolts at most.

So finding the voltage at one input pin determines

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 59. Differential Input to Differential Output

Amplifier

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

Depending on the source and load, input and output

the negative input is evaluated at the extremes of

termination can be accomplished by adding RIT and

VOUT+.

RO.

As the gain of the op amp increases, the input

common-mode voltage becomes closer and closer to

Amplifier

the input common-mode voltage of the source.

The THS4520 can be used to amplify and convert

single-ended input signals to differential output

signals. A basic block diagram of the circuit is shown

The output common-mode voltage is set by the

in Figure 60 (CM input not shown). The gain of the

voltage at the CM pin. The internal common-mode

circuit is again set by RF divided by RG.

control circuit maintains the output common-mode

voltage within 0.25-mV offset (typical) from the set

voltage, when set within

±0.5 V of mid-supply. 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 61 is

representative of the CM input. The internal CM

circuit has about 230 MHz of bandwidth, which is

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