THS4502CD datasheet(24/40 Pages) TI

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THS4502

THS4503

SLOS352D − APRIL 2002 − REVISED JANUARY 2004

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Figure 98

−

VOCM

14 Bit/40 MSps

5 V

µF 0.1 µF

THS4501

µF

ADS5421

0.1

µF

Using the THS4501 With the ADS5421

Riso

FULLY DIFFERENTIAL LINE DRIVERS

The THS4500 family of amplifiers can be used as

high-frequency, high-swing differential line drivers. Their

high power supply voltage rating (16.5 V absolute

maximum) allows operation on a single 12-V or a single

15-V supply. The high supply voltage, coupled with the

ability to provide differential outputs enables the ability to

drive 26 VPP into reasonably heavy loads (250 Ω or

greater). The circuit in Figure 99 illustrates the THS4500

family of devices used as high speed line drivers. For line

driver applications, close attention must be paid to thermal

design constraints due to the typically high level of power

dissipation.

Figure 99

VOCM

15 V

−

0.1

µF

THS4500/2

VDD

VOD = 26 VPP

Riso

Fully Differential Line Driver With High Output Swing

FILTERING WITH FULLY DIFFERENTIAL

AMPLIFIERS

Similar to their single-ended counterparts, fully differential

amplifiers have the ability to couple filtering functionality

with voltage gain. Numerous filter topologies can be based

on fully differential amplifiers. Several of these are outlined

in A Differential Circuit Collection, (literature number

SLOA064) referenced at the end of this data sheet. The

circuit below depicts a simple two-pole low-pass filter

applicable to many different types of systems. The first

pole is set by the resistors and capacitors in the feedback

paths, and the second pole is set by the isolation resistors

and the capacitor across the outputs of the isolation

resistors.

Figure 100

Rg1

Rf1

Rf2

−

Riso

CF2

CF1

Rg2

Riso

A Two-Pole, Low-Pass Filter Design Using a Fully

Differential Amplifier With Poles Located at:

P1 = (2

πRfCF)−1 in Hz and P2 = (4πRisoC)−1 in Hz

Often times, filters like these are used to eliminate

broadband noise and out-of-band distortion products in

signal acquisition systems. It should be noted that the

increased load placed on the output of the amplifier by the

second low-pass filter has a detrimental effect on the

distortion performance. The preferred method of filtering

is using the feedback network, as the typically smaller

capacitances required at these points in the circuit do not

load the amplifier nearly as heavily in the pass-band.

SETTING THE OUTPUT COMMON-MODE

VOLTAGE WITH THE VOCM INPUT

The output common-mode voltage pin provides a critical

function to the fully differential amplifier; it accepts an input

voltage and reproduces that input voltage as the output

common-mode voltage. In other words, the VOCM input

provides the ability to level-shift the outputs to any voltage

inside the output voltage swing of the amplifier.

A description of the input circuitry of the VOCM pin is shown

below to facilitate an easier understanding of the VOCM

interface requirements. The VOCM pin has two 50-kΩ

resistors between the power supply rails to set the default

output common-mode voltage to

midrail. A voltage

applied to the VOCM pin alters the output common-mode

voltage as long as the source has the ability to provide

enough current to overdrive the two 50-k

Ω resistors. This

phenomenon is depicted in the VOCM equivalent circuit

diagram. The table contains some representative

examples to aid in determining the current drive

requirement for the VOCM voltage source. This parameter

is especially important when using the reference voltage

THS4502CD Datasheet(PDF) 24 Page - Texas Instruments

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