THS4501CDG4 datasheet(27/48 Pages) TI1

27 / 48 page

R =50kW

VS+

VS-

VOCM

IIN

IIN =

2 VOCM

- S+ - VS-

VOCM =2.5V

RG1

RG2

RF1

RF2

2.5-VDC

DCCurrentPathtoGround

I2 =

VOCM

RF2 +RG2

I1 =

VOCM

RF1+RG1+RS ||RT

THS4500

THS4501

www.ti.com

SLOS350F

– APRIL 2002 – REVISED OCTOBER 2011

detrimental effect on the distortion performance. The

capacitance is a reasonable value for eliminating a

preferred method of filtering is to use the feedback

great deal of broadband interference, but additional,

network,

the

typically

smaller

capacitances

tuned decoupling capacitors should be considered if a

required at these points in the circuit do not load the

specific source of electromagnetic or radio frequency

amplifier nearly as heavily in the passband.

interference is present elsewhere in the system.

Information on the ac performance (bandwidth, slew

rate) of the VOCM circuitry is included in the Electrical

SETTING THE OUTPUT COMMON-MODE

Characteristics and Typical Characterisitcs sections.

VOLTAGE WITH THE VOCM INPUT

Since the VOCM pin provides the ability to set an

The output common-mode voltage pin provides a

output

common-mode

voltage,

the

ability

for

critical function to the fully differential amplifier; it

increased

power

dissipation

exists.

While

this

accepts an input voltage and reproduces that input

possibility does not pose a performance problem for

voltage as the output common-mode voltage. In other

the

amplifier,

can

cause

additional

power

words, the VOCM input provides the ability to level-shift

dissipation of which the system designer should be

the outputs to any voltage inside the output voltage

aware. The circuit shown in Figure 107 demonstrates

swing of the amplifier.

an example of this phenomenon. For a device

A description of the input circuitry of the VOCM pin is

operating on a single 5-V supply with an input signal

shown

Figure

106

facilitate

easier

referenced

around

ground

and

output

understanding of the VOCM interface requirements.

common-mode voltage of 2.5 V, a dc potential exists

The VOCM pin has two 50-kΩ resistors between the

between the outputs and the inputs of the device. The

power

supply

rails

set

the

default

output

amplifier sources current into the feedback network in

common-mode voltage to midrail. A voltage applied to

order to provide the circuit with the proper operating

the VOCM pin alters the output common-mode voltage

point. While there are no serious effects on the circuit

as long as the source has the ability to provide

performance, the extra power dissipation may need to

enough current to overdrive the two 50-k

Ω resistors.

be included in the system power budget.

This phenomenon is depicted in the VOCM equivalent

circuit diagram. Current drive is especially important

when

using

the

reference

voltage

analog-to-digital converter to drive VOCM. Output

current drive capabilities differ from part to part, so a

voltage

buffer

may

necessary

some

applications.

Figure 106. Equivalent Input Circuit for VOCM

Figure 107. Depiction of DC Power Dissipation

Caused By Output Level-Shifting in a DC-Coupled

Circuit

By design, the input signal applied to the VOCM pin

propagates to the outputs as a common-mode signal.

As shown in Figure 106, the VOCM input has a high

impedance associated with it, dictated by the two

50-k

Ω resistors. While the high impedance allows for

relaxed drive requirements, it also allows the pin and

any associated PCB traces to act as an antenna. For

this reason, a decoupling capacitor is recommended

on this node for the sole purpose of filtering any

high-frequency noise that could couple into the signal

path through the VOCM circuitry. A 0.1-μF or 1-μF

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