SETTING THE OUTPUT COMMON-MODE

VOCM =2.5V

5V

VS

RS

RG1

RG2

RF1

RF2

+

RT

+

RS

2.5-VDC

2.5-VDC

DCCurrentPathtoGround

DCCurrentPathtoGround

I2 =

VOCM

RF2

G2

+R

I1 =

VOCM

RF1

G1

S

T

+R

+R ||R

-

-

R =50kW

R =50kW

V

S+

V

S-

V

OCM

I

IN

I

IN =

2V

V

V

-

-

OCM

S

S+

-

R

THS4504

THS4505

SLOS363D – AUGUST 2002 – REVISED MAY 2008 ......................................................................................................................................................... www.ti.com

Often times, filters like these are used to eliminate

for the sole purpose of filtering any high frequency

broadband noise and out-of-band distortion products

noise that could couple into the signal path through

in signal acquisition systems. It should be noted that

the VOCM circuitry. A 0.1-µF or 1-µF capacitance is a

the increased load placed on the output of the

reasonable value for eliminating a great deal of

amplifier

by

the

second

low-pass

filter

has

a

broadband

interference,

but

additional,

tuned

detrimental effect on the distortion performance. The

decoupling capacitors should be considered if a

preferred method of filtering is using the feedback

specific source of electromagnetic or radio frequency

network,

as

the

typically

smaller

capacitances

interference is present elsewhere in the system.

required at these points in the circuit do not load the

Information on the ac performance (bandwidth, slew

amplifier nearly as heavily in the pass-band.

rate) of the VOCM circuitry is included in the

specification table and graph section.

Since the VOCM pin provides the ability to set an

VOLTAGE WITH THE VOCM INPUT

output

common-mode

voltage,

the

ability

for

increased power dissipation exists. While this does

The output common-mode voltage pin provides a

not pose a performance problem for the amplifier, it

critical function to the fully differential amplifier; it

can cause additional power dissipation of which the

accepts an input voltage and reproduces that input

system designer should be aware. The circuit shown

voltage as the output common-mode voltage. In other

in Figure 84 demonstrates an example of this

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

phenomenon. For a device operating on a single 5-V

the outputs to any voltage inside the output voltage

supply with an input signal referenced around ground

swing of the amplifier.

and an output common-mode voltage of 2.5 V, a dc

A description of the input circuitry of the VOCM pin is

potential exists between the outputs and the inputs of

shown below to facilitate an easier understanding of

the device. The amplifier sources current into the

the VOCM interface requirements. The VOCM pin has

feedback network in order to provide the circuit with

two 50-k

Ω resistors between the power supply rails to

the proper operating point. While there are no serious

set the default output common-mode voltage to

effects on the circuit performance, the extra power

midrail. A voltage applied to the VOCM pin alters the

dissipation may need to be included in the system

output common-mode voltage as long as the source

power budget.

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.

Current drive is especially important when using the

reference voltage of an analog-to-digital converter to

drive VOCM. Output current drive capabilities differ

from part to part, so a voltage buffer may be

necessary in some applications.

Figure 84. Depiction of DC Power Dissipation

Figure 83. Equivalent Input Circuit for VOCM

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 the equivalent circuit diagram, 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 printed-circuit

board traces to act as an antenna. For this reason, a

decoupling capacitor is recommended on this node

24

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