THS4502CD datasheet(25/40 Pages) TI

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THS4502

THS4503

SLOS352D − APRIL 2002 − REVISED JANUARY 2004

www.ti.com

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 101

R = 50 k

Ω

R = 50 k

Ω

VS+

VS−

VOCM

IIN

IIN =

2 VOCM − VS+ − VS−

Equivalent Input Circuit for VOCM

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 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

capacitance is a reasonable value for eliminating a great

deal of broadband interference, but additional, tuned

decoupling capacitors should be considered if a specific

source of electromagnetic or radio frequency interference

is present elsewhere in the system. Information on the ac

performance (bandwidth, slew rate) of the VOCM circuitry

is included in the specification table and graph section.

Since the VOCM pin provides the ability to set an output

common-mode voltage, the ability for increased power

dissipation exists. While this does not pose a performance

problem for the amplifier, it can cause additional power

dissipation of which the system designer should be aware.

The circuit shown in Figure 102 demonstrates

example of this phenomenon. For a device operating on

a single 5-V supply with an input signal referenced around

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

dc potential exists between the outputs and the inputs of

the device. The amplifier sources current into

the

feedback network in order to provide the circuit with the

proper operating point. While there are no serious effects

on the circuit performance, the extra power dissipation

may need to be included in the system’s power budget.

Figure 102

VOCM = 2.5 V

5 V

Rg1

Rg2

Rf1

Rf2

−

2.5-V DC

DC Current Path to Ground

I2 =

VOCM

Rf2 + Rg2

Depiction of DC Power Dissipation Caused By

Output Level-Shifting in a DC-Coupled Circuit

I1 =

VOCM

Rf1+ Rg1 + RS || RT

SAVING POWER WITH POWER-DOWN

FUNCTIONALITY

The THS4500 family of fully differential amplifiers contains

devices that come with and without the power-down

option. Even-numbered

devices

have

power-down

capability, which is described in detail here.

The power-down pin of the amplifiers defaults to the

positive supply voltage in the absence of an applied

voltage (i.e. an internal pullup resistor is present), putting

the amplifier in the power-on mode of operation. To turn off

the amplifier in an effort to conserve power, the

power-down pin can be driven towards the negative rail.

The threshold voltages for power-on and power-down are

relative to the supply rails and given in the specification

tables. Above the enable threshold voltage, the device is

on. Below the disable threshold voltage, the device is off.

Behavior in between these threshold voltages is not

specified.

Note that this power-down functionality is just that; the

amplifier consumes less power in power-down mode. The

power-down mode is not intended to provide a

high-impedance output. In other words, the power-down

functionality is not intended to allow use as a 3-state bus

driver. When in power-down mode, the impedance looking

back into the output of the amplifier is dominated by the

feedback and gain setting resistors.

The time delays associated with turning the device on and

off are specified as the time it takes for the amplifier to

reach 50% of the nominal quiescent current. The time

delays are on the order of microseconds because the

amplifier moves in and out of the linear mode of operation

in these transitions.

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