THS4302RGTR datasheet(13/28 Pages) TI2

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APPLICATION CIRCUITS

Driving an Analog-to-Digital Converter With the

THS4302

100

Ω

49.9

Ω

µF

47 pF

0.1

µF

VS+

50-

Ω Source

30.1

Ω

FB = Ferrite Bead

*2.5 V

* = Low Impedance

ADS5422

14-Bit, 63 Msps

RISO 0.1 µF

24.9

Ω

THS4302

49.9

Ω

µF

47 pF

0.1

µF

VS+

50-

Ω Source

30.1

Ω

FB = Ferrite Bead

*2.5 V

* = Low Impedance

24.9

Ω

16.5

Ω

THS4302

SLOS403H – OCTOBER 2002 – REVISED AUGUST 2006

THS4302

The THS4302 amplifier can be used to drive high-

performance

analog-to-digital

converters.

Two

example circuits are presented below.

The first circuit uses a wideband transformer to

convert a single-ended input signal into a differential

signal. The amplified signal from the output of the

THS4302 is fed through a low-pass filter, via an

isolation resistor and an ac-coupling capacitor, to the

transformer.

For applications without signal content at dc, this

method

driving

ADCs

useful.

Where

information content is required, the THS4500 family

of fully differential amplifiers may be applicable.

Figure 45. DC-Coupled Single Supply Operation

Saving Power With Power-Down Functionality

The THS4302 features a power-down pin (PD) which

lowers the quiescent current from 37 mA down to

800

μA, ideal for reducing system power.

The power-down pin of the amplifier defaults to the

positive supply voltage in the absence of an applied

voltage, 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

Figure 46. Driving an ADC Via a Transformer

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

The second circuit depicts single-ended ADC drive.

as a 3-state bus driver. When in power-down mode,

While not recommended for optimum performance

the impedance looking back into the output of the

using converters with differential inputs, satisfactory

amplifier is dominated by the feedback and gain

performance can sometimes be achieved with single-

setting resistors, but the output impedance of the

ended input drive. An example circuit is shown here

device itself varies depending on the voltage applied

for reference.

to the outputs.

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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