THS4271 datasheet(21/41 Pages) TI

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Power Supply Decoupling Techniques and

Power-Down Reference Pin Operation

APPLICATION CIRCUITS

Driving an Analog-to-Digital Converter With the

THS4211

THS4215

SLOS400D – SEPTEMBER 2002 – REVISED NOVEMBER 2004

NO. OF CHANNELS

PACKAGES

tive rail. The threshold voltages for power-on and

power-down are relative to the supply rails, and are

Single (8-pin)

THS4215D, THS4215DGN, and

THS4215DRB

given in the specification tables. Above the Enable

Threshold Voltage, the device is on. Below the

Disable Threshold Voltage, the device is off. Behavior

between these threshold voltages is not specified.

Recommendations

Note that this power-down functionality is just that;

Power supply decoupling is a critical aspect of any

the amplifier consumes less power in power-down

high-performance amplifier design process. Careful

mode. The power-down mode is not intended to

decoupling provides higher quality ac performance

provide a high- impedance output. In other words, the

(most notably improved distortion performance). The

power-down functionality is not intended to allow use

following guidelines ensure the highest level of per-

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

formance.

the impedance looking back into the output of the

1. Place decoupling capacitors as close to the

amplifier is dominated by the feedback and gain

power supply inputs as possible, with the goal of

setting resistors, but the output impedance of the

minimizing the inductance of the path from

device itself varies depending on the voltage applied

ground to the power supply.

to the outputs.

2. Placement priority should put the smallest valued

The time delays associated with turning the device on

capacitors closest to the device.

and off are specified as the time it takes for the

3. Use of solid power and ground planes is rec-

amplifier to reach 50% of the nominal quiescent

ommended to reduce the inductance along power

current. The time delays are on the order of

supply return current paths, with the exception of

microseconds because the amplifier moves in and out

the areas underneath the input and output pins.

of the linear mode of operation in these transitions.

4. Recommended values for power supply decoup-

ling include a bulk decoupling capacitor (6.8 to 22

µF), a mid-range decoupling capacitor (0.1 µF)

In addition to the power-down pin, the THS4215 also

and a high frequency decoupling capacitor (1000

features a reference pin (REF) which allows the user

pF) for each supply. A 100 pF capacitor can be

to control the enable or disable power-down voltage

used across the supplies as well for extremely

levels applied to the PD pin. Operation of the

high frequency return currents, but often is not

reference pin as it relates to the power-down pin is

required.

described below.

In most split-supply applications, the reference pin will

be connected to ground. In some cases, the user

may want to connect it to the negative or positive

THS4211

supply rail. In either case, the user needs to be aware

of the voltage level thresholds that apply to the

The THS4211 can be used to drive high-performance

power-down pin. The table below illustrates the re-

analog-to-digital converters. Two example circuits are

lationship between the reference voltage and the

presented below.

power-down thresholds.

The first circuit uses a wideband transformer to

POWER-DOWN PIN VOLTAGE

convert a single-ended input signal into a differential

REFERENCE

DEVICE

signal. The differential signal is then amplified and

VOLTAGE

DISABLED

ENABLED

filtered by two THS4211 amplifiers. This circuit pro-

VS– to 0.5 (VS– + VS+)

≤ Ref + 1.0 V

≥ Ref + 1.8 V

vides

low

intermodulation

distortion,

suppressed

even-order distortion, 14 dB of voltage gain, a 50-

Ω

0.5 (VS– + VS+) to VS+

≤ Ref – 1.5 V

≥ Ref – 1 V

input impedance, and a single-pole filter at 100 MHz.

The recommended mode of operation is to tie the

For applications without signal content at dc, this

reference pin to mid-rail, thus setting the threshold

method of driving ADCs can be very useful. Where dc

levels to mid-rail +1.0 V and midrail +1.8 V.

information content is required, the THS4500 family

of fully differential amplifiers may be applicable.

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