LTC1409CSW datasheet(11/20 Pages) LINER

11 / 20 page

LTC1409

APPLICATI

I FOR ATIO

bandwidth must be greater than 20MHz to ensure

adequate small-signal settling for full throughput rate. If

slower op amps are used, more settling time can be

provided by increasing the time between conversions.

The best choice for an op amp to drive the LTC1409 will

depend on the application. Generally applications fall into

two categories: AC applications where dynamic specifi-

cations are most critical, and time domain applications

where DC accuracy and settling time are most critical.

The following list is a summary of the op amps that are

suitable for driving the LTC1409, more detailed informa-

tion is available in the Linear Technology databooks and

the LinearView

TM CD-ROM.

® 1220: 30MHz unity-gain bandwidth voltage feed-

back amplifier.

±5V to ±15V supplies. Excellent DC

specifications, 90ns settling to 0.5LSB.

LT1223: 100MHz video current feedback amplifier. 6mA

supply current.

±5V to ±15V supplies. Low distortion up

to and above 400kHz. Low noise. Good for AC

applications.

LT1227: 140MHz video current feedback amplifier. 10mA

supply current

±5V to ±15V supplies. Lowest distortion

at frequencies above 400kHz. Low noise. Best for AC

applications.

LT1229/LT1230: Dual and quad 100MHz current feed-

back amplifiers.

±2V to ±15V supplies. Low noise. Good

AC specs. 6mA supply current for each amplifier.

LT1360: 37MHz voltage feedback amplifier. 3.8mA sup-

ply current. Good AC/DC specs.

±5V to ±15V supplies.

70ns settling to 0.5LSB.

LT1363: 50MHz, 450V/

µs op amps. 6.3mA supply cur-

rent. Good AC/DC specs. 60ns settling to 0.5LSB.

LT1364/LT1365: Dual and quad 50MHz, 450V/

µs op

amps. 6.3mA supply current per amplifier. 60ns settling

to 0.5LSB.

Input Filtering

The noise and the distortion of the input amplifier and

other circuitry must be considered since they will add to

the LTC1409 noise and distortion. The small-signal band-

width of the sample-and-hold circuit is 20MHz. Any noise

or distortion products that are present at the analog inputs

will be summed over this entire bandwidth. Noisy input

circuitry should be filtered prior to the analog inputs to

minimize noise. A simple 1-pole RC filter is sufficient for

many applications. For example, Figure 7 shows a 1000pF

capacitor from + AIN to ground and a 100Ω source resistor

to limit the input bandwidth to 1.6MHz. The 1000pF

capacitor also acts as a charge reservoir for the input

sample-and-hold and isolates the ADC input from sam-

pling glitch sensitive circuitry. High quality capacitors and

resistors should be used since these components can add

distortion. NPO and silver mica type dielectric capacitors

have excellent linearity. Carbon surface mount resistors

can also generate distortion from self heating and from

damage that may occur during soldering. Metal film

surface mount resistors are much less susceptible to both

problems.

When high amplitude unwanted signals are close in fre-

quency to the desired signal frequency, a multiple pole filter

LTC1409

+AIN

–AIN

VREF

REFCOMP

AGND

LTC1409 • F07

LTC1560-1

µF

0.1

µF

0.1

µF

–5V

VIN

Figure 7b. 500kHz 5th Order Elliptic Lowpass Filter

LinearView is a trademark of Linear Technology Corporation.

LTC1409

+AIN

–AIN

VREF

REFCOMP

AGND

LTC1409 • F07b

µF

1000pF

Ω

ANALOG INPUT

Figure 7a. RC Input Filter

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