LTC1418AIN datasheet(12/28 Pages) LINER

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LTC1418

APPLICATIONS INFORMATION

amplifier. 2.2V to

±15V supplies. Good AC performance,

input noise voltage = 12nV/

√Hz (typ).

LT1630/LT1631: 30MHz, 10V/

µs, Dual/Quad Rail-to-Rail

Input and Output Precision Op Amps. 3.5mA supply

current per amplifier. 2.7V to

±15V supplies. Best AC

performance, input noise voltage = 6nV/

√Hz (typ),

THD = – 86dB at 100kHz.

Input Filtering

The noise and the distortion of the input amplifier and

other circuitry must be considered since they will add to

the LTC1418 noise and distortion. The small-signal band-

width of the sample-and-hold circuit is 5MHz. 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 2000pF

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

to limit the input bandwidth to 800kHz. The 2000pF

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.

Input Range

The

±2.048V and 0V to 4.096V input ranges of the

LTC1418 are optimized for low noise and low distortion.

Most op amps also perform well over these ranges,

allowing direct coupling to the analog inputs and eliminat-

ing the need for special translation circuitry.

Some applications may require other input ranges. The

LTC1418 differential inputs and reference circuitry can

accommodate other input ranges often with little or no

additional circuitry. The following sections describe the

reference and input circuitry and how they affect the input

range.

INTERNAL REFERENCE

The LTC1418 has an on-chip, temperature compensated,

curvature corrected, bandgap reference which is factory

trimmed to 2.500V. It is internally connected to a reference

amplifier and is available at Pin 3. A 8k resistor is in series

with the output so that it can be easily overdriven in

applications where an external reference is required, see

Figure 8. The reference amplifier compensation pin

(REFCOMP, Pin 4) must be connected to a capacitor to

ground. The reference is stable with capacitors of 1

µF or

greater. For the best noise performance, a 10

µF in parallel

with a 0.1

µF ceramic is recommended.

The VREF pin can be driven with a DAC or other means

to provide input span adjustment. The reference should

be kept in the range of 2.25V to 2.75V for specified

linearity.

LTC1418

AIN

–

VREF

REFCOMP

AGND

ANALOG INPUT

100

Ω

1418 F07

2000pF

µF

Figure 7. RC Input Filter

ANALOG

INPUT

1418 F08

µF

0.1

µF

VIN

VOUT

LT1460

LTC1418

AIN

–

VREF

REFCOMP

AGND

VDD

Figure 8. Using the LT1460 as an External Reference

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