LTC1408 datasheet(14/20 Pages) LINER

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LTC1408

1408fa

APPLICATIO S I FOR ATIO

INPUT FILTERING AND SOURCE IMPEDANCE

The noise and the distortion of the input amplifier and

other circuitry must be considered since they will add to

the LTC1408 noise and distortion. The small-signal band-

width of the sample-and-hold circuit is 50MHz. 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 1 shows a 47pF

capacitor from CHO+ to ground and a 51

Ω source resistor

to limit the net input bandwidth to 30MHz. The 47pF ca-

pacitor also acts as a charge reservoir for the input sample-

and-hold and isolates the ADC input from sampling-glitch

sensitive circuitry. High quality capacitors and resistors

should be used since these components can add distor-

tion. NPO and silvermica type dielectric capacitors have

excellent linearity. Carbon surface mount resistors can

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

frequency to the desired signal frequency a multiple pole

filter is required.

High external source resistance, combined with 13pF of

input capacitance, will reduce the rated 50MHz input band-

width and increase acquisition time beyond 39ns.

INPUT RANGE

The analog inputs of the LTC1408 may be driven fully

differentially with a single supply. Either input may swing

up to VCC, provided the differential swing is no greater than

2.5V with BIP (Pin 29) Low, or

±1.25V with (BIP Pin 29)

High. The 0V to 2.5V range is also ideally suited for single-

ended input use with single supply applications. The

common mode range of the inputs extend from ground to

the supply voltage VCC. If the difference between the CH+

and CH– at any input pair exceeds 2.5V (unipolar) or 1.25V

(bipolar), the output code will stay fixed at positive full-

scale, and if this difference goes below 0V (unipolar) or –

1.25V (bipolar), the output code will stay fixed at negative

full-scale.

INTERNAL REFERENCE

The LTC1408 has an on-chip, temperature compensated,

bandgap reference that is factory trimmed near 2.5V to

obtain a precise 2.5V input span. The reference amplifier

output VREF, (Pin 23) must be bypassed with a capacitor

to ground. The reference amplifier is stable with capaci-

tors of 1

µF or greater. For the best noise performance, a

µF ceramic or a 10µF tantalum in parallel with a 0.1µF

ceramic is recommended. The VREF pin can be overdriven

with an external reference as shown in Figure 2. The volt-

age of the external reference must be higher than the 2.5V

of the open-drain P-channel output of the internal refer-

ence. The recommended range for an external reference

is 2.55V to VDD. An external reference at 2.55V will see a

DC quiescent load of 0.75mA and as much as 3mA

Figure 1. RC Input Filter

LTC1408

CH0+

CH0–

VREF

GND

1408 F01

µF

47pF*

Ω*

CH1+

CH1–

47pF*

*TIGHT TOLERANCE REQUIRED TO AVOID

APERTURE SKEW DEGRADATION

Ω*

ANALOG

INPUT

ANALOG

INPUT

Figure 2. External Reference

LTC1408

VREF

GND

1408 F02

µF

3V REF

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