LTC1403 Datasheet, PDF(10/20 Page) Linear Technology – Serial 12-Bit/14-Bit, 2.8Msps Sampling ADCs with Shutdown

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LTC1403 Datasheet, PDF (10/20 Pages) Linear Technology – Serial 12-Bit/14-Bit, 2.8Msps Sampling ADCs with Shutdown

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LTC1403/LTC1403A

APPLICATIO S I FOR ATIO

51â¦

47pF

10ÂµF

1 AIN+

2 AINâ

LTC1403/

LTC1403A

VREF

11 GND

1403A F01

Figure 1. RC Input Filter

3VREF

10ÂµF

3 VREF

LTC1403/

LTC1403A

11 GND

1403A F02

Figure 2

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 LTC1403/LTC1403A noise and distortion. The small-

signal bandwidth 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 AIN+ to ground and a 51â¦

source resistor to limit the input bandwidth to 47MHz. The

47pF 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 silvermica type dielectric capacitors

have excellent linearity. Carbon surface mount resistors

can generate distortion from self heating and from dam-

age that may occur during soldering. Metal film surface

mount resistors are much less susceptible to both prob-

lems. When high amplitude unwanted signals are close in

frequency to the desired signal frequency, a multiple pole

filter is required. High external source resistance, com-

bined with the 13pF of input capacitance, will reduce the

rated 50MHz bandwidth and increase acquisition time

beyond 39ns.

INPUT RANGE

The analog inputs of the LTC1403/LTC1403A may be

driven fully differentially with a single supply. Each input

may swing up to 3VP-P individually. In the conversion

range, the noninverting input of each channel is always up

to 2.5V more positive than the inverting input of each

channel. 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 VDD. If the difference be-

tween the AIN+ and AINâ inputs exceeds 2.5V, the output

code will stay fixed at all ones and if this difference goes

below 0V, the ouput code will stay fixed at all zeros.

INTERNAL REFERENCE

The LTC1403/LTC1403A has an on-chip, temperature com-

pensated, bandgap reference that is factory trimmed near

2.5V to obtain 2.5V input span. The reference amplifier

output VREF, (Pin 3) must be bypassed with a capacitor to

ground. The reference amplifier is stable with capacitors

of 1ÂµF or greater. For the best noise performance, a 10ÂµF

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

is recommended. The VREF pin can be overdriven with an

external reference as shown in Figure 2. The voltage of the

external reference must be higher than the 2.5V of the

class A pull-up output of the internal reference. 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 during conversion.

1403af

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