LTC1414 データシートの表示（PDF） - Linear Technology

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LTC1414

14-Bit, 2.2Msps, Sampling A/D Converter

Linear Technology 

LTC1414

APPLICATIONS INFORMATION

R1

RP

1:N

50Ω

1 AIN+

ANALOG

INPUT

RS

R2

50Ω

C1

500pF 2 AIN–

3

LTC1414

VREF

1µF

10µF

4

REFCOMP

5

AGND

LTC1414 • F08

Figure 8. If a Transformer Coupled Input is Required,

this Circuit Provides a Simple Solution

Input Filtering

The noise and the distortion of the input amplifier and

other circuitry must be considered since they will add to

the LTC1414 noise and distortion. The small-signal band-

width of the sample-and-hold circuit is 40MHz. 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 9 shows a 500pF capacitor from AIN+

to ground and a 100Ω source resistor to limit the input

bandwidth to 3.2MHz. The 500pF capacitor also acts as a

charge reservoir for the input sample-and-hold and iso-

lates the ADC input from sampling glitch-sensitive cir-

cuitry. High quality capacitors and resistors should be

used since poor quality components can add distortion.

NPO and silver mica type dielectric capacitors have excel-

lent 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.

100Ω

INPUT

1 AIN+

500pF

2 AIN–

3

LTC1414

VREF

10µF

4

REFCOMP

5

AGND

LTC1414 • F09

Figure 9. An RC Filter Reduces the ADC’s 40MHz

Bandwidth to 3.2MHz and Filters Out Wideband Noise

Which May Be Present in the Input Signal

Input Range

The ±2.5V input range of the LTC1414 is optimized for low

noise and low distortion. Most op amps also perform best

over this same range, allowing direct coupling to the

analog inputs and eliminating the need for special transla-

tion circuitry.

Some applications may require other input ranges. The

LTC1414 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 LTC1414 has an on-chip, temperature compensated,

curvature corrected, bandgap reference that is factory

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

amplifier and is available at VREF (Pin 3), see Figure 10. A

2k resistor is in series with the output so that it can be

easily overdriven by an external reference or other cir-

cuitry. The reference amplifier multiplies the voltage at the

VREF pin by 1.625 to create the required internal reference

voltage. This provides buffering between the VREF pin and

the high speed capacitive DAC. The reference amplifier

compensation pin, REFCOMP (Pin 4) must be bypassed

with a capacitor to ground. The reference amplifier is

stable with capacitors of 1µF or greater. For the best noise

performance, a 10µF ceramic or 10µF tantalum in parallel

with a 0.1µF ceramic is recommended.

3 VREF

2.500V

R1

2k BANDGAP

REFERENCE

4.0625V

4 REFCOMP

10µF

+

REFERENCE

AMP

–

R2

40k

R3

5 AGND

64k

LTC1414

1414 F10

Figure 10. LTC1414 Reference Circuit

11

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