LTC1412C データシートの表示（PDF） - Linear Technology

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LTC1412C

12-Bit, 3Msps, Sampling A/D Converter

Linear Technology 

LTC1412

APPLICATIONS INFORMATION

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 7 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

circuitry. High quality capacitors and resistors should be

used since these components can add distortion. NPO and

silver mica type dielectric capacitors have excellent linear-

ity. Carbon surface mount resistors can also generate

distortion from self heating and from damage that may

occur during soldering. Metal film surface mount resis-

tors are much less susceptible to both problems.

When high amplitude unwanted signals are close in

frequency to the desired signal frequency, a multiple pole

100Ω

ANALOG INPUT

500pF

1 AIN+

2 AIN–

LTC1412

3

VREF

10µF

4

REFCOMP

5

AGND

1412 F07a

Figure 7a. RC Input Filter

1

8

1 AIN+

2

7

VIN

LTC1560-1

3

6

2 AIN–

LTC1412

3

VREF

4

– 5V

5

5V

4

REFCOMP

0.1µF

0.1µF 10µF

5

AGND

1412 F07b

Figure 7b. 1MHz Fifth-Order Elliptic Lowpass Filter

filter is required. Figure 7b shows a simple implementa-

tion using an LTC1560-1 fifth-order elliptic continuous

time filter.

Input Range

The ±2.5V input range of the LTC1412 is optimized for low

noise and low distortion. Most op amps also perform best

over this same range, allowing direct coupling to the ana-

log inputs and eliminating the need for special translation

circuitry.

Some applications may require other input ranges. The

LTC1412 differential inputs and reference circuitry can ac-

commodate other input ranges often with little or no addi-

tional circuitry. The following sections describe the reference

and input circuitry and how they affect the input range.

Internal Reference

The LTC1412 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 8a. A

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

easily overdriven by an external reference or other cir-

cuitry, see Figure 8b. The reference amplifier gains the

voltage at the VREF pin by 1.625 to create the required

internal reference voltage. This provides buffering be-

tween 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 refer-

ence 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

REFERENCE

AMP

R2

10µF

40k

R3

5 AGND

64k

LTC1412

1412 F08a

Figure 8a. LTC1412 Reference Circuit

11

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