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LTC1419AISW

14-Bit, 800ksps Sampling A/D Converter with Shutdown

Linear Technology 

LTC1419

APPLICATIONS INFORMATION

many applications. For example, Figure 7 shows a 1000pF

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

to limit the input bandwidth to 1.6MHz. The 1000pF

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 ±2.5V input range of the LTC1419 is optimized for low

noise and low distortion. Most op amps also perform well

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

LTC1419 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 LTC1419 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

5V

VIN

LT1019A-2.5

VOUT

+

ANALOG

INPUT

1

+AIN

2

–AIN

3

VREF

LTC1419

4

REFCOMP

10µF

0.1µF

5

AGND

1419 F08b

Figure 8b. Using the LT1019-2.5 as an External Reference

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

easily overdriven by an external reference or other

circuitry, 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.

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

shown in Figure 9. This is useful in applications where the

peak input signal amplitude may vary. The input span of

the ADC can then be adjusted to match the peak input

signal, maximizing the signal-to-noise ratio. The filtering

of the internal LTC1419 reference amplifier will limit the

bandwidth and settling time of this circuit. A settling time

of 5ms should be allowed for after a reference adjustment.

3 VREF

2.500V

R1

2k BANDGAP

REFERENCE

4.0625V

4 REFCOMP

10µF

5 AGND

REFERENCE

AMP

R2

40k

R3

64k

LTC1419

1419 F08a

Figure 8a. LTC1419 Reference Circuit

LTC1450

ANALOG INPUT

1.25V TO 3V

DIFFERENTIAL

1

+AIN

2

– AIN

LTC1419

1.25V TO 3V 3 VREF

10µF

4

REFCOMP

5

AGND

1419 F09

Figure 9. Driving VREF with a DAC

1419fb

11

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