LTC1609 datasheet(13/22 Pages) LINER

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LTC1403-1/LTC1403A-1

14031fc

APPLICATIONS INFORMATION

internalreference.Therecommendedrangeforanexternal

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.

INPUT SPAN VERSUS REFERENCE VOLTAGE

The differential input range has a unipolar voltage span

that equals the difference between the voltage at the ref-

erence buffer output VREF at Pin 3, and the voltage at the

ground(ExposedPadGround).Thedifferentialinputrange

of the ADC is ±1.25V when using the internal reference.

The internal ADC is referenced to these two nodes. This

relationship also holds true with an external reference.

DIFFERENTIAL INPUTS

The LTC1403-1/LTC1403A-1 have a unique differential

sample-and-hold circuit that allows inputs from ground

to VDD. The ADC will always convert the bipolar differ-

ence of AIN+ – AIN–, independent of the common mode

voltage at the inputs. The common mode rejection holds

up at extremely high frequencies, see Figure 4. The only

requirement is that both inputs not go below ground or

exceed VDD. Integral nonlinearity errors (INL) and differ-

ential nonlinearity errors (DNL) are largely independent

of the common mode voltage. However, the offset error

will vary. The change in offset error is typically less than

0.1% of the common mode voltage.

Figure 5 shows the ideal input/output characteristics for

the LTC1403-1/LTC1403A-1. The code transitions occur

midway between successive integer LSB values (i.e.,

0.5LSB, 1.5LSB, 2.5LSB, FS – 1.5LSB). The output code

is 2’s complement with 1LSB = 2.5V/16384 = 153µV for

the LTC1403A-1, and 1LSB = 2.5V/4096 = 610µV for the

LTC1403-1. The LTC1403A-1 has 1LSB RMS of random

white noise. Figure 6a shows the LTC1819 converting a

single ended input signal to differential input signals for

optimum THD and SFDR performance as shown in the

FFT plot (Figure 6b).

Figure 4

CMRR vs Frequency

FREQUENCY (Hz)

100

–20

–40

–60

–80

–100

–120

10k

100k

14031 F04

10M 100M

Figure 5

LTC1403-1/LTC1403A-1 Transfer

Characteristic

INPUT VOLTAGE (V)

14031 F05

011...111

011...110

011...101

100...000

100...001

100...010

FS – 1LSB

–FS

AIN–

LTC1403A-1

AIN+

47pF TO

1000pF

1µF

0.1µF

–5V

1µF

1.5VCM

499

47pF TO

1000pF

0.1µF

VIN

1.25VP-P

MAX

1403A F06a

1/2 LT1819

Figure 6a. The LT1819 Driving the LTC1403A-1 Differentially

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