LTC1420IGN datasheet(9/20 Pages) LINER

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LTC1420

VCM

SENSE

VREF

1420 F02

–

LOGIC

2.5V

REFERENCE

2.048V

µF

ADC

VOUT

VIN

LT1019A-2.5

1420 F03a

µF

VREF

SENSE

VCM

LTC1420

1420 F03b

µF

VREF

LTC1420

SENSE

VCM

2.048V

–

LTC1450

Figure 2. Reference Circuit

Figure 3a. Using the LT1019-2.5 As an

External Reference; Input Range =

±1.25V

Figure 3b. Driving VREF with a DAC

APPLICATIONS INFORMATION

VREF and VREF can be driven by an external voltage. With

two additional resistors, VREF can be set to any voltage

between 2.048V and 4.5V.

An external reference or a DAC can be used to drive VREF

over a 0V to 5V range (Figures 3a and 3b). The input

impedance of the VREF pin is 1k, so a buffer may be

required for high accuracy. Driving VREF with a DAC is

useful in applications where the peak input signal ampli-

tude may vary. The input span of the ADC can then be

adjusted to match the peak input signal, maximizing the

signal-to-noise ratio.

Both the VCM and VREF pins must be bypassed with

capacitors to ground. For best performance, 1

µF or larger

ceramic capacitors are recommended. For the case of

external circuitry driving VREF, a smaller capacitor can be

used at VREF so the input range can be changed quickly. In

this case, a 0.05

µF or larger ceramic capacitor is accept-

able.

The VCM pin is a low output impedance 2.5V reference that

can be used by external circuitry. For single 5V supply

applications it is convenient to connect – AIN directly to the

VCM pin.

Driving the Analog Inputs

The differential inputs of the LTC1420 are easy to drive.

The inputs may be driven differentially or single-ended

(i. e., the – AIN input is held at a fixed value). The – AIN and

+AIN inputs are simultaneously sampled and any com-

mon mode signal is reduced by the high common mode

rejection of the sample-and-hold circuit. Any common

mode input value is acceptable as long as the input pins

stay between VDD and VSS. During conversion, the analog

inputs are high impedance. At the end of conversion, the

inputs draw a small current spike while charging the

sample-and-hold.

For superior dynamic performance in dual supply mode,

the LTC1420 should be operated with the analog inputs

centered at ground, and in single supply mode the inputs

should be centered at 2.5V. If required, the analog inputs

can be driven differentially via a transformer. Refer to

Table 2 for a summary of the analog input and reference

configurations and their relative advantages.

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