ISL267452

11

FN8255.0

July 26, 2012

Analog Input

The ISL267452 features a fully differential input with a nominal

full-scale range equal to twice the applied VREF voltage. Each

a total differential input of 2*VREF (refer to Figure 21).

Differential signaling offers several benefits over a single-ended

input, such as:

• Doubling of the full-scale input range (and therefore the

dynamic range)

• Improved even order harmonic distortion

• Better noise immunity due to common mode rejection

Figure 22 shows the relationship between the reference voltage

and the full-scale input range for two different values of VREF.

Note that there is a trade-off between VREF and the allowable

common mode input voltage (VCM). The full-scale input range is

proportional to VREF; therefore the VCM range must be limited

for larger values of VREF in order to keep the absolute maximum

and minimum voltages on the VIN+ and VIN– pins within

specification. Figures 23 and 24 illustrate this relationship for 5V

and 3V operation, respectively. The dashed lines show the

theoretical VCM range based solely on keeping the VIN+ and

VIN– pins within the supply rails. Additional restrictions are

imposed due to the required headroom of the input circuitry,

resulting in practical limits shown by the shaded area.

Voltage Reference Input

An external low-noise reference voltage must be applied to the VREF

pin to set the full-scale input range of the converter. The reference

input accepts voltages ranging from 0.1V to 2.2V for 3V operation

and 0.1V to 3.5V for 5V operation. The device is specified with a

reference voltage of 2.5V for 5V operation and 2.0V for 3V

operation.

Figures 26 and 27 illustrate possible voltage reference options for

the ISL267452. Figure 26 uses the precision ISL21090 voltage

reference, which exhibits exceptionally low drift and low noise. The

ISL21090 must use a power supply greater than 4.7V. The VREF

input pin of the ISL267452 uses very low current, so the decoupling

capacitor can be small (0.1µF).

Figure 27 illustrates the ISL21010 voltage reference being used

with the ISL267452. The ISL21010 series voltage references have

higher noise and drift than the ISL26090 devices, but they consume

very low operating current and are excellent for battery-powered

applications.

FIGURE 21. DIFFERENTIAL INPUT SIGNALING

FIGURE 22. RELATIONSHIP BETWEEN VREF AND FULL-SCALE RANGE

ISL267452

VIN

+

VIN

–

3.0

5.0

2.0

1.0

4.0

VIN+

VIN–

VCM

VREF=2V

3.0

5.0

2.0

1.0

4.0

VIN+

VIN–

VCM

VREF=2.5V

t

V

t

V

FIGURE 23. RELATIONSHIP BETWEEN VREF AND VCM FOR VDD = 5V

FIGURE 24. RELATIONSHIP BETWEEN VREF AND VCM FOR VDD = 3V

3.0

5.0

2.0

1.0

4.0

VREF

VCM

0.5

1.0

1.5

2.0

2.5

3.0

3.5

3.25V

1.75V

4.25V

1.5

2.0

1.0

0.5

VREF

VCM

0.25 0.50 0.75 1.00

2.5

2.00

1.25 1.50 1.75

2.5

1.0V

2.0V