ISL267452_14 Datasheet, PDF (11/16 Pages) Intersil Corporation – 12-Bit, 555kSPS SAR ADC

ISL267452
Analog Input
The ISL267452 features a fully differential input with a nominal
full-scale range equal to twice the applied VREF voltage. Each
input swings VREF V_P-P, 180° out-of-phase from one another for
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
VREF_P-P
VIN+
ISL267452
V_CM
VREF_P-P
VIN–
FIGURE 21. DIFFERENTIAL INPUT SIGNALING
Figure 22 shows the relationship between the reference voltage
and the full-scale input range for two different values of VREF.
V
5.0
4.0
VIN–
VIN+
3.0
2.0VP-P
VCM
2.0
1.0
t
VREF = 2V
V
5.0
VIN–
4.0
2.5VP-P
VIN+
VCM
3.0
2.0
1.0
t
VREF = 2.5V
FIGURE 22. RELATIONSHIP BETWEEN VREF AND FULL-SCALE RANGE
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.
VCM
5.0
4.25V
4.0
3.0
3.25V
2.0
1.75V
1.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5
VREF
FIGURE 23. RELATIONSHIP BETWEEN VREF AND VCM FOR VDD = 5V
VCM
2.5
2.5
2.0
2.0V
1.5
1.0
1.0V
0.5
0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
VREF
FIGURE 24. RELATIONSHIP BETWEEN VREF AND VCM FOR VDD = 3V
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.
11
FN8255.0
July 26, 2012