12 FN7708.2 June 28, 2012 Note that there is a trade-off between VREF and the allowable common mode input voltage (VCM). " />
參數資料
型號: ISL267440IUZ
廠商: Intersil
文件頁數: 4/18頁
文件大?。?/td> 0K
描述: IC INTERFACE
標準包裝: 980
系列: *
ISL267440, ISL267450A
12
FN7708.2
June 28, 2012
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 25 and 26 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 27 and 28 illustrate possible voltage reference options
for the ISL267440/ISL26750A. Figure 27 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 ISL267XX devices uses very
low current, so the decoupling capacitor can be small (0.1F).
Figure 28 illustrates the ISL21010 voltage reference being used
with these ADCs. 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 24. RELATIONSHIP BETWEEN VREF AND FULL-SCALE RANGE
FIGURE 25. RELATIONSHIP BETWEEN VREF AND VCM FOR VDD = 5V
3.0
5.0
2.0
1.0
4.0
VIN+
VIN–
VCM
2.0VP-P
VREF=2V
3.0
5.0
2.0
1.0
4.0
VIN+
VIN–
VCM
2.5VP-P
VREF=2.5V
t
V
t
V
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
FIGURE 26. RELATIONSHIP BETWEEN VREF AND VCM FOR VDD = 3V
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
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