ISL267817_14 Datasheet, PDF (12/18 Pages) Intersil Corporation – 12-Bit Differential Input 200kSPS SAR ADC

ISL267817
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 28 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 +IN and –IN pins within
specification. Figures 29 and 30 illustrate this relationship for
single-ended and differential inputs, respectively.
V
5.0
4.0
+IN
3.0
2.0Vpp
2.0
–IN
VCM
1.0
t
VREF = 2V
V
5.0
4.0
2.5Vpp
+IN
3.0
–IN
VCM
2.0
1.0
t
VREF = 2.5V
FIGURE 28. RELATIONSHIP BETWEEN VREF AND FULL-SCALE RANGE
VCM
5.0
4.0
4.0
3.0
2.8
2.0
2.2
SINGLE-ENDED
1.0
0.0
-0.3
-1.0
0.5 1.0 1.5 2.0 2.5
FIGURE 29. RELATIONSHIP BETWEEN VREF AND VCM FOR
SINGLE-ENDED INPUTS (+VCC = 5V)
VCM
5.0
4.0
3.0
2.75
2.0
D IF F E R E N T IA L
1.0
0.95
0.0
-0.3
-1.0
0.5 1.0 1.5 2.0 2.5
FIGURE 30. RELATIONSHIP BETWEEN VREF AND VCM FOR
DIFFERENTIAL INPUTS (+VCC = 5V)
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 2.5V; however the
device is specified with a reference voltage of 2.5V.
Figures 31 and 32 illustrate possible voltage reference options
for the ISL267817. Figure 31 uses the precision ISL21090
voltage reference which exhibits exceptionally low drift and low
noise. The VREF input pin of the ISL267817 devices uses very low
current, so the decoupling capacitor can be small (0.1µF).
Figure 32 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.
12
FN7877.2
April 19, 2012