
ISLA214S50
17
FN7973.2
April 25, 2013
Analog Input
A single fully differential input (VINP/VINN) connects to the
sample and hold amplifier (SHA) of each unit ADC. The ideal
full-scale input voltage is 2.0V, centered at the VCM voltage as
Best performance is obtained when the analog inputs are driven
differentially. The common-mode output voltage, VCM, should be
used to properly bias the inputs as shown in Figures
34 through
36. An RF transformer will give the best noise and distortion
performance for wideband and/or high intermediate frequency
(IF) inputs. Two different transformer input schemes are shown in
This dual transformer scheme is used to improve common-mode
rejection, which keeps the common-mode level of the input
matched to VCM. The value of the shunt resistor should be
determined based on the desired load impedance. The
differential input resistance of the ISLA214S50 is 600
Ω.
The SHA design uses a switched capacitor input stage (see
Figure
48), which creates current spikes when the sampling
capacitance is reconnected to the input voltage. This causes a
disturbance at the input which must settle before the next
sampling point. Lower source impedance will result in faster
settling and improved performance. Therefore a 2:1 or 1:1
transformer and low shunt resistance are recommended for
optimal performance.
A differential amplifier, as shown in the simplified block diagram
in Figure
36, can be used in applications that require
DC-coupling. In this configuration, the amplifier will typically
dominate the achievable SNR and distortion performance.
Intersil’s new ISL552xx differential amplifier family can also be
FIGURE 32. TYPICAL SNR AND SFDR PERFORMANCE vs TEMPERATURE, DEVICE CALIBRATED AT +85°C, fIN = 105MHz
Temperature Calibration (Continued)
0
10
20
30
40
50
60
70
80
90
100
65
70
75
80
85
TEMPERATURE (°C)
S
N
R
(dBFS)
AND
S
F
DR
(dBc)
SFDR AT 500MSPS
SFDR AT 350MSPS
SNR AT 500MSPS
SNR AT 350MSPS
FIGURE 33. ANALOG INPUT RANGE
1.0
1.8
0.6
0.2
1.4
VINP
VINN
VCM
1.0V
FIGURE 34. TRANSFORMER INPUT FOR GENERAL PURPOSE
APPLICATIONS
ADT1-1WT
0.1F
ADC
VCM
ADT1-1WT
1000pF
FIGURE 35. TRANSMISSION-LINE TRANSFORMER INPUT FOR
HIGH IF APPLICATIONS
ADC
VCM
1000pF
TX-2-5-1
ADTL1-12
FIGURE 36. DIFFERENTIAL AMPLIFIER INPUT
ADC