ANALOG INPUT DRIVEN BY TRANSFORMER
The ADS5220 can be driven by a transformer, which pro-
vides signal AC-coupling and allows a signal conversion from
single-ended input to differential output, or from single-ended
input to single-ended output. Using a transformer offers a
number of advantages. As a passive component, it does not
add to the total noise and has better harmonics in wide
frequency bands, compared to an op amp driver. By using a
step-up transformer, further signal amplification can be real-
ized; as a result, the signal swing from the source can be
reduced. For transformer selection, it is important to carefully
examine the application requirements and determine the
correct model, the desired impedance ratio, and frequency
characteristics. Furthermore, the appropriate model must
support the targeted distortion level and should not exhibit
any core saturation at full-scale voltage levels. A variety of
miniature RF transformers from different manufacturers (such
as Mini-Circuits, Coilcraft, or Trak) can be selected.
24.9Ω
24.9Ω
1:n
IN
IN
ADS5220
22pF
RS
RT
0.1µF
1.7kΩ
1.5kΩ
1.5V
3.3V
FIGURE 3. Transformer-Coupled Differential Input Configura-
tion of ADS5220.
Figure 3 shows a transformer-coupled input configuration of
the ADS5220. The ADS5220 receives a differential AC signal
from the output of the transformer and common-mode volt-
age of +1.5V from the center tap. A source termination
resistor, RT , is required, which may be placed at the primary
or secondary side of the transformer to satisfy the termina-
tion requirements of the source impedance, RS. The circuit
also shows the use of an additional RC low-pass filter placed
in series with each converter input to attenuate some of the
wideband noise. The resistor values are typically in the range
of 10Ω to 50Ω, and capacitors are in the range of 10pF to
100pF for individual application requirements.
ANALOG INPUT DRIVEN BY AMPLIFIER
The ADS5220 can be driven by an operational amplifier with
DC or AC signal coupling, as shown in Figure 4 and Figure
5. In Figure 4, the THS4503, a differential amplifier, is used
to convert a single-ended input into a differential output with
a gain of 2. The THS4503 provides an output common-mode
voltage set by VOCM pin, and is DC-coupled to the input of
ADS5220. A low-pass filter can be created by adding small
capacitors (for example, 10pF) in parallel with the feedback
resistors of the THS4503 as needed for some applications.
10pF(1)
+5V
392Ω
187Ω
24.9Ω
IN
50Ω
60.4Ω
ADS5220
VOCM
THS4503
22pF
24.9Ω
Source
IN
0.1µF
392Ω
215Ω
10pF(1)
–5V
1.7kΩ
1.5kΩ
NOTE: (1) optional.
1.5V
3.3V
FIGURE 4. Using the THS4503 Differential Amplifier (Gain = 2) to Drive the ADS5220 in a DC-Coupled Configuration.
ADS5220
SBAS261A
11
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