+5V
XFR
1:n
RS
RIN
RIN
Optional
Bandpass
Filter
IF
VIN (IF)
IN
IN
Amplifier
CIN
ADS5421
RT
CM
NOTE: Supply bypassing not shown.
+
0.1µF
2.2µF
FIGURE 4. Driving the ADS5421 with a Low-Distortion IF Amplifier and a Transformer Suited for IF Sampling Applications.
TRANSFORMER-COUPLED, SINGLE-ENDED-TO-
DIFFERENTIAL CONFIGURATION
The circuit also shows the use of an additional RC low-pass
filter placed in series with each converter input. This optional
filter can be used to set a defined corner frequency and
attenuate some of the wideband noise. The actual compo-
nent values would need to be tuned for individual application
requirements. As a guideline, resistor values are typically in
the range of 10Ω to 50Ω, and capacitors in the range of 10pF
to 100pF. In any case, the RIN and CIN values should have
a low tolerance. This will ensure that the ADS5421 sees
closely matched source impedances.
For applications in which the input frequency is limited to
approximately 10MHz (e.g., baseband), a high-speed opera-
tional amplifier may be used. The OPA847 is configured for the
noninverting mode; this amplifies the single-ended input signal
and drives the primary of a RF transformer (see Figure 5). To
maintain the very low distortion performance of the OPA847, it
may be advantageous to set the full-scale input range of the
ADS5421 to 3Vp-p or 2Vp-p (refer to the Reference section for
details on selecting the converter’s full-scale range).
+5V –5V
+5V
RG
RS
RIN
0.1µF
VIN
1:n
OPA847
IN
CIN
RT
RIN
ADS5421
R1
IN
CM
V
CM ≈ +2.5V
R2
+
0.1µF
2.2µF
FIGURE 5. Converting a Single-Ended Input Signal into a Differential Signal Using a RF Transformer.
ADS5421
SBAS237D
11
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