ADS5421_14 Datasheet, PDF (11/26 Pages) Texas Instruments – 14-Bit, 40MHz Sampling ANALOG-TO-DIGITAL CONVERTER

V_IN (IF)
Optional
Bandpass
Filter
IF
Amplifier
R_S
XFR
1:n
R_IN
R_{T RIN}
NOTE: Supply bypassing not shown.
+5V
IN
C_IN
ADS5421
IN
CM
+
2.2µF
0.1µF
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
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 3V_PP.
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 R_IN and C_IN values should have
a low tolerance. This will ensure that the ADS5421 sees
closely matched source impedances.
+5V –5V
V_IN
OPA847
R₁
R₂
R_G
R_S
0.1µF
1:n
R_IN
R_{T RIN}
+5V
IN
C_IN
IN
V_CM ≈ +2.5V
ADS5421
CM
+
2.2µF
0.1µF
FIGURE 5. Converting a Single-Ended Input Signal into a Differential Signal Using a RF Transformer.
ADS5421
11
SBAS237E
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