ADS808 Datasheet, PDF (10/19 Pages) Burr-Brown (TI) – 12-Bit, 70MHz Sampling ANALOG-TO-DIGITAL CONVERTER

Some differential driver circuits may allow setting an appropri-
ate common-mode voltage directly at the driver input. This will
simplify the interface to the ADS808 and eliminate the external
biasing resistors and the coupling capacitors. Texas Instru-
ments offers a line of fully differential high-speed amplifiers
(refer to our web site at www.ti.com). The THS4150, for
example, may be used for input frequencies from DC to
approximately 10MHz, for which the part maintains good
distortion performance, providing a 2Vp-p (max) output swing
on ±5V supplies. Combining a differential driver circuit with a
step-up transformer can lead to significant improvement of the
distortion performance (see Figure 6).
Transformer Coupled Interface Circuits
If the application allows for AC-coupling, but requires a signal
conversion from a single-ended source to drive the ADS808
differentially, using a transformer offers a number of advan-
tages. As a passive component, it does not add to the total
noise; plus by using a step-up transformer, further signal
amplification can be realized. As a result, the signal swing
out of the amplifier driving the transformer can be reduced,
leading to more headroom for the amplifier and improved
distortion performance.
One possible interface solution that uses a transformer is
given in Figure 4. The input signal is assumed to be an
Intermediate Frequency (IF) and bandpass filtered prior to
the IF amplifier. Dedicated IF amplifiers, for example the
RF2312 or MAR-6, are fixed-gain broadband amplifiers and
feature a very high bandwidth, a low-noise figure, and a high
intercept point at the expense of high quiescent currents of
50-120mA. The IF amplifier may be AC-coupled or directly
connected to the primary side of the transformer.
A variety of miniature RF transformers are readily available
from different manufacturers, i.e., Mini-Circuits, Coilcraft, or
Trak. When choosing a 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. Since the
transformer does not appreciably load the ladder, its center
tap can be directly tied to the CM pin of the converter, as
shown in Figure 4. The value of termination resistor (R_T)
should be chosen to satisfy the termination requirements of
the source impedance (R_S). It can be calculated using the
equation R_T = n² • R_S to ensure proper impedance matching.
Transformer Coupled, Single-Ended to
Differential Configuration
For applications in which the input frequency is limited to
about 40MHz (e.g., baseband), the wideband, current-feed-
back, operational amplifier OPA685 may be used. As shown
in Figure 5, the OPA685 is configured for the noninverting
mode, amplifies the single-ended input signal, and drives the
primary of an RF transformer. To maintain the very low
distortion performance of the OPA685, it may be advanta-
geous to reduce the full-scale input range (FSR) of the
ADS808 from 2Vp-p to 1.5Vp-p or 1Vp-p (refer to the “Ref-
erence” section for details on selecting the converter’s full-
scale range).
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
+V_S
V_IN (IF)
Optional
Bandpass
Filter
IF
Amp
–V_S
R_S
0.1µF
1:n
XFMR
R_IN
R_{T RIN}
V_CM +2.5V
+5V
C_IN
C_IN
IN
ADS808
IN
CM
+
4.7µF
0.1µF
FIGURE 4. Driving the ADS808 with a Low Distortion RF Amplifier and a Transformer Suited for IF Sampling Applications.
+V –V
V_IN
OPA685
R₁
R₂
R_G
R_S
0.1µF
1:n
XFMR
R_IN
R_{T RIN}
V_CM +2.5V
+5V
C_IN
C_IN
IN
ADS808
IN
CM
+
2.2µF
0.1µF
FIGURE 5. Converting a Single-Ended Input Signal into a Differential Signal Using a RF Transformer.
10
ADS808
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SBAS179C