Datasheet

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600 mA f
s
125MSPS
(5)
ADS5500
SBAS303F – DECEMBER 2003 – REVISED FEBRUARY 2007
can be selected depending on the application. An RF
gain block amplifier, such as TI's THS9001, can also
be used with an RF transformer for very high input
frequency applications. The THS4503 is a
recommended differential input/output amplifier.
Table 4 lists the recommended amplifiers.
When using single-ended operational amplifiers
(such as the THS3201, THS3202, OPA847, or
OPA695) to provide gain, a three-amplifier circuit is
recommended with one amplifier driving the primary
of an RF transformer and one amplifier in each of the
legs of the secondary driving the two differential
inputs of the ADS5500. These three amplifier circuits
Figure 42. Transformer Input to Convert
minimize even-order harmonics. For very high
Single-Ended Signal to Differential Signal
frequency inputs, an RF gain block amplifier can be
used to drive a transformer primary; in this case, the
transformer secondary connections can drive the
The single-ended signal is fed to the primary winding
input of the ADS5500 directly, as shown in
of an RF transformer. Since the input signal must be
Figure 42 , or with the addition of the filter circuit
biased around the common-mode voltage of the
shown in Figure 43 .
internal circuitry, the common-mode voltage (V
CM
)
from the ADS5500 is connected to the center-tap of
Figure 43 illustrates how RIN and CIN can be placed
the secondary winding. To ensure a steady low-noise
to isolate the signal source from the switching inputs
V
CM
reference, best performance is obtained when
of the ADC and to implement a low-pass RC filter to
the CM (pin 17) output is filtered to ground with 0.1
limit the input noise in the ADC. It is recommended
µ F and 0.001- µ F low-inductance capacitors.
that these components be included in the ADS5500
circuit layout when any of the amplifier circuits
Output V
CM
(pin 17) is designed to directly drive the
discussed previously are used. The components
ADC input. When providing a custom CM level, be
allow fine-tuning of the circuit performance. Any
aware that the input structure of the ADC sinks a
mismatch between the differential lines of the
common-mode current in the order of 600 µ A (300
ADS5500 input produces a degradation in
µ A per input) at 125 Msps. Equation 5 describes the
performance at high input frequencies, mainly
dependency of the common-mode current and the
characterized by an increase in the even-order
sampling frequency:
harmonics. In this case, special care should be taken
to keep as much electrical symmetry as possible
between both inputs.
Where:
Another possible configuration for lower-frequency
f
S
> 2 Msps.
signals is the use of differential input/output
amplifiers that can simplify the driver circuit for
This equation helps to design the output capability
applications requiring dc-coupling of the input.
and impedance of the driving circuit accordingly.
Flexible in their configurations (see Figure 44 ), such
When it is necessary to buffer or apply a gain to the
amplifiers can be used for single-ended-
incoming analog signal, it is possible to combine
to-differential conversion, signal amplification.
single-ended operational amplifiers with an RF
transformer, or to use a differential input/output
amplifier without a transformer, to drive the input of
the ADS5500. TI offers a wide selection of
single-ended operational amplifiers (including the
THS3201, THS3202, OPA847, and OPA695) that
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