Datasheet
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R
0
50
W
Z
0
50
W
1:1
INP
INM
CM
ADT1−1WT
R
50
W
1nF 0.1
m
F
AC Signal
Source
10
W
25
W
25
W
ADS5542
400mA f
S
(in MSPS)
80 MSPS
(2)
ADS5542
SBAS308D – MAY 2004 – REVISED FEBRUARY 2007
This differential input topology produces a high level of ac-performance for high sampling rates. It also results in
a very high usable input bandwidth, especially important for high intermediate-frequency (IF) or undersampling
applications. The ADS5542 requires each of the analog inputs (INP, INM) to be externally biased around the
common-mode level of the internal circuitry (CM, pin 17). For a full-scale differential input, each of the differential
lines of the input signal (pins 19 and 20) swings symmetrically between CM + 0.575 V and CM – 0.575 V. This
means that each input is driven with a signal of up to CM ± 0.575 V, so that each input has a maximum
differential signal of 1.15 V
PP
for a total differential input signal swing of 2.3 V
PP
. The maximum swing is
determined by the two reference voltages, the top reference (REFP, pin 29), and the bottom reference (REFM,
pin 30).
The ADS5542 obtains optimum performance when the analog inputs are driven differentially. The circuit shown
in Figure 38 shows one possible configuration using an RF transformer.
Figure 38. Transformer Input to Convert Single-Ended Signal to Differential Signal
The single-ended signal is fed to the primary winding of an RF transformer. Since the input signal must be
biased around the common-mode voltage of the internal circuitry, the common-mode voltage (V
CM
) from the
ADS5542 is connected to the center-tap of the secondary winding. To ensure a steady low-noise V
CM
reference,
best performance is obtained when the CM (pin 17) output is filtered to ground with 0.1 µ F and 0.001- µ F
low-inductance capacitors.
Output V
CM
(pin 17) is designed to directly drive the ADC input. When providing a custom CM level, be aware
that the input structure of the ADC sinks a common-mode current in the order of 400 µ A (200 µ A per input) at 80
MSPS. Equation 2 describes the dependency of the common-mode current and the sampling frequency:
Where:
f
S
> 2MSPS.
This equation helps to design the output capability and impedance of the driving circuit accordingly.
When it is necessary to buffer or apply a gain to the incoming analog signal, it is possible to combine
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 ADS5542. Texas Instruments offers a wide selection of single-ended
operational amplifiers (including the THS3201, THS3202, OPA695, and OPA847) that can be selected
depending on the application. An RF gain block amplifier, such as Texas Instruments THS9001, can also be
used with an RF transformer for 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, OPA695, or OPA847) 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
ADS5542. These three amplifier circuits minimize even-order harmonics. For high 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 input of the ADS5542 directly, as shown in Figure 38 , or with the addition of the filter circuit shown
in Figure 39 .
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