Datasheet

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R
IN
R
IN
C
IN
0.1
µ
F
R
T
100
0.1
µ
F
1000pF
1:1
R
S
100
OPA695
R
1
400
A
V
= 8V/V
(18dB)
R
2
57.5
V
IN
INP
INM
CM
5V+5V
10
ADS5541
Production Data
ADS5541
SBAS307C MAY 2004 REVISED FEBRUARY 2007
When it is necessary to buffer or apply a gain to the Figure 41 illustrates how R
IN
and C
IN
can be placed
incoming analog signal, it is possible to combine to isolate the signal source from the switching inputs
single-ended operational amplifiers with an RF of the ADC and to implement a low-pass RC filter to
transformer, or to use a differential input/output limit the input noise in the ADC. It is recommended
amplifier without a transformer, to drive the input of that these components be included in the ADS5541
the ADS5541. Texas Instruments offers a wide circuit layout when any of the amplifier circuits
selection of single-ended operational amplifiers discussed previously are used. The components
(including the THS3201 , THS3202 , OPA847 , and allow fine-tuning of the circuit performance. Any
OPA695 ) that can be selected depending on the mismatch between the differential lines of the
application. An RF gain block amplifier, such as ADS5541 input produces a degradation in
Texas Instruments THS9001 , can also be used with performance at high input frequencies, mainly
an RF transformer for high input frequency characterized by an increase in the even-order
applications. The THS4503 is a recommended harmonics. In this case, special care should be taken
differential input/output amplifier. Table 4 lists the to keep as much electrical symmetry as possible
recommended amplifiers. between both inputs.
When using single-ended operational amplifiers Another possible configuration for lower-frequency
(such as the THS3201 , THS3202 , OPA847 , or signals is the use of differential input/output
OPA695 ) to provide gain, a three-amplifier circuit is amplifiers that can simplify the driver circuit for
recommended with one amplifier driving the primary applications requiring dc-coupling of the input.
of an RF transformer and one amplifier in each of the Flexible in their configurations (see Figure 42 ), such
legs of the secondary driving the two differential amplifiers can be used for single-ended-to-differential
inputs of the ADS5520. These three amplifier circuits conversion signal amplification.
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 ADS5541 directly, as shown in
Figure 40 , or with the addition of the filter circuit
shown in Figure 41 .
Table 4. Recommended Amplifiers to Drive the Input of the ADS5520
INPUT SIGNAL FREQUENCY RECOMMENDED AMPLIFIER TYPE OF AMPLIFIER USE WITH TRANSFORMER
DC to 20MHz THS4503 Differential In/Out Amp No
DC to 50MHz OPA847 Operational Amp Yes
OPA695 Operational Amp Yes
10MHz to 120MHz THS3201 Operational Amp Yes
THS3202 Operational Amp Yes
Over 100MHz THS9001 RF Gain Block Yes
Figure 41. Converting a Single-Ended Input Signal to a Differential Signal Using an RF Transformer
23
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