Datasheet

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R
IN
R
IN
C
IN
0.1
m
F
R
T
0.1
m
F
1000pF
1:1
R
S
OPA695
R
1
A
V
= 8V/V
(18dB)
R
2
V
IN
INP
INM
CM
-
5V+5V
ADS5542
25 W
25 W
10 W
100 W
100 W
57.5 W
400 W
R
F
R
G
V
OCM
INP
INM
CM
R
F
R
G
R
S
0.1
m
F
0.1
m
F
10
m
F
1
m
F
R
T
+3.3V
+5V
0.1
m
F10
m
F
-
5V
THS4503
R
IN
R
IN
10
W
ADS5542
14-Bit / 80 MSPS
ADS5542
SBAS308D – MAY 2004 – REVISED FEBRUARY 2007
Figure 39 illustrates how R
IN
and C
IN
can be placed to isolate the signal source from the switching inputs of the
ADC and to implement a low-pass RC filter to limit the input noise in the ADC. It is recommended that these
components be included in the ADS5542 circuit layout when any of the amplifier circuits discussed previously
are used. The components allow fine-tuning of the circuit performance. Any mismatch between the differential
lines of the ADS5542 input produces a degradation in performance at high input frequencies, mainly
characterized by an increase in the even-order harmonics. In this case, special care should be taken to keep as
much electrical symmetry as possible between both inputs.
Another possible configuration for lower-frequency signals is the use of differential input/output amplifiers that
can simplify the driver circuit for applications requiring dc-coupling of the input. Flexible in their configurations
(see Figure 40 ), such amplifiers can be used for single-ended-to-differential conversion signal amplification.
Table 4. Recommended Amplifiers to Drive the Input of the ADS5542
INPUT SIGNAL FREQUENCY RECOMMENDED AMPLIFIER TYPE OF AMPLIFIER USE WITH TRANSFORMER?
DC to 20 MHz THS4503 Differential In/Out Amp No
DC to 50 MHz OPA847 Operational Amp Yes
DC to 100 MHz THS4509 Differential In/Out Amp No
OPA695 Operational Amp Yes
10 MHz to 120 MHz THS3201 Operational Amp Yes
THS3202 Operational Amp Yes
Over 100 MHz THS9001 RF Gain Block Yes
Figure 39. Converting a Single-Ended Input Signal to a Differential Signal Using an RF Transformer
Figure 40. Using the THS4503 with the ADS5542
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