ADS5510
www.ti.com
SLAS499–JANUARY 2007
Table 4. Recommended Amplifiers to Drive the Input of the ADS5510
INPUT SIGNAL FREQUENCY
DC to 20 MHz
RECOMMENDED AMPLIFIER
THS4503
TYPE OF AMPLIFIER
Differential In/Out Amp
Operational Amp
USE WITH TRANSFORMER?
No
Yes
No
DC to 50 MHz
OPA847
DC to 100 MHz
THS4509
Differential In/Out Amp
Operational Amp
OPA695
Yes
Yes
Yes
Yes
10 MHz to 120 MHz
Over 100 MHz
THS3201
Operational Amp
THS3202
Operational Amp
THS9001
RF Gain Block
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
ADS5510. 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 ADS5510 directly, as shown in Figure 25, or with the addition of the filter circuit shown
in Figure 26.
Figure 26 illustrates how RIN and CIN 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 ADS5510 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 ADS5510 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 27), such amplifiers can be used for single-ended-to-differential conversion signal amplification.
−
+5V 5V
RS
RIN
RIN
Ω
100
µ
0.1 F
VIN
1:1
INP
INM
OPA695
RT
100
CIN
1000pF
ADS5510
Ω
R1
Ω
400
CM
Ω
10
AV = 8V/V
(18dB)
R2
57.5
Ω
µ
0.1 F
Figure 26. Converting a Single-Ended Input Signal to a Differential Signal Using an RF Transformer
20
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