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AD8351 Datasheet PDF : 19 Pages
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Data Sheet
AD8351
Figure 35 illustrates a surface acoustic wave (SAW) filter interface.
Many SAW filters are inherently differential, allowing for a low
loss output match. In this example, the SAW filter requires a 50 Ω
source impedance to provide the desired center frequency and
Q. The series L shunt C output network provides a 150 Ω to
50 Ω impedance transformation at the desired frequency of
operation. The impedance transformation is illustrated on a
Smith chart in Figure 36.
It is possible to drive a single-ended SAW filter by connecting
the unused output to ground using the appropriate terminating
resistance. The overall gain of the system is reduced by 6 dB
because only half of the signal is available to the input of the
SAW filter.
50
25
100
35
30
RL = 1000Ω
25
20
RL = 150Ω
15
RL = 500Ω
10
5
0
10
100
RG (Ω)
Figure 38. Gain Selection
7
1000
10
200
500
50Ω
SERIES L
150Ω
0
500
SHUNT C 200
100
6
RL = 1000Ω
5
4
RL = 500Ω
3
2
50
25
10
Figure 36. Smith Chart Representation of SAW Filter Output Matching Network
1
RL = 150Ω
0
10
100
1000
RG (Ω)
Figure 39. Feedback Resistor Selection
50Ω
0.1µF
0.1µF
ADC DRIVING
50Ω RG
0.1µF
25Ω
AD8351
RF
0.1µF RL
Figure 37. Single-Ended Application
SINGLE-ENDED-TO-DIFFERENTIAL OPERATION
The AD8351 can easily be configured as a single-ended-to-
differential gain block, as illustrated in Figure 37. The input signal
is ac-coupled and applied to the INHI input. The unused input
is ac-coupled to ground. Select the values of C1 through C4 such
that their reactances are negligible at the desired frequency of
operation. To balance the outputs, an external feedback resistor,
RF, is required. To select the gain resistor and the feedback resistor,
refer to Figure 38 and Figure 39. From Figure 38, select an RG for
the required dB gain at a given load. Next, select from Figure 39
an RF resistor for the selected RG and load.
Even though the differential balance is not perfect under these
conditions, the distortion performance is still impressive. Figure 13
and Figure 14 show the second and third harmonic distortion
performance when driving the input of the AD8351 using a
single-ended 50 Ω source.
The circuit in Figure 40 represents a simplified front end of the
AD8351 driving the AD6645, which is a 14-bit, 105 MSPS ADC.
For optimum performance, the AD6645 and the AD8351 are
driven differentially. The resistors R1 and R2 present a 50 Ω
differential input impedance to the source with R3 and R4
providing isolation from the analog-to-digital input. The gain
setting resistor for the AD8351 is RG. The AD6645 presents a
1 kΩ differential load to the AD8351 and requires a 2.2 V p-p
differential signal between AIN and AIN for a full-scale output.
This AD8351 circuit then provides the gain, isolation, and source
matching for the AD6645. The AD8351 also provides a balanced
input, not provided by the balun, to the AD6645, which is essential
for second-order cancellation. The signal generator is bipolar,
centered around ground. Connecting the VOCM pin (Pin 10 on
the MSOP and Pin 13 on the LFCSP) of the AD8351 to the VREF
pin of the AD6645 sets the common-mode output voltage of the
AD8351 at 2.4 V. This voltage is bypassed with a 0.1 μF capacitor.
Increasing the gain of the AD8351 increases the system noise and
thus decrease the SNR but does not significantly affect the
distortion. The circuit in Figure 40 can provide SFDR performance
of better than −90 dBc with a 10 MHz input and −80 dBc with a
70 MHz input at a gain of 10 dB.
Rev. D | Page 13 of 19
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