Datasheet
AD8370 Data Sheet
Rev. B | Page 20 of 28
V
S
R
S
2
R
S
2
R
L
2
R
L
2
Z
S
2
Z
S
2
Z
P
V
S
R
S
R
L
Z
S
Z
P
SOURCE LOAD
BALANCED
CONFIGURATION
SINGLE-ENDED
EQUIVALENT
03692-053
Figure 55. Single-Ended-to-Differential Network Conversion
As an example, a second-order, Butterworth, low-pass filter
design is presented where the differential load impedance is
1200 Ω, and the padded source impedance of the AD8370 is
assumed to be 120 Ω. The normalized series inductor value for
the 10-to-1, load-to-source impedance ratio is 0.074 H, and the
normalized shunt capacitor is 14.814 F. For a 70 MHz cutoff
frequency, the single-ended equivalent circuit consists of a
200 nH series inductor followed by a 27 pF capacitor. To realize
the balanced equivalent, simply split the 200 nH inductor in
half to realize the network shown in Figure 56.
03692-054
V
S
R
S
2
R
S
R
L
R
S
2
R
L
2
R
L
2
27pF
V
S
R
S
= 120Ω
R
L
= 1200Ω
200nH
100nH
100nH
27pF
BALANCED
CONFIGURATION
DE-NORMALIZED
SINGLE-ENDED
EQUIVALENT
V
S
R
S
= = 0.1
R
L
= 1Ω
L
N
= 0.074H
C
N
14.814F
NORMALIZED
SINGLE-ENDED
EQUIVALENT
= 60Ω
= 60Ω
= 600Ω
= 600Ω
f
C
= 70MHz
f
C
= 1Hz
Figure 56. Second-Order, Butterworth, Low-Pass Filter Design Example
A complete design example is shown in Figure 58. The AD8370
is configured for single-ended-to-differential conversion with
the input terminated down to present a single-ended 75 Ω input.
A sixth-order Chebyshev differential filter is used to interface
the output of the AD8370 to the input of the AD9430
170 MSPS, 12-bit ADC. The filter minimizes aliasing effects
and improves harmonic distortion performance.
The input of the AD9430 is terminated with a 1.5 kΩ resistor so
that the overall load presented to the filter network is ~1 kΩ.
The variable gain of the AD8370 extends the useable dynamic
range of the ADC. The measured intermodulation distortion of
the combination is presented in Figure 57 at 42 MHz.
–130
–120
–100
–60
–40
–20
–10
0
–80
–110
–70
–50
–30
–90
dBFS
0 10 20 30 40 50 60 70
FREQUENCY (MHz)
03692-055
Figure 57. FFT Plot of Two-Tone Intermodulation Distortion at
42 MHz for the Circuit in Figure 58
In Figure 57, the intermodulation products are comparable to
the noise floor of the ADC. The spurious-free dynamic range of
the combination is better than 66 dB for a 70 MHz measurement
bandwidth.
3 V OPERATION
It is possible to operate the AD8370 at voltages as low as 3 V
with only minor performance degradation. Table 6 gives typical
specifications for operation at 3 V.
Table 6.
Parameter Typical (70 MHz, R
L
= 100 Ω)
Output IP3 +23.5 dBm
P1dB +12.7 dBm
−3 dB Bandwidth 650 MHz (HG 127)
IMD3 −82 dBc (R
L
= 1 kΩ)