AD8375ACPZ-WP(Rev0) View Datasheet(PDF) - Analog Devices
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Description
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AD8375ACPZ-WP Datasheet PDF : 24 Pages
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AD8375
BROADBAND OPERATION
The AD8375 uses an open-collector output structure that
requires dc bias through an external bias network. Typically,
choke inductors are used to provide bias to the open-collector
outputs. Choke inductors work well at signal frequencies where
the impedance of the choke is substantially larger than the target
ac load impedance. In broadband applications, it may not be
possible to find large enough choke inductors that offer enough
reactance at the lowest frequency of interest while offering a
high enough self resonant frequency (SRF) to support the
maximum bandwidth available from the device. The circuit in
Figure 36 can be used when frequency response below 10 MHz
is desired. This circuit replaces the bias chokes with bias resistors.
The bias resistor has the disadvantage of a greater IR drop, and
requires a supply rail that is several volts above the local 5 V
supply used to power the device. Additionally, it is necessary
to account for the ac loading effect of the bias resistors when
designing the output interface. Whereas the gain of the AD8375
is load dependent, RL, in parallel with R1 + R2, should equal the
optimum 150 Ω target load impedance to provide the expected
ac performance depicted in the data sheet. Additionally, to
ensure good output balance and even-order distortion
performance, it is essential that R1 = R2.
ETC1-1-13
5V
37.5Ω
0.1µF
SET TO
5V
VR
R1 0.1µF
50Ω
0.1µF AD8375
RL
0.1µF
37.5Ω
5
R2
VR
A0 TO A4
Figure 36. Single-Ended Broadband Operation with Resistive Pull-Ups
Using the formula for R1 (Equation 1), the values of R1 = R2
that provide a total presented load impedance of 150 Ω can be
found. The required voltage applied to the bias resistors, VR,
can be found by using the VR formula (Equation 2).
R1 = 75 × RL
(1)
RL − 150
and
VR = R1 × 40 × 10−3 + 5
(2)
For example, in the extreme case where the load is assumed to
be high impedance, RL = ∞, the equation for R1 reduces to R1 =
75 Ω. Using the equation for VR, the applied voltage should be
VR = 8 V. The measured single-tone low frequency harmonic
distortion for a 2 V p-p output using 75 Ω resistive pull-ups is
provided in Figure 37.
–80
–82
HD2
–84
–86
–88
–90
HD3
–92
–94
–96
0
5
10
15
20
FREQUENCY (MHz)
Figure 37. Harmonic Distortion vs. Frequency Using Resistive Pull-Ups
ADC INTERFACING
The AD8375 is a high output linearity variable gain amplifier
that is optimized for ADC interfacing. The output IP3 and noise
floor essentially remain constant vs. the 24 dB available gain
range. This is a valuable feature in a variable gain receiver where
it is desirable to maintain a constant instantaneous dynamic
range as the receiver gain is modified. The output noise density
is typically around 20 nV/√Hz, which is comparable to 14-/16-
bit sensitivity limits. The two-tone IP3 performance of the
AD8375 is typically around 50 dBm. This results in SFDR levels
of better than 86 dB when driving the AD9445 up to 140 MHz.
There are several options available to the designer when using
the AD8375. The open-collector output provides the capability
of driving a variety of loads. Figure 38 shows a simplified
wideband interface with the AD8375 driving a AD9445. The
AD9445 is a 14-bit 125 MSPS analog-to-digital converter with a
buffered wideband input, which presents a 2 kΩ differential
load impedance and requires a 2 V p-p differential input swing
to reach full scale.
ETC1-1-13
0.1µF
5V
1µH 0.1µF
L
(SERIES) 0.1µF 33Ω
50Ω
37.5Ω
5V
0.1µF AD8375
82Ω
1µH 0.1µF
L
(SERIES) 0.1µF 33Ω
37.5Ω
82Ω
5
VIN+
AD9445 14
14-BIT ADC
VIN–
A0 TO A4
Figure 38. Wideband ADC Interfacing Example Featuring the AD9445
Rev. 0 | Page 14 of 24
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