Datasheet

AD8334-EVAL
Rev. 0 | Page 3 of 8
CONFIGURING THE INPUT IMPEDANCE
The board is built and tested using the components shown
in black (see
Figure 2). Provisions are made for optional
components (shown in gray) that can be installed at the user’s
discretion. As shipped, the input impedances of the low noise
amplifiers (LNAs) are configured for 50 Ω to match the output
impedance of most signal generators and network analyzers.
Input impedances up to 6 kΩ can be realized by changing the
values of the feedback resistors, R
FB
1, R
FB
2, R
FB
3, R
FB
4, and shunt
capacitors, C6, C8, C10, and C12. See the AD8331/AD8332/
AD8334 data sheet for additional details on this circuit feature.
For reference,
Table 1 lists standard values of 1% resistors for
some typical values of input impedance. Of course, if the user
has determined that the source impedance falls between these
values, the feedback resistor value can be calculated accordingly.
Note that the board is designed to accept standard surface-
mount, size 0603 components.
Table 1. LNA External Component Values for Common
Source Impedances
R
IN
(Ω) R
FB1
, R
FB2
, R
FB3
, R
FB4
(Ω, ±1%) C6, C8, C10, C12 (pF)
50 274 22
75 412 12
100 562 8
200 1.13 k 1.2
500 3.01 k No capacitor
6 k No resistor No capacitor
Driving the VGA from an External Source, or Using the
LNA to Drive an External Load
Appropriate components can be installed if the user wishes to
drive the VGA directly from an external source or to evaluate
the LNA output. If the LNA is used to drive off-board loads
or cables, small value series resistors (47 Ω to 100 Ω) are
recommended for LNA decoupling. These can be installed
in the R10, R11, R14, R15, R18, R19, R22, and R23 spaces.
Provisions are made for surface-mount SMA connectors that
can be used for driving from either direction. If the LNA is not
used, it is recommended that the capacitors, C16, C17, C21,
C22, C26, C27, C31, and C32, be carefully removed to avoid
driving the outputs of the LNAs.
06225-002
Figure 2. Assembly
Using the Clamp Circuit
The board is shipped with no resistors installed in the spaces
provided for clamp-circuit operation. Note that each pair of
channels shares a clamp resistor, and the value is selected the
same as it is in the
AD8331/AD8332/AD8334 data sheet. If the
output clamping is desired, the resistors are installed in R49 and
R50. The peak-to-peak clamping level is application dependent,
and appropriate values are listed in the AD8331/AD8332/
AD8334 data sheet.
Viewing Signals
The preferred signal detector is a high impedance differential
probe, such as the Tektronix P6247, 1 GHz differential probe,
connected to the 2-pin headers (VO1, VO2, VO3, or VO4), as
shown in
Figure 4. The low capacitance of this probe has the
least effect on the performance of the device of any detection
method tried. The probe can also be used for monitoring input
signals at IN1, IN2, IN3, or IN4. It can also be used for probing
other circuit nodes; however, beware that the 200 kΩ input
impedance can affect certain circuits.
Differential-to-single-ended transformers are provided for
single-ended output connections. Note that series resistors are
provided to protect against accidental output overload should a
50 Ω load be connected to the connector. Of course, the effect
of these resistors is to limit the bandwidth. If the load connected
to the SMA is > 500 Ω, the 237 Ω series resistors, RX1, RX2, RX3,
RX4, RX5, RX6, RX7, and RX8, can be replaced with 0 Ω values.