AD8332ACP-R2 datasheet(48/56 Pages) AD

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AD8331/AD8332/AD8334

Rev. G | Page 48 of 56

CONFIGURING THE INPUT IMPEDANCE

The board is built and tested using the components shown in

black in Figure 115. Provisions are made for optional components

(shown in gray) that can be installed at user discretion. As

shipped, the input impedances of the low noise amplifiers (LNAs)

are configured for 50 Ω to match the output impedances of most

signal generators and network analyzers. Input impedances up to

6 kΩ can be realized by changing the values of the feedback

resistors, RFB1, RFB2, RFB3, RFB4, and shunt capacitors, C6, C8, C10,

and C12. For reference, Table 12 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 12. LNA External Component Values for Common

Source Impedances

RIN (Ω)

RFB1, RFB2, RFB3, RFB4 (Ω, ±1%)

C6, C8, C10, C12 (pF)

274

412

100

562

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 wants 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.

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. If the output clamping is

desired, the resistors are installed in R49 and R50. The peak-to-

peak clamping level is application dependent.

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 116. 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 be used for probing

other circuit nodes; however, be aware 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.

Figure 114. AD8334-EVALZ Assembly

MEASUREMENT SETUP

The basic board connections for measuring bandwidth are

shown in Figure 116. A 5 V, 200 mA (minimum) power supply

is required, and a low noise voltage reference supply is required

for VGAIN.

BOARD LAYOUT

The evaluation board circuitry uses four conductor layers. The

two inner layers are ground, and all interconnecting circuitry is

located on the outer layers. Figure 117 to Figure 120 illustrate

the copper patterns.

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