MAX2740 Evaluation Kit

4

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VGA and FGA

The inputs and outputs of both circuits are differential.

The VGA has a useful gain-control range of >50dB. The

FGA was designed to deliver 40dB of differential gain

at the second IF frequency of 15.42MHz. In the appli-

cation, the FGA differential inputs are received from the

VGA outputs through a balanced lowpass filter circuit.

The FGA’s differential output is designed to drive a dig-

itizer with a typical load impedance of 4k

Ω differential.

PC Board Layout/Construction

The MAX2740 EV kit can serve as a board layout guide.

Keep PC board trace lengths as short as possible to

minimize parasitics. Keep decoupling capacitors close

to the device, with a low inductance via connection to

the ground plane. The GLSOUT signal must not be

routed parallel to the REF input to avoid loss of frequen-

cy lock due to coupling between the two signals.

The MAX2740 EV kit PC board uses 14mil-wide traces

for 50

Ω traces. The PC board has an 8mil layer profile

to the ground plane on FR4, with a dielectric constant

of 4.5, and 75mil trace-to-ground-plane spacing.