Data Sheet

AD8313

EVALUATION BOARD

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Rev. F | 20 of 23

SCHEMATIC AND LAYOUT

Figure 44 shows the schematic of the AD8313 evaluation board.

Note that uninstalled components are indicated as open. This board

contains the AD8313 as well as the AD8009 current-feedback

operational amplifier.

This is a 4-layer board (top and bottom signal layers, ground, and

power). The top layer silkscreen and layout are shown in Figure

42 and Figure 43. A detailed drawing of the recommended PCB

footprint for the MSOP package and the pads for the matching

components are shown in Figure 45.

The vacant portions of the signal and power layers are filled out

with ground plane for general noise suppression. To ensure a

low impedance connection between the planes, there are multiple

through-hole connections to the RF ground plane. While the ground

planes on the power and signal planes are used as general-pur-

pose ground returns, any RF grounds related to the input matching

network (for example, C2) are returned directly to the RF internal

ground plane.

GENERAL OPERATION

The AD8313 should be powered by a single supply in the range

of 2.7 V to 5.5 V. The power supply to each AD8313 VPOS pin is

decoupled by a 10 Ω resistor and a 0.1 µF capacitor. The AD8009

can run on either single or dual supplies, +5 V to ±6 V. Both the

positive and negative supply traces are decoupled using a 0.1 µF

capacitor. Pads are provided for a series resistor or inductor to

provide additional supply filtering.

The two signal inputs are ac-coupled using 680 pF high quality

RF capacitors (C1, C2). A 53.6 Ω resistor across the differential

signal inputs (INHI, INLO) combines with the internal 900 Ω input

impedance to give a broadband input impedance of 50.6 Ω. This

termination is not optimal from a noise perspective due to the

Johnson noise of the 53.6 Ω resistor. Neither does it account

for the AD8313’s reactive input impedance nor for the decrease

over frequency of the resistive component of the input impedance.

However, it does allow evaluation of the AD8313 over its complete

frequency range without having to design multiple matching net-

works.

For optimum performance, a narrow-band match can be implement-

ed by replacing the 53.6 Ω resistor (labeled L/R) with an RF

inductor and replacing the 680 pF capacitors with appropriate val-

ues. The Narrow-Band LC Matching Example at 100 MHz section

includes a table of recommended values for selected frequencies

and explains the method of calculation.

Switch 1 is used to select between power-up and power-down

modes. Connecting the PWDN pin to ground enables normal oper-

ation of the AD8313. In the opposite position, the PWDN pin can

be driven externally (SMA connector labeled ENBL) to either device

state, or it can be allowed to float to a disabled device state.

The evaluation board comes with the AD8313 configured to operate

in RSSI/measurement mode. This mode is set by the 0 Ω resistor

(R11), which shorts the VOUT and VSET pins to each other. When

using the AD8009, the AD8313 logarithmic output appears on the

SMA connector labeled VOUT. Using only the AD8313, the log

output can be measured at TP1 or the SMA connector labeled

VSET.

USING THE AD8009 OPERATIONAL

AMPLIFIER

The AD8313 can supply only 400 µA at VOUT. It is also sensitive

to capacitive loading, which can cause inaccurate measurements,

especially in applications where the AD8313 is used to measure the

envelope of RF bursts.

The AD8009 alleviates both of these issues. It is an ultrahigh speed

current feedback amplifier capable of delivering over 175 mA of

load current, with a slew rate of 5,500 V/µs, which results in a rise

time of 545 ps, making it ideal as a pulse amplifier.

The AD8009 is configured as a buffer amplifier with a gain of 1.

Other gain options can be implemented by installing the appropriate

resistors at R10 and R12.

Various output filtering and loading options are available using

R5, R6, and C6. Note that some capacitive loads may cause the

AD8009 to become unstable. It is recommended that a 42.2 Ω

resistor be installed at R5 when driving a capacitive load. More

details can be found in the AD8009 data sheet.

VARYING THE LOGARITHMIC SLOPE

The slope of the AD8313 can be increased from its nominal value

of 18 mV/dB to a maximum of 40 mV/dB by removing R11, the

0 Ω resistor, which shorts VSET to VOUT. VSET and VOUT are

now connected through the 20 kΩ potentiometer. The AD8009 must

be configured for a gain of 1 to accurately vary the slope of the

AD8313.

OPERATING IN CONTROLLER MODE

To put the AD8313 into controller mode, R7 and R11 should be

removed, breaking the link between VOUT and VSET. The VSET

pin can then be driven externally via the SMA connector labeled

VSET.

RF BURST RESPONSE

The VOUT pin of the AD8313 is very sensitive to capacitive loading,

as a result care must be taken when measuring the device’s

response to RF bursts. For best possible response time measure-

ments it is recommended that the AD8009 be used to buffer the

output from the AD8313. No connection should be made to TP1,

the added load will effect the response time.