CN-0072

Circuit Note

Rev. 0 | Page 2 of 3

CIRCUIT DESCRIPTION

The ADL5513 logarithmic detector dynamic range can be

extended by adding a separate VGA whose gain control input is

the gain control range of the VGA. For the overall measurement

to remain linear in dB, the VGA must provide a linear-in-dB

(exponential) gain control function. The VGA gain must

decrease with an increase in its gain bias in the same way as the

ADL5513. Alternatively, an inverting op amp with suitable level

shifting can be used. It is convenient to select a VGA that needs

only a single 5.0 V supply and is capable of generating a single-

ended output. All of these conditions are met by the AD8368.

Figure 1 shows the simplified schematic. Using the inverse gain

mode (MODE pin low) of the AD8368, its gain decreases on a

slope of 37.5 mV/dB to a minimum value of −12 dB for a gain

voltage (VGAIN) of 1.0 V. The voltage, VGAIN, required by the

AD8368 is 50% of the output of the ADL5513. To scale this

voltage, it is necessary to insert a voltage divider at the output of

the ADL5513. Over the 1.5 V range from the output of the

ADL5513, the gain of the AD8368 varies by (0.5 × 1.5 V)÷

(37.5 mV/dB), or 20 dB. Combined with the 75 dB gain span (at

120 MHz) of the ADL5513, this results in a 1.5 V change in

Due to the amplification of out-of-band noise by the AD8368, a

band-pass filter is used between the AD8368 and ADL5513 to

increase the low-signal sensitivity. The VGA amplifies low

power signals and attenuates high power signals to fit them in

the detectable range of the ADL5513. If an amplifier with higher

gain and lower noise figure is used, better than 90 dB sensitivity

can be achieved for use in a receive-signal-strength-indicator

(RSSI) application.

Figure 2 shows the data results of the extended dynamic range

Due to the high frequencies and wide dynamic ranges involved,

the circuit must be constructed on a multilayer PC board that

has at least one layer for the ground plane. Each IC power pin

must be decoupled with a 0.1 µF low inductance ceramic

capacitor as shown in Figure 1. Some power pins have an

additional smaller value decoupling capacitor (low ESL) for

extremely high frequencies. The “0 Ω” resistors on the

schematic represent optional small resistor values (less than

10 Ω) or ferrite beads that may be added for more decoupling.

The data sheets for the ADL5513 and the AD8368 show the

optimal layouts of the respective evaluation boards.

On the underside of both the ADL5513 and the AD8368 chip

scale package there is an exposed paddle. This paddle is

internally connected to the ground of the chip. Solder the

paddle to the low impedance ground plane on the PCB to

ensure specified electrical performance and to provide thermal

relief. It is also recommended that the ground planes on all

layers under the paddle be stitched together with vias to reduce

thermal impedance.

1.750

1.625

1.500

1.375

1.250

1.125

1.000

0.875

0.750

0.625

0.500

0.375

0.250

3.0

2.0

1.0

0

–1.0

–2.0

–3.0

2.5

1.5

0.5

–0.5

–1.5

–2.5

–90

–70

–50

–30

–10

0

20

–80

–60

–40

–20

10

ERROR +25°C

ERROR –40°C

ERROR +85°C

Figure 2. Output and Conformance for the AD8368/ADL5513 Extended

Dynamic Range Circuit at 120 MHz

COMMON VARIATIONS

The application circuit described here can be modified for any

IF frequency within the operating range of the AD8368 and

ADL5513 by selecting the appropriate band-pass filter. For

operation above 800 MHz, the ADL5330 can be used in place of

the AD8368. The ADL5330 VGA can be used at frequencies

between 10 MHz and 3 GHz. Using the ADL5330 also requires

an inverter between the ADL5330 gain pin and the gain control

voltage from the ADL5513. A single-supply operational

amplifier such as the AD8061 can be configured for use as an

inverter in this application.

LEARN MORE

MT-073 Tutorial, High Speed Variable Gain Amplifiers (VGAs).

Analog Devices.

MT-077 Tutorial, Log Amp Basics. Analog Devices.

MT-078 Tutorial, High Frequency Log Amps. Analog Devices.

MT-101 Tutorial, Decoupling Techniques. Analog Devices.

Data Sheets

AD8061 Data Sheet.

AD8368 Data Sheet.

ADL5330 Data Sheet.

ADL5513 Data Sheet.

REVISION HISTORY

7/09—Revision 0: Initial Version