ADL5513ACPZ-R2 datasheet(22/25 Pages) AD

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ADL5513

Data Sheet

Rev. A | Page 22 of 25

INCREASING THE DYNAMIC RANGE OF THE

ADL5513

The ADL5513 dynamic range can be extended by adding a standa-

lone VGA, whose gain control input is derived directly from

VOUT. This extends the dynamic range by the gain control range

of the VGA. In order 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. Alterna-

tively, 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 46 shows

the 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, that is required by the AD8368 is 50% of the

output of the ADL5513. To scale this voltage, it is necessary to

install 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. Com-

bined with the 75 dB gain span (at 120 MHz) of the ADL5513,

this results in a 95 dB variation for a 1.5 V change in VOUT.

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

band-pass filter was inserted between the AD8368 and ADL5513

to increase the low end 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 an RSSI application.

Figure 45 shows data results of the extended dynamic range at

120 MHz with error in VOUT.

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

–1.0

–2.0

–3.0

2.5

1.5

0.5

–0.5

–1.5

–2.5

–90

–70

–50

–30

–10

–80

–60

–40

–20

PIN (dBm)

VOUT +25°C

VOUT –40°C

VOUT +85°C

ERR0R +25°C

ERR0R –40°C

ERR0R +85°C

Figure 45. Output and Conformance for the AD8368/ADL5513

Extended Dynamic Range Circuit

INPT

ENBL

VPSI

OCOM

ICOM

MODE

VPSI

VPSO

VPSI

OUTP

AD8368

VPOS1

10kΩ

215Ω

0Ω

10nH

10nF

1nF

0.1µF

C10

1nF

5.6pF

1nF

C12

1nF

C15

0.1µF

INPUT 10nF

VPOS3

0Ω

C12

1nF

C15

0.1µF

VPOS2

BAND-PASS

120MHz

52.3Ω

VPOS

2 INHI

3 INLO

VPOS

VSET

TADJ

VOUT

COMM

ADL5513

VPOS

GND

1kΩ

47nF

100pF

1000pF

0.1uF

100pF

0.1uF

VTADJ = 0.89V

VPOS

1kΩ

VPOS

VPOS1

VPOS

VPOS2 VPOS3

Figure 46. ADL5513 with 95 dB Dynamic Range

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