Data Sheet

ADL5513

APPLICATIONS INFORMATION

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Rev. B | 22 of 25

INCREASING THE DYNAMIC RANGE OF THE

ADL5513

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

alone 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. 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 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. Combined 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 AD8368, a band-

pass filter was inserted between the AD8368 and ADL5513 to

increase the low end sensitivity. The VGA amplifies low power sig-

nals 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.

Figure 45. Output and Conformance for the AD8368/ADL5513 Extended

Dynamic Range Circuit

Figure 46. ADL5513 with 95 dB Dynamic Range