Data Sheet

ADL5506

APPLICATIONS INFORMATION

analog.com

Rev. C | 16 of 22

BASIC CONNECTIONS

Figure 42 shows the basic connections for measurement mode. A

supply voltage of 2.5 V to 5.5 V is required. Decouple the supply

to the VPOS pin with a low inductance 0.1 µF surface-mount

ceramic capacitor. A series resistor of about 10 Ω can be added;

this resistor slightly reduces the supply voltage to the ADL5506 and

depends on the load resistance at the output to ground. Avoid its

use in applications where the power supply voltage is very low. A

series inductor provides similar power supply filtering with minimal

drop in supply voltage.

Figure 42. Basic Connections

The ADL5506 has an internal input coupling capacitor. This elimi-

nates the need for external ac coupling. In this example, a broad-

band input match is achieved by connecting a 52.3 Ω resistor

between RFIN and ground. This resistance combines with the inter-

nal input impedance to give an overall broadband input resistance

of 50 Ω. Several other coupling methods are possible; these are

described in the Input Coupling Options section.

Ensure that the load resistance on VLOG is not lower than 600

Ω so that the full-scale output can be generated with the limited

available sourcing current of 4 mA. Figure 43 shows the logarithmic

conformance under the same conditions.

Figure 43. VLOG and Log Conformance Error vs. Input Level at 900 MHz

TRANSFER FUNCTION IN TERMS OF SLOPE

AND INTERCEPT

The transfer function of the ADL5506 is characterized in terms of

its slope and intercept. The logarithmic slope is defined as the

change in the RSSI output voltage for a 1 dB change at the input.

For the ADL5506, the slope is nominally 18 mV/dB. Therefore, a

10 dB change at the input results in a change at the output of

approximately 180 mV. Figure 43 shows the range over which the

device maintains its constant slope. The dynamic range can be

defined as the range over which the error remains within a certain

band, usually ±1 dB or ±3 dB. In Figure 43 for example, the ±1 dB

dynamic range is approximately 46 dB (from −44 dBm to +2 dBm).

The intercept is the point at which the extrapolated linear response

intersects the horizontal axis (see Figure 43). Using the slope and

intercept, calculate the output voltage for any input level within the

specified input range, or calculate the input level from the output

voltage by the following complementary equations:

VLOG = VSLOPE × (PIN – PO)

(1)

PIN = (VLOG/VSLOPE) + PO

(2)

where:

VLOG is the demodulated and filtered RSSI output, in V.

VSLOPE is the logarithmic slope, expressed in V/dB.

PIN is the input signal, expressed in decibels relative to some

reference level (dBm in this case).

PO is the logarithmic intercept, expressed in decibels relative to the

same reference level.

For example, at an input level of −27 dBm, the VLOG output voltage

is

VLOG = 0.018 V/dB × [−27 dBm − (−56 dBm)] = 0.522 V

LOG CONFORMANCE ERROR CALCULATION

Log conformance error is expressed in terms of the deviation in

the output voltage between the measured VLOG and the VLOG

calculated with an ideal log transformation function. Ideally, the

measured VLOG output at a particular input power, as plotted in

Figure 44, must not deviate from the calculated value of VLOG at

that same input power. Setting the measured VLOG to the right side

of the preceding equation and rearranging yields

0

)

(







O

IN

SLOPE

IN

MEASURED

LOG

P

P

V

P

V

(3)

In actuality, this does not always calculate to zero. The finite

calculation that results is the log conformance error, as follows:

0

)

(

)

(









O

IN

SLOPE

IN

MEASURED

LOG

IN

P

P

V

P

V

P

Error

(4)

where Error is in dB.