AD8307 datasheet(19/24 Pages) AD

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Data Sheet

AD8307

Rev. F | Page 19 of 24

SLOPE AND INTERCEPT ADJUSTMENTS

Where higher calibration accuracy is needed, the adjustments

shown in Figure 37 can be used, either singly or in combination.

The log slope is lowered to 20 mV/dB by shunting the nominally

12.5 kΩ on-chip load resistor (see Figure 31) with 50 kΩ, adjusted

by VR1. The calibration range is ±10% (18 mV/dB to 22 mV/dB),

including full allowance for the variability in the value of the

internal load. The adjustment can be made by alternately applying

two input levels, provided by an accurate signal generator, spaced

over the central portion of the log amp’s dynamic range, for

example, −60 dBm and 0 dBm. An AM modulated signal at the

center of the dynamic range can also be used. For a modulation

depth, M, expressed as a fraction, the decibel range between the

peaks and troughs over one cycle of the modulation period is

given by









log

(7)

For example, using an rms signal level of −40 dBm with a 70%

modulation depth (M = 0.7), the decibel range is 15 dB, as the

signal varies from −47.5 dBm to −32.5 dBm.

The log intercept is adjustable over a ±3 dB range, which is

sufficient to absorb the worst-case intercept error in the AD8307,

plus some system level errors. For greater range, set RS to zero.

VR2 is adjusted while applying an accurately known CW signal

near the lower end of the dynamic range to minimize the effect

of any residual uncertainty in the slope. For example, to position

the intercept to −80 dBm, a test level of −65 dBm can be applied

and VR2 adjusted to produce a dc output of 15 dB above zero at

25 mV/dB, which is 0.3 V.

C2 = CC

C1 = CC

20mV/dB

±10%

AD8307

INPUT

–75dBm TO

+16dBm

INP VPS ENB INT

INM COM OFS OUT

NC = NO CONNECT

4.7Ω

VR2

50kΩ

0.1µF

VP, 2.7V TO 5.5V

AT ~8mA

±3dB

VR1

50kΩ

32.4kΩ

FOR VP = 3V, RS = 20kΩ

VP = 5V, RS = 51kΩ

Figure 37. Slope and Intercept Adjustments

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