AD8309 datasheet(16/20 Pages) AD

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REV. B

AD8309

–16–

Slope and Intercept Adjustment

The AD8309 provides limited opportunities for adjustment of

its basic scaling parameters, which are controlled to within tight

limits through robust design. In applications involving the ob-

servation of measured signal levels on a DVM a slope of 10 mV

per decade is convenient: the reading is then directly in deci-

bels, needing only the positioning of the decimal point. This

may be simply achieved and at the same time trimmed to this

exact value using the scheme shown in Figure 35. A large filter

capacitor CFILT may be added as shown when the voltage is to

be measured on a DVM; this lowers the fluctuation in the lower-

order display digits.

A precision attenuator or signal generator is required to provide

several test levels at 10 dB intervals. The adjustment may also

made using an AM modulated signal, at about the center of the

dynamic range. 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

∆dB = 20 log

10 (1+M)/(1–M)

(12)

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 output would

thus be adjusted to have a peak-to-peak amplitude of 150 mV.

VLOG

VPS2

PADL

LMHI

LMLO

PADL

FLTR

LMDR

COM2

VPS1

PADL

INHI

INLO

PADL

COM1

ENBL

AD8309

RSSI OUTPUT

10mV/dB

10%

0.1 F

CFILT

10 F

DVM

VR1

SLOPE

LIMITER

MAY BE

DISABLED

FOR RSSI

ONLY MODE

8.87k

INPUT

COUPLING

Figure 35. Trimming Slope to 10 mV/dB

± 10%

The intercept can be adjusted by the use of the auxiliary circuit

shown in Figure 36, without changing the slope, which remains

20 mV/dB. This circuit provides a range of about

±4 dB on a

nominal intercept of –113 dBV (–100 dBm), with a fairly low

residual temperature sensitivity (+0.008 dB/

°C). This is suffi-

cient to absorb the worst-case intercept error in the AD8309

plus system-level gain errors. VR2 is adjusted while applying an

accurately known CW signal near the lower end of the dynamic

range, in order to minimize the effect of any residual uncer-

tainty in the slope. For example, to position the intercept to

exactly –100 dBm, a test level of –60 dBm may be applied and

VR2 adjusted to produce a dc output of 40 dB above the inter-

cept, which is +0.8 V. This trim can optionally be combined

with the slope trim described above.

VLOG

VPS2

PADL

LMHI

LMLO

PADL

FLTR

LMDR

COM2

VPS1

PADL

INHI

INLO

PADL

COM1

ENBL

AD8309

0.1 F

RLIM

RSSI

0.1 F

47k

9.6k

VR2

INTERCEPT

IN914

OR SIMILAR

Figure 36. Trimming Intercept to –113 dBV

± 4 dB

APPLICATIONS

The AD8309 is a versatile and easily applied log-limiting amp.

Being complete, it can be used with very few external compo-

nents, and most applications can be accommodated using the

simple connections shown in the preceding section. A few ex-

amples of more specialized applications are provided here.

Log Amp with High Slope Voltage

Where a higher RSSI slope voltage is required, and/or complete

calibration with good temperature stability and minimal interac-

tion between trims, the interface shown in Figure 37 may be

used. Note that at 50 mV/dB, the full 100 dB dynamic range of

the AD8309 requires a 5 V swing. This can be provided by a

single supply operational amplifier having a rail-to-rail output

stage and operating from a 6 V supply. Where a lower range is

sufficient, or when using the 40 mV/dB option, a 5 V supply will

be adequate.

In this application, the supply current into the VPS2 pin is only

slightly dependent on the current delivered to the load resis-

tance, RL, so a voltage dropping resistor, RD, may be added to

lower the supply to the AD8309, which can meet all of its speci-

fications with a 2.7 V supply. The lower chip dissipation and

the resulting reduction in operating temperature will minimize

degradation of noise figure at high ambient temperatures. RD is

calculated as follows:

LIM

−

≥

100

Ω

(13)

which allows for operation at ambient temperatures up to +85

°C.

Table II may be used to select the component values for various

different operating conditions. The slope adjustment range is

±10% and the intercept adjustment range is ±3 dB. Since the

intercept offset bias is derived from the supply, there is a sensi-

tivity to this voltage. Where supply stability is poor, a regulator

may be needed to bias VR2 and R4.

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