AD8310

Rev. F | Page 17 of 24

()

+VS

(2.7V–5.5V)

Step 4: Calculate LM

The matching inductor required to provide both LIN and LO is

the parallel combination of these.

O

IN

O

IN

M

L

L

L

L

L

+

=

(7)

With LIN = 1.8 μH and LO = 356 nH, the value of LM to complete

this example of a match of 50 Ω at 100 MHz is 297.2 nH.

The nearest standard value of 270 nH can be used with only a

slight loss of matching accuracy. The voltage gain at resonance

depends only on the ratio of impedances, as given by

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

=

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

S

IN

S

IN

R

R

R

R

GAIN

log

10

log

20

(8)

SLOPE AND INTERCEPT ADJUSTMENTS

Where system (that is, software) calibration is not available, the

adjustments shown in Figure 33 can be used, either singly or in

combination, to trim the absolute accuracy of the AD8310.

The log slope can be raised or lowered by VR1; the values

shown provide a calibration range of ±10% (22.6 mV/dB to

27.4 mV/dB), which includes full allowance for the variability in

the value of the internal resistances. The adjustment can be

made by alternately applying two fixed input levels, provided by

an accurate signal generator, spaced over the central portion of

the dynamic range, for example, −60 dBV and −20 dBV.

Alternatively, an AM-modulated signal at about the center of

the dynamic range can 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

M

M

+

+

=

Δ

1

1

log

20

dB

10

(9)

For example, using a generator output of −40 dBm with a 70%

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

the signal varies from −47.5 dBm to −32.5 dBm.

The log intercept is adjustable by VR2 over a −3 dB range with

the component values shown. VR2 is adjusted while applying

an accurately known CW signal, preferably 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

can be adjusted to produce a dc output of 15 dB above 0 at

24 mV/dB, which is 360 mV.

0.01

μF

4.7

Ω

52.3

Ω

NC = NO CONNECT

C1

0.01

μF

NC

INHI ENBL

BFIN VPOS

INLO COMM OFLT VOUT

AD8310

VOUT (RSSI)

SIGNAL

INPUT

10k

Ω

C2

0.01

μF

25k

Ω

VR1

10k

Ω

RS

VR2

100k

Ω

FOR VPOS = 3V, RS = 500kΩ

FOR VPOS = 5V, RS = 850kΩ

24mV/dB ±10%

1234

8765

Figure 33. Slope and Intercept Adjustments

INCREASING THE SLOPE TO A FIXED VALUE

It is also possible to increase the slope to a new fixed value and,

therefore, to increase the change in output for each decibel of

input change. A common example of this is the need to map the

output swing of the AD8310 into the input range of an analog-

to-digital converter (ADC) with a rail-to-rail input swing.

Alternatively, a situation might arise when only a part of the

total dynamic range is required (for example, just 20 dB) in an

application where the nominal input level is more tightly

constrained, and a higher sensitivity to a change in this level is

required. Of course, the maximum output is limited by either

the load resistance and the maximum output current rating of

25 mA or by the supply voltage (see the Specifications section).

The slope can easily be raised by adding a resistor from VOUT

to BFIN, as shown in Figure 34. This alters the gain of the

output buffer, by means of stable positive feedback, from its

normal value of 4 to an effective value that can be as high as 16,

corresponding to a slope of 100 mV/dB.

VS

(2.7V–5.5V)

0.01

μF

4.7

Ω

52.3

Ω

NC = NO CONNECT

C1

0.01

μF

NC

INHI ENBL BFIN VPOS

INLO COMM OFLT VOUT

AD8310

VOUT 100mV/dB

SIGNAL

INPUT

C2

0.01

μF

8765

RSLOPE

12.1k

Ω

1234

Figure 34. Raising the Slope to 100 mV/dB

The resistor, RSLOPE, is set according to the equation

Slope

R

SLOPE

mV/dB

24

1 −

k

22

.

9

Ω

=

(10)