AD8310

Rev. F | Page 15 of 24

INPUT LEVEL (dBV)

–5

–120

20

–100

(–87dBm)

–80

–60

–40

–20

0

(+13dBm)

5

4

–1

–2

–3

–4

2

0

3

1

10MHz

50MHz

100MHz

±3dB DYNAMIC RANGE

±1dB DYNAMIC RANGE

Figure 30. Log Conformance Error vs. Input Level at 10 MHz,

50 MHz, and 100 MHz

TRANSFER FUNCTION IN TERMS OF SLOPE AND

INTERCEPT

The transfer function of the AD8310 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 AD8310, slope is nominally 24 mV/dB. Therefore, a

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

approximately 240 mV. The plot of log conformance shows the

range over which the device maintains its constant slope. The

dynamic range of the log amp is defined as the range over

which the slope remains within a certain error band, usually

±1 dB or ±3 dB. In Figure 30, for example, the ±1 dB dynamic

range is approximately 95 dB (from +4 dBV to −91 dBV).

The intercept is the point at which the extrapolated linear

response would intersect the horizontal axis (see Figure 29).

For the AD8310, the intercept is calibrated to be −108 dBV

(−95 dBm). Using the slope and intercept, the output voltage

can be calculated for any input level within the specified input

range using the following equation:

VOUT

= VSLOPE × (PIN − PO)

(3)

where:

VOUT

is the demodulated and filtered RSSI output.

VSLOPE

is the logarithmic slope expressed in V/dB.

PIN

is the input signal expressed in dB relative to some reference

level (either dBm or dBV in this case).

PO

is the logarithmic intercept expressed in dB relative to the

same reference level.

For example, for an input level of −33 dBV (−20 dBm), the

output voltage is

VOUT

= 0.024 V/dB × (−33 dBV − (−108 dBV)) = 1.8 V (4)

dBV vs. dBm

The most widely used convention in RF systems is to specify

power in dBm, decibels above 1 mW in 50 Ω. Specification of

the log amp input level in terms of power is strictly a concession

to popular convention. As mentioned previously, log amps do

not respond to power (power absorbed at the input), but to the

input voltage. The use of dBV, defined as decibels with respect

to a 1 V rms sine wave, is more precise. However, this is still

ambiguous, because waveform is also involved in the response

of a log amp, which, for a complex input such as a CDMA

signal, does not follow the rms value exactly. Because most

users specify RF signals in terms of power (more specifically, in

dBm/50 Ω) both dBV and dBm are used to specify the perform-

ance of the AD8310, showing equivalent dBm levels for the

special case of a 50 Ω environment. Values in dBV are

converted to dBm re 50 Ω by adding 13 dB.

Table 4. Correction for Signals with Differing Crest Factors

Signal Type

Correction Factor1 (dB)

Sine wave

0

Square wave or dc

−3.01

Triangular wave

0.9

GSM channel (all time slots on)

0.55

CDMA channel (forward link, nine

channels on)

3.55

CDMA channel (reverse link)

0.5

PDC channel (all time slots on)

0.58

1 Add to the measured input level.

INPUT MATCHING

Where higher sensitivity is required, an input matching network

is useful. Using a transformer to achieve the impedance trans-

formation also eliminates the need for coupling capacitors,

lowers the offset voltage generated directly at the input, and

balances the drive amplitude to INLO and INHI.

The choice of turns ratio depends somewhat on the frequency.

At frequencies below 50 MHz, the reactance of the input

capacitance is much higher than the real part of the input

impedance. In this frequency range, a turns ratio of about 1:4.8

lowers the input impedance to 50 Ω, while raising the input

voltage lowers the effect of the short-circuit noise voltage by the

same factor. The intercept is also lowered by the turns ratio; for

a 50 Ω match, it is reduced by 20 log10 (4.8) or 13.6 dB. The total

noise is reduced by a somewhat smaller factor, because there is a

small contribution from the input noise current.