Datasheet AD8310 (Analog Devices) - 15

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AD8310 5 dBV vs. dBm 4 ±3dB DYNAMIC RANGE
The most widely used convention in RF systems is to specify
3
power in dBm, decibels above 1 mW in 50 Ω. Specification of
2
the log amp input level in terms of power is strictly a concession
±1dB DYNAMIC RANGE 1
to popular convention. As mentioned previously, log amps do
10MHz
not respond to power (power absorbed at the input), but to the
0 ROR (dB)
input voltage. The use of dBV, defined as decibels with respect
R E –1
to a 1 V rms sine wave, is more precise. However, this is still
–2
ambiguous, because waveform is also involved in the response
50MHz –3
of a log amp, which, for a complex input such as a CDMA
–4
signal, does not follow the rms value exactly. Because most
100MHz
users specify RF signals in terms of power (more specifically, in
–5 –120 –100 –80 –60 –40 –20 0 20
dBm/50 Ω) both dBV and dBm are used to specify the perform-
(–87dBm) (+13dBm) INPUT LEVEL (dBV)
01084-030 ance of the AD8310, showing equivalent dBm levels for the Figure 30. Log Conformance Error vs. Input Level at 10 MHz, special case of a 50 Ω environment. Values in dBV are 50 MHz, and 100 MHz converted to dBm re 50 Ω by adding 13 dB.
TRANSFER FUNCTION IN TERMS OF SLOPE AND INTERCEPT Table 4. Correction for Signals with Differing Crest Factors Signal Type Correction Factor1 (dB)
The transfer function of the AD8310 is characterized in terms Sine wave 0 of its slope and intercept. The logarithmic slope is defined as the Square wave or dc −3.01 change in the RSSI output voltage for a 1 dB change at the input. Triangular wave 0.9 For the AD8310, slope is nominally 24 mV/dB. Therefore, a GSM channel (all time slots on) 0.55 10 dB change at the input results in a change at the output of CDMA channel (forward link, nine 3.55 approximately 240 mV. The plot of log conformance shows the channels on) range over which the device maintains its constant slope. The CDMA channel (reverse link) 0.5 dynamic range of the log amp is defined as the range over PDC channel (all time slots on) 0.58 which the slope remains within a certain error band, usually 1 ±1 dB or ±3 dB. In Figure 30, for example, the ±1 dB dynamic Add to the measured input level. range is approximately 95 dB (from +4 dBV to −91 dBV).
INPUT MATCHING
The intercept is the point at which the extrapolated linear Where higher sensitivity is required, an input matching network response would intersect the horizontal axis (see Figure 29). is useful. Using a transformer to achieve the impedance trans- For the AD8310, the intercept is calibrated to be −108 dBV formation also eliminates the need for coupling capacitors, (−95 dBm). Using the slope and intercept, the output voltage lowers the offset voltage generated directly at the input, and can be calculated for any input level within the specified input balances the drive amplitude to INLO and INHI. range using the following equation: The choice of turns ratio depends somewhat on the frequency. VOUT = VSLOPE × (PIN − PO) (3) At frequencies below 50 MHz, the reactance of the input capacitance is much higher than the real part of the input where: impedance. In this frequency range, a turns ratio of about 1:4.8 VOUT is the demodulated and filtered RSSI output. lowers the input impedance to 50 Ω, while raising the input VSLOPE is the logarithmic slope expressed in V/dB. voltage lowers the effect of the short-circuit noise voltage by the PIN is the input signal expressed in dB relative to some reference same factor. The intercept is also lowered by the turns ratio; for level (either dBm or dBV in this case). a 50 Ω match, it is reduced by 20 log P 10 (4.8) or 13.6 dB. The total O is the logarithmic intercept expressed in dB relative to the noise is reduced by a somewhat smaller factor, because there is a same reference level. small contribution from the input noise current. 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) Rev. F | Page 15 of 24 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION PROGRESSIVE COMPRESSION SLOPE AND INTERCEPT CALIBRATION OFFSET CONTROL PRODUCT OVERVIEW ENABLE INTERFACE INPUT INTERFACE OFFSET INTERFACE OUTPUT INTERFACE USING THE AD8310 BASIC CONNECTIONS TRANSFER FUNCTION IN TERMS OF SLOPE AND INTERCEPT dBV vs. dBm INPUT MATCHING NARROW-BAND MATCHING GENERAL MATCHING PROCEDURE Step 1: Tune Out CIN Step 2: Calculate CO and LO Step 3: Split CO into Two Parts Step 4: Calculate LM SLOPE AND INTERCEPT ADJUSTMENTS INCREASING THE SLOPE TO A FIXED VALUE OUTPUT FILTERING LOWERING THE HIGH-PASS CORNER FREQUENCY OF THE OFFSET COMPENSATION LOOP APPLICATIONS INFORMATION CABLE-DRIVING DC-COUPLED INPUT EVALUATION BOARD DIE INFORMATION OUTLINE DIMENSIONS ORDERING GUIDE