Datasheet AD8313 (Analog Devices) - 20

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Data Sheet AD8313
In Figure 41, an emitter-follower provides the current gain,
EFFECT OF WAVEFORM TYPE ON INTERCEPT
when a 100 Ω load can readily be driven to full-scale output. Although specified for input levels in dBm (dB relative to While a high ß transistor such as the BC848BLT1 (min ß = 200) 1 mW), the AD8313 responds to voltage and not to power. A is recommended, a 2 kΩ pull-up resistor between VOUT and direct consequence of this characteristic is that input signals of +VS can provide additional base current to the transistor. equal rms power but differing crest factors produce different
+VS
results at the log amp’s output.
R1

MIN = 200 10 +V 1 S VPOS VOUT 8 BC848BLT1
Different signal waveforms vary the effective value of the log
0.1

F AD8313 13k OUTPUT
amp’s intercept upward or downward. Graphically, this looks
2 INHI VSET 7 RL
like a vertical shift in the log amp’s transfer function. The
100 10k 3 INLO COMM 6
device’s logarithmic slope, however, is in principle not affected.
R3 10
For example, if the AD8313 is being fed alternately from a
+VS 4 VPOS PWDN 5 0.1

F
continuous wave and from a single CDMA channel of the same 01085-C-041 rms power, the AD8313 output voltage differs by the equivalent Figure 41. Output Current Drive Boost Connection of 3.55 dB (64 mV) over the complete dynamic range of the In addition to providing current gain, the resistor/potentiometer device (the output for a CDMA input being lower). combination between VSET and the emitter of the transistor Table 7 shows the correction factors that should be applied to increases the log slope to as much as 45 mV/dB, at maximum measure the rms signal strength of a various signal types. A resistance. This gives an output voltage of 4 V for a 0 dBm continuous wave input is used as a reference. To measure the input. If no increase in the log slope is required, VSET can be rms power of a square wave, for example, the mV equivalent of connected directly to the emitter of the transistor. the dB value given in the table (18 mV/dB × 3.01 dB) should be subtracted from the output voltage of the AD8313.
Table 7. Shift in AD8313 Outp ut for Signals with Differing Crest Factors Correction Factor Signal Type (Add to Output Reading)
CW Sine Wave 0 dB Square Wave or DC −3.01 dB Triangular Wave +0.9 dB GSM Channel (All Time Slots On) +0.55 dB CDMA Channel +3.55 dB PDC Channel (All Time Slots On) +0.58 dB Gaussian Noise +2.51 dB Rev. E | Page 19 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 Description Typical Performance Characteristics Circuit Description Interfaces Power-Down Interface, PWDN Signal Inputs, INHI, INLO Logarithmic/Error Output, VOUT Setpoint Interface, VSET Applications Information Basic Connections for Log (RSSI) Mode Operating in Controller Mode Input Coupling Narrow-Band LC Matching Example at 100 MHz Adjusting the Log Slope Increasing Output Current Effect of Waveform Type on Intercept Evaluation Board Schematic and Layout General Operation Using the AD8009 Operational Amplifier Varying the Logarithmic Slope Operating in Controller Mode RF Burst Response Outline Dimensions Ordering Guide