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AD8314ARM-REEL Datasheet(PDF) 10 Page - Analog Devices |
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AD8314ARM-REEL Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page AD8314 –10– REV. 0 INPUT AMPLITUDE – dBV 1.2 0 –70 0 –60 (–47dBm) –50 –40 –30 –20 –10 (+3dBm) 1.0 0.8 0.6 0.4 0.2 VS = 3V RT = 52.3 3 –3 2 1 0 –1 –2 1dB DYNAMIC RANGE 3dB DYNAMIC RANGE INTERCEPT Figure 29. VUP and Log Conformance Error vs. Input Level vs. Input Level at 900 MHz Transfer Function in Terms of Slope and Intercept The transfer function of the AD8314 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 AD8314, slope is nominally 21.5 mV/dB. So a 10 dB change at the input results in a change at the output of approxi- mately 215 mV. The plot of Log-Conformance (Figure 29) shows the range over which the device maintains its constant slope. The dynamic range can be defined as the range over which the error remains within a certain band, usually ±1 dB or ±3 dB. In Figure 29, for example, the ±1 dB dynamic range is approxi- mately 50 dB (from –13 dBV to –63 dBV). The intercept is the point at which the extrapolated linear response would intersect the horizontal axis (Figure 29). Using the slope and intercept, the output voltage can be calculated for any input level within the specified input range using the equation: VUP = VSLOPE × (PIN – PO) where VUP is the demodulated and filtered RSSI output, VSLOPE is the logarithmic slope, expressed in V/dB, PIN is the input sig- nal, expressed in decibels relative to some reference level (either dBm or dBV in this case) and PO is the logarithmic intercept, expressed in decibels relative to the same reference level. For example, at an input level of –40 dBV (–27 dBm), the output voltage will be VOUT = 0.020 V/dB × (–40 dBV – (–63 dBV )) = 0.46 V dBV vs. dBm The most widely used convention in RF systems is to specify power in dBm, that is, decibels above 1 mW in 50 Ω. Specification of log amp input levels in terms of power is strictly a concession to popular convention; they do not respond to power (tacitly “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 pre- cise, although this is still not unambiguous because waveform is also involved in the response of a log amp, which, for a complex input (such as a CDMA signal), will not follow the rms value exactly. Since most users specify RF signals in terms of power— more specifically, in dBm/50 Ω—we use both dBV and dBm in specifying the performance of the AD8314, 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. Filter Capacitor The video bandwidth of both V_UP and V_DN is approximately 3.5 MHz. In CW applications where the input frequency is much higher than this, no further filtering of the demodulated signal will be required. Where there is a low-frequency modulation of the carrier amplitude, however, the low-pass corner must be reduced by the addition of an external filter capacitor, CF (see Figure 28). The video bandwidth is related to CF by the equation Video Bandwidth kpF C F = ×× + 1 24 4 10 π .( ) Ω Operating in Controller Mode Figure 30 shows the basic connections for operation in the con- troller mode and Figure 31 shows a block diagram of a typical controller mode application. The feedback from V_UP to VSET is broken and the desired setpoint voltage is applied to VSET from the controlling source (often this will be a DAC). VDN will rail high (2.2 V on a 3.3 V supply, 1.9 V on a 2.7 V supply) when the applied power is less than the value corresponding to the set- point voltage. When the input power slightly exceeds this value, VDN would, in the absence of the loop via the power amplifier gain pin, decrease rapidly toward ground. In the closed loop, however, the reduction in VDN causes the power amplifier to re- duce its output. This restores a balance between the actual power level sensed at the input of the AD8314 and the demanded value determined by the setpoint. This assumes that the gain control sense of the variable gain element is positive, that is, an increas- ing voltage from V_DN will tend to increase gain. The output swing and current sourcing capability of V_DN are shown in Figures 19 and 20. 1 2 3 4 ENBL RFIN AD8314 8 7 6 5 VSET FLTR V DN VPOS COMM V UP VS VDN VS INPUT VSET CF 0.1 F 52.3 Figure 30. Basic Connections for Operation in Controller Mode DAC FLTR V UP VSET AD8314 DIRECTIONAL COUPLER POWER AMPLIFIER RF INPUT GAIN CONTROL VOLTAGE RFIN V DN CF 52.3 Figure 31. Typical Controller Mode Application |
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