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AD8367-EVAL 데이터시트(PDF) 10 Page - Analog Devices |
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AD8367-EVAL 데이터시트(HTML) 10 Page - Analog Devices |
10 / 16 page REV. 0 AD8367 –10– Power and Voltage Metrics Although power is the traditional metric used in the analysis of cascaded systems, most active circuit blocks fundamentally respond to voltage. The relationship between power and voltage is defined by the impedance level. When input and output imped- ance levels are the same, power gain and voltage gain are identical. However, when impedance levels change between input and output, they differ. Thus, one must be very careful to use the appropriate gain for system chain analyses. Quantities such as OIP3 are quoted in dBV rms as well as dBm referenced to 200 Ω. The dBV rms unit is defined as decibels relative to 1 V rms. In a 200 Ω environment, the conversion from dBV rms to dBm requires the addition of 7 dB to the dBV rms value. For example, a +2 dBV rms level corresponds to +9 dBm. Noise and Distortion Since the AD8367 consists of a passive variable attenuator followed by a fixed gain amplifier, the noise and distortion characteristics as a function of the gain voltage are easily pre- dicted. The input-referred noise increases in proportion to the attenuation level. Figure 4 shows noise figure, NF, as a func- tion of VGAIN for the MODE pin pulled high. The minimum NF of 7.5 dB occurs at maximum gain and increases 1 dB for every 1 dB reduction in gain. In receiver applications, the minimum NF should occur at the maximum gain where the received signal presumably is weak. At higher levels, a lower gain is needed, and the increased NF becomes less important. VGAIN – V 0 1.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 60 50 –30 10 0 –10 –20 30 20 40 60 50 –30 10 0 –10 –20 30 20 40 NF IIP3 Figure 4. Noise Figure and Input Third Order Inter- cept vs. Gain (RSOURCE 200 Ω) The input-referred distortion varies in a similar manner to the noise. Figure 4 illustrates how the third-order intercept point at the input, IIP3, behaves as a function of VGAIN. The highest IIP3 of 20 dBV rms (27 dBm re 200 Ω) occurs at minimum gain. The IIP3 then decreases 1 dB for every 1 dB increase in gain. At lower levels, a degraded IIP3 is acceptable. Overall, the dynamic range, represented by the difference between IIP3 and NF, remains reasonably constant as a function of gain. The output distortion and compression are essentially independent of the gain. At low gains, when the input level is high, input overload may occur, causing premature distortion. Output Centering The output level is centered midway between ground and the supply if the DECL pin is left floating. Alternatively, the out- put level may be set by driving the DECL pin with the desired reference level. As shown in Figure 5, the loop acts to suppress deviations from the reference at outputs below its corner frequency while not affecting signals above it. The maximum corner frequency with no external capacitor is 500 kHz. The corner frequency can be lowered arbitrarily by adding an exter- nal capacitor, CHP: f C HP HP (kHz) (nF)+ . = 10 002 (3) A capacitor at pin DECL is recommended to decouple the reference level to which the output is centered. gm HPFL DECL AV 1 VMID VOUT FROM INPUT MAIN AMPLIFIER CHP Figure 5. The dc output level is centered to mid supply by a control loop whose corner frequency is determined by CHP. RMS Detection The AD8367 contains a square-law detector that senses the output signal and compares it to a calibrated set-point of 354 mV rms which corresponds to a 1 V p-p sine wave. Any difference between the output and set-point generates a current which is integrated by an external capacitor, CAGC, connected from the DETO pin to ground, to provide an AGC control voltage. There is also an internal 5 pF capacitor on the DETO pin. The resulting voltage is used as an AGC bias. For this appli- cation, the MODE pin is pulled low and the DETO pin is tied to the GAIN pin. The output signal level is then regu- lated to 354 mV rms. The AGC bias represents a calibrated rms measure of the received signal strength (RSSI). Since in the AGC mode the output signal is forced to the 354 mV rms set-point (–9.02 dBV rms), Equation 2 can be recast to express the strength of the received signal, VIN-RMS, in terms of the AGC bias VDETO, VV IN RMS DETO − =− + × (dBV rms) 54 02 50 . (4) where –54.02 dBV rms –45 dB 9.02 dBV rms. For small changes in input signal level, VDETO responds with a characteristic single-pole time constant, τAGC, which is propor- tional to CAGC, τµ AGC AGC C ( s) (nF) =× 10 (5) where the internal 5 pF capacitor has been lumped with the external capacitor to give CAGC. |
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