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AD600 Datasheet(PDF) 14 Page - Analog Devices |
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AD600 Datasheet(HTML) 14 Page - Analog Devices |
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14 / 20 page  AD600/AD602 REV. A –14– 100 dB to 120 dB RMS Responding Constant Bandwidth AGC Systems with High Accuracy dB Outputs The next two applications double as both AGC amplifiers and measurement systems. In both, precise gain offsets are used to achieve either (1) a very high gain linearity of ±0.1 dB over the full 100 dB range, or (2) the optimal signal-to-noise ratio at any gain. A 100 dB RMS/AGC System with Minimal Gain Error (Parallel Gain with Offset) Figure 25 shows an rms-responding AGC circuit, which can equally well be used as an accurate measurement system. It accepts inputs of 10 µV to 1 V rms (–100 dBV to 0 dBV) with generous overrange. Figure 26 shows the logarithmic output, VLOG, which is accurately scaled 1 V per decade, that is, 50 mV/dB, with an intercept (VLOG = 0) at 3.16 mV rms (–50 dBV). Gain offsets of ±2 dB have been introduced between the amplifiers, provided by the ±62.5 mV introduced by R6–R9. These offsets cancel a small gain ripple which arises in the X-AMP from its finite interpolation error, which has a period of 18 dB in the individual VCA sections. The gain ripple of all three amplifier sections without this offset (in which case the gain errors simply add) is shown in Figure 27; it is still a remarkably low ±0.25 dB over the 108 dB range from 6 µV to 1.5 V rms. However, with the gain offsets connected, the gain linearity remains under ±0.1 dB over the specified 100 dB range (Figure 28). 5 3 –5 1 µV10µV 10V 1V 100mV 10mV 1mV 100 µV 4 2 0 1 –1 –3 –2 –4 INPUT SIGNAL – V RMS Figure 26. VLOG Plotted vs. VIN for Figure 25‘s Circuit Showing 120 dB AGC Range INPUT 1V RMS MAX (SINE WAVE) 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 REF A1 A2 C1HI A1CM A1OP VPOS VNEG A2OP A2CM C2HI C1LO A1HI A1LO GAT1 GAT2 A2LO A2HI C2LO U1 AD600 +5V DEC –5V DEC U3A 1/4 AD713 R2 487 Ω VOUT C5 22 µF 1 2 3 4 5 6 7 14 13 12 11 10 9 8 NC NC NC –5V DEC U3C R11 46.4k Ω +5V DEC NC NC NC 1/4 AD713 +316.2mV VINP VNEG CAVG VLOG BFOP BFIN VPOS COMM LDLO V U4 AD636 NC = NO CONNECT C1 0.1 µF R3 200 Ω U3B 1/4 AD713 R1 133k Ω C2 0.1 µF R5 1.58k Ω R4 133k Ω C3 220pF 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 REF A1 A2 C1HI A1CM A1OP VPOS VNEG A2OP A2CM C2HI C1LO A1HI A1LO GAT1 GAT2 A2LO A2HI C2LO U2 AD600 +5V DEC –5V DEC C4 2 µF R6 10k Ω +5V –5V R7 127 Ω R8 127 Ω R9 10k Ω –2dB –62.5mV 0dB +2dB +62.5mV 0.1 µF 0.1 µF FB –5V +5V POWER SUPPLY DECOUPLING NETWORK +5V DEC –5V DEC FB R10 3.16k Ω C6 4.7 µF VLOG Q1 2N3906 R14 301k Ω R12 11.3k Ω R13 3.01k Ω R15 19.6k Ω R16 6.65k Ω +5V DEC RMS Figure 25. RMS Responding AGC Circuit with 100 dB Dynamic Range |
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