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AD8313 датащи(PDF) 11 Page - Analog Devices |
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AD8313 датащи(HTML) 11 Page - Analog Devices |
11 / 24 page AD8313 Rev. D | Page 11 of 24 CIRCUIT DESCRIPTION The AD8313 is an 8-stage logarithmic amplifier, specifically designed for use in RF measurement and power amplifier control applications at frequencies up to 2.5 GHz. A block diagram is shown in Figure 22. For a detailed description of log amp theory and design principles, refer to the AD8307 data sheet. ++ + + + AD8313 VOUT VSET COMM PWDN GAIN BIAS BAND GAP REFERENCE SLOPE CONTROL INTERCEPT CONTROL EIGHT 8dB 3.5GHz AMPLIFIER STAGES 8dB 8dB VPOS INHI INLO VPOS 8dB 8dB NINE DETECTOR CELLS CINT LP I →V V →I 1 8 7 6 5 2 3 4 Figure 22. Block Diagram A fully differential design is used. Inputs INHI and INLO (Pins 2 and 3) are internally biased to approximately 0.75 V below the supply voltage, and present a low frequency impedance of nominally 900 Ω in parallel with 1.1 pF. The noise spectral density referred to the input is 0.6 nV/√Hz, equivalent to a voltage of 35 V rms in a 3.5 GHz bandwidth, or a noise power of −76 dBm re: 50 Ω. This sets the lower limit to the dynamic range; the Applications section shows how to increase the sensitivity by using a matching network or input transformer. However, the low end accuracy of the AD8313 is enhanced by specially shaping the demodulation transfer characteristic to partially compensate for errors due to internal noise. Each of the eight cascaded stages has a nominal voltage gain of 8 dB and a bandwidth of 3.5 GHz. Each stage is supported by precision biasing cells that determine this gain and stabilize it against supply and temperature variations. Since these stages are direct-coupled and the dc gain is high, an offset compensation loop is included. The first four stages and the biasing system are powered from Pin 4, while the later stages and the output inter- faces are powered from Pin 1. The biasing is controlled by a logic interface PWDN (Pin 5); this is grounded for normal operation, but may be taken high (to VS) to disable the chip. The threshold is at VPOS/2 and the biasing functions are enabled and disabled within 1.8 µs. Each amplifier stage has a detector cell associated with its output. These nonlinear cells perform an absolute value (full- wave rectification) function on the differential voltages along this backbone in a transconductance fashion; their outputs are in current-mode form and are thus easily summed. A ninth detector cell is added at the input of the AD8313. Since the midrange response of each of these nine detector stages is separated by 8 dB, the overall dynamic range is about 72 dB (Figure 23). The upper end of this range is determined by the capacity of the first detector cell, and occurs at approximately 0 dBm. The practical dynamic range is over 70 dB to the ±3 dB error points. However, some erosion of this range can occur at temperature and frequency extremes. Useful operation to over 3 GHz is possible, and the AD8313 remains serviceable at 10 MHz, needing only a small amount of additional ripple filtering. INPUT AMPLITUDE (dBm) 2.0 –80 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 –70 –60 –50 –40 –30 –20 –10 0 5 4 3 2 1 0 –1 –2 –3 –4 –5 –90 INTERCEPT = –100dBm SLOPE = 18mV/dB Figure 23. Typical RSSI Response and Error vs. Input Power at 1.9 GHz The fluctuating current output generated by the detector cells, with a fundamental component at twice the signal frequency, is filtered first by a low-pass section inside each cell, and then by the output stage. The output stage converts these currents to a voltage, VOUT, at VOUT (Pin 8), which can swing rail-to-rail. The filter exhibits a 2-pole response with a corner at approximately 12 MHz and full-scale rise time (10% to 90%) of 40 ns. The residual output ripple at an input frequency of 100 MHz has an amplitude of under 1 mV. The output can drive a small resistive load; it can source currents of up to 400 µA, and sink up to 10 mA. The output is stable with any capacitive load, though settling time could be impaired. The low frequency incremental output impedance is approximately 0.2 Ω. In addition to its use as an RF power measurement device (that is, as a logarithmic amplifier), the AD8313 may also be used in controller applications by breaking the feedback path from VOUT to VSET (Pin 7), which determines the slope of the output (nominally 18 mV/dB). This pin becomes the setpoint input in controller modes. In this mode, the voltage VOUT remains close to ground (typically under 50 mV) until the decibel equivalent of the voltage VSET is reached at the input, when VOUT makes a rapid transition to a voltage close to VPOS (see the Operating in Controller Mode section). The logarithmic intercept is nominally positioned at −100 dBm (re: 50 Ω); this is effective in both the log amp mode and the controller mode. |
Аналогичный номер детали - AD8313_04 |
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Аналогичное описание - AD8313_04 |
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