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AD8309ARU-REEL7 Datasheet(PDF) 10 Page - Analog Devices |
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AD8309ARU-REEL7 Datasheet(HTML) 10 Page - Analog Devices |
10 / 20 page REV. B AD8309 –10– sensitivity to disturbances on the supply lines. With careful design, the sensitivities to many other parametric variations, and the effects of temperature and supply voltage, can be reduced to negligible proportions. A/0 A/0 A/0 gm gm gm gm STAGE 1 STAGE 2 STAGE N – RSLOPE VLOG VLIM VIN +TOP-END DETECTORS CURRENT-SUMMING LINE Figure 24. Basic Log Amp Structure Using A/0 Stages and Transconductance (gm) Cells for Summing The output of each gain cell has an associated transconductance (gm) cell, which converts the differential output voltage of the cell to a pair of differential currents; these are summed by sim- ply connecting the outputs of all the gm (detector) stages in parallel. The total current is then converted back to a voltage by a transresistance stage, which determines the slope of the loga- rithmic output. This general scheme is depicted, in a simplified single-sided form, in Figure 24. Additional detectors, driven by a passive attenuator, may be added to extend the top end of the dynamic range. The slope voltage may now be decoupled from the knee-voltage EK = 2kT/q, which is inherently PTAT. The detector stages are biased with currents (not shown in the Figure) which can be derived from a band-gap reference and thus be stable with tem- perature. This is the architecture used in the AD8309. It affords complete control over the magnitude and temperature behavior of the logarithmic slope. A further step is yet needed to achieve the demodulation response, required in a log-limiter amp is to convert an alternating input into a quasi- dc baseband output. This is achieved by modifying the gm cells used for summation purposes to implement the rectification function. Early log amps based on the progressive compression technique used half-wave rectifiers, which made post-detection filtering difficult. The AD640 was the first com- mercial monolithic log amp to use a full-wave rectifier; this proprietary practice has been used in all subsequent Analog Devices types. We can model these detectors as being essentially linear gm cells, but producing an output current that is independent of the sign of the voltage applied to the input. That is, they implement the absolute-value function. Since the output from the later A/0 stages closely approximates an amplitude symmetric square wave for even moderate input levels, the current output from each detec- tor is almost constant over each period of the input. Somewhat earlier detectors stages in the chain produce a waveform having only very brief “dropouts” at twice the input frequency. Only those detectors nearest the log amp’s input produce a low level waveform that is approximately sinusoidal. When all these (cur- rent mode) outputs are summed, the resulting signal has a wave- form which is readily filtered, to provide a low residual ripple on the output. Intercept Calibration Monolithic log amps from Analog Devices incorporate accurate means to position the intercept voltage VX (or equivalent sine- wave power for a demodulating log amp, when driven at a spe- cific impedance level). Using the scheme shown in Figure 24, the value of the intercept level departs considerably from that predicted by the simple theory. Nevertheless, the intrinsic inter- cept voltage is still proportional to EK, which is PTAT (propor- tional to absolute temperature). Recalling that the addition of an offset to the output produces an effect which is indistinguishable from a change in the posi- tion of the intercept, it will be apparent that we can cancel the “left-right” motion of VX resulting from the temperature varia- tion of EK by simply adding an offset at its demodulated output having the required temperature behavior. The precise temperature-shaping of the intercept-positioning offset can result in a log amp having stable scaling parameters, making it a true measurement device, for example, as a calibrated Received Signal Strength Indicator (RSSI). In this application, one is more interested in the value of the output for an input waveform which is often sinusoidal (CW). The input level be stated as an equivalent power, in dBm, but it is essential to know the impedance level at which this “power” is presumed to be measured. In an impedance of 50 Ω, 0 dBm (1 mW) corre- sponds to a sinusoidal amplitude of 316.2 mV (223.6 mV rms). For the AD8309, the intercept may be specified in dBm when the input impedance is lowered to 50 Ω, by the addition of a shunt resistor of 52.3 Ω, in which case it occurs at –95 dBm. However, the response is actually to the voltage at the input, not the power in the termination resistor, and should be specified in dBV. A –95 dBm sine input across a 50 Ω resistance corre- sponds to an amplitude of 5.6 µV, or –108 dBV, where 0 dBV is specified as a sine waveform of 1 V rms, that is, 2.8 V p-p. Note that a log amp’s intercept is a function of waveform. For example, a square-wave input will read 6 dB higher than a sine- wave of the same amplitude, and a Gaussian noise input 0.5 dB higher than a sine wave of the same rms value. Further, a log amp driven by the sum of two sinusoidal voltages of equal am- plitude will show an output that is only 2.1 dB higher than the response for a single sine wave drive, rather than the 3 dB that might be expected if the device truly responded to input power. These are characteristics exhibited by all demodulating log amps. Dynamic Range The lower end of the dynamic range is determined largely by the thermal noise floor, measured at the input of the amplifier chain. For the AD8309, the short-circuit input-referred noise-spectral density is 1.1 nV/ √Hz, and 1.275 nV/√Hz when driven from a net source impedance of 25 Ω (a terminated 50 Ω). This corre- sponds to a noise power of –78 dBm in a 500 MHz bandwidth. The upper end of the dynamic range is extended upward by the addition of top-end detectors driven by a tapped attenuator. These smaller signals are applied to additional full-wave detectors whose outputs are summed with those of the main detectors. With care in design, this extension in the dynamic range can be ‘seamless’ over the full frequency range. For the AD8309 it amounts to a further 48 dB. When using a supply of 4.5 V or greater, an input amplitude of 4 V can be accommodated, corre- sponding to a power level of +22 dBm in 50 Ω. (A larger input voltage may cause damage.) |
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