Datasheet AD8309 (Analog Devices) - 10

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