Datasheet AD8310 (Analog Devices) - 10

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AD8310 SLOPE AND INTERCEPT CALIBRATION OFFSET CONTROL
All monolithic log amps from Analog Devices use precision In a monolithic log amp, direct coupling is used between the design techniques to control the logarithmic slope and stages for several reasons. First, it avoids the need for coupling intercept. The primary source of this calibration is a pair of capacitors, which typically have a chip area at least as large as accurate voltage references that provide supply- and that of a basic gain cell, considerably increasing die size. Second, temperature-independent scaling. The slope is set to 24 mV/dB the capacitor values predetermine the lowest frequency at which by the bias chosen for the detector cells and the subsequent gain the log amp can operate. For moderate values, this can be as of the postdetector output interface. With this slope, the full high as 30 MHz, limiting the application range. Third, the 95 dB dynamic range can be easily accommodated within the parasitic back-plate capacitance lowers the bandwidth of the output swing capacity, when operating from a 2.7 V supply. cell, further limiting the scope of applications. Intercept positioning at −108 dBV (−95 dBm re 50 Ω) has likewise been chosen to provide an output centered in the However, the very high dc gain of a direct-coupled amplifier available voltage range. raises a practical issue. An offset voltage in the early stages of the chain is indistinguishable from a real signal. If it were as Precise control of the slope and intercept results in a log amp high as 400 μV, it would be 18 dB larger than the smallest ac with stable scaling parameters, making it a true measurement signal (50 μV), potentially reducing the dynamic range by this device as, for example, a calibrated received signal strength amount. This problem can be averted by using a global feedback indicator (RSSI). In this application, the input waveform is path from the last stage to the first, which corrects this offset in invariably sinusoidal. The input level is correctly specified in a similar fashion to the dc negative feedback applied around an dBV. It can alternatively be stated as an equivalent power, in op amp. The high frequency components of the feedback signal dBm, but in this case, it is necessary to specify the impedance in must, of course, be removed to prevent a reduction of the HF which this power is presumed to be measured. In RF practice, it gain in the forward path. is common to assume a reference impedance of 50 Ω, in which 0 dBm (1 mW) corresponds to a sinusoidal amplitude of An on-chip filter capacitor of 33 pF provides sufficient suppres- 316.2 mV (223.6 mV rms). However, the power metric is sion of HF feedback to allow operation above 1 MHz. The −3 dB correct only when the input impedance is lowered to 50 Ω, point in the high-pass response is at 2 MHz, but the usable range either by a termination resistor added across INHI and INLO, extends well below this frequency. To further lower the frequency or by the use of a narrow-band matching network. range, an external capacitor can be added at OFLT (Pin 3). For example, 300 pF lowers it by a factor of 10. Note that log amps do not inherently respond to power, but to the voltage applied to their input. The AD8310 presents a Operation at low audio frequencies requires a capacitor of about nominal input impedance much higher than 50 Ω (typically 1 μF. Note that this filter has no effect for input levels well above 1 kΩ at low frequencies). A simple input matching network the offset voltage, where the frequency range would extend can considerably improve the power sensitivity of this type of down to dc (for a signal applied directly to the input pins). The log amp. This increases the voltage applied to the input and, dc offset can optionally be nulled by adjusting the voltage on therefore, alters the intercept. For a 50 Ω reactive match, the the OFLT pin (see the Applications Information section). voltage gain is about 4.8, and the whole dynamic range moves down by 13.6 dB. The effective intercept is a function of wave- form. For example, a square-wave input reads 6 dB higher than a sine wave of the same amplitude, and a Gaussian noise input reads 0.5 dB higher than a sine wave of the same rms value. Rev. F | Page 10 of 24 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION PROGRESSIVE COMPRESSION SLOPE AND INTERCEPT CALIBRATION OFFSET CONTROL PRODUCT OVERVIEW ENABLE INTERFACE INPUT INTERFACE OFFSET INTERFACE OUTPUT INTERFACE USING THE AD8310 BASIC CONNECTIONS TRANSFER FUNCTION IN TERMS OF SLOPE AND INTERCEPT dBV vs. dBm INPUT MATCHING NARROW-BAND MATCHING GENERAL MATCHING PROCEDURE Step 1: Tune Out CIN Step 2: Calculate CO and LO Step 3: Split CO into Two Parts Step 4: Calculate LM SLOPE AND INTERCEPT ADJUSTMENTS INCREASING THE SLOPE TO A FIXED VALUE OUTPUT FILTERING LOWERING THE HIGH-PASS CORNER FREQUENCY OF THE OFFSET COMPENSATION LOOP APPLICATIONS INFORMATION CABLE-DRIVING DC-COUPLED INPUT EVALUATION BOARD DIE INFORMATION OUTLINE DIMENSIONS ORDERING GUIDE