Datasheet AD641 (Analog Devices) - 10

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AD641 SIGNAL MAGNITUDE
often referred to as the logarithmic offset. For dc or square wave The AD641 is a calibrated device. It is, therefore, important to inputs, VX is 1 mV so the numerical value of XdBV is –60, and be clear in specifying the signal magnitude under all waveform Equation (4) becomes conditions. For dc or square wave inputs there is, of course, no I ambiguity. Bounded periodic signals, such as sinusoids and OUT = 50 µA (InputdBV + 60) Equation (5) triwaves, can be specified in terms of their simple amplitude Alternatively, for a sinusoidal input measured in dBm (power in (peak value) or alternatively by their rms value (which is a mea- dB above 1 mW in a 50 Ω system) the output can be written sure of power when the impedance is specified). It is generally bet- ter to define this type of signal in terms of its amplitude because IOUT = 50 µA (InputdBm + 44) Equation (6) the AD641 response is a consequence of the input voltage, not because the intercept for a sine wave expressed in volts rms is at power. However, provided that the appropriate value of inter- 1.414 mV (from Table I) or –44 dBm. cept for a specific waveform is observed, rms measures may be used. Random waveforms can only be specified in terms of rms
OPERATION OF A SINGLE AD641
value because their peak value may be unbounded, as is the case Figure 24 shows the basic connections for a single device, using for Gaussian noise. These must be treated on a case-by-case 100 Ω load resistors. Output A is a negative going voltage with a basis. The effective intercept given in Table I should be used for slope of –100 mV per decade; output B is positive going with a Gaussian noise inputs. slope of +100 mV per decade. For applications where absolute On the other hand, for bounded signals the amplitude can be calibration of the intercept is essential, the main output (from expressed either in volts or dBV (decibels relative to 1 V). For LOG OUT, Pin 14) should be used; the LOG COM output can example, a sine wave or triwave of 1 mV amplitude can also be then be grounded. To evaluate the demodulation response, a defined as an input of –60 dBV, one of 100 mV amplitude as simple low pass output filter having a time constant of roughly –20 dBV, and so on. RMS value is usually expressed in dBm 500 µs (3 dB corner of 320 Hz) is provided by a 4.7 µF (–20% (decibels above 1 mW) for a specified impedance level. Through- +80%) ceramic capacitor (Erie type RPE117-Z5U-475-K50V) out this data sheet we assume a 50 Ω environment, the customary placed across the load. A DVM may be used to measure the impedance level for high speed systems, when referring to signal pow- averaged output in verification tests. The voltage compliance at ers in dBm. Bearing in mind the above discussion of the effect of Pins 13 and 14 extends from 0.3 V below ground up to 1 V waveform on the intercept calibration of the AD641, it will be below +VS. Since the current into Pin 14 is from –0.2 mA at apparent that a sine wave at a power of, say, –10 dBm will not zero signal to +2.3 mA when fully limited (dc input of >300 mV) produce the same output as a triwave or square wave of the the output never drops below –230 mV. On the other hand, the same power. Thus, a sine wave at a power level of –10 dBm has current out of Pin 13 ranges from –0.2 mA to +2.3 mA, and if an rms value of 70.7 mV or an amplitude of 100 mV (that is, √2 desired, a load resistor of up to 2 kΩ can be used on this output; times as large, the ratio of amplitude to rms value for a sine the slope would then be 2 V per decade. Use of the LOG COM wave), while a triwave of the same power has an amplitude output in this way provides a numerically correct decibel read- which is √3 or 1.73 times its rms value, or 122.5 mV. ing on a DVM (+100 mV = +1.00 dB). Board layout is very important. The AD641 has both high gain
“Intercept” and “Logarithmic Offset”
If the signals are expressed in dBV, we can write the output and wide bandwidth; therefore every signal path must be very current in a simpler form, as: carefully considered. A high quality ground plane is essential, but it should not be assumed that it behaves as an equipotential IOUT = 50 µA (InputdBV – XdBV) Equation (4) plane. Even though the application may only call for modest bandwidth, each of the three differential signal interface pairs where InputdBV is the input voltage amplitude (not rms) in dBV (SIG IN, Pins l and 20, SIG OUT, Pins 10 and 11, and LOG, and XdBV is the appropriate value of the intercept (for a given wave- Pins 13 and 14) must have their own “starred” ground points to form) in dBV. This form shows more clearly why the intercept is avoid oscillation at low signal levels (where the gain is highest).
DENOTES A SHORT, DIRECT CONNECTION 10 TO THE GROUND PLANE.
V
+5V ALL UNMARKED CAPACITORS ARE 0.1 OUTPUT A
m
F CERAMIC (SEE TEXT). OUTPUT B NC 20 19 18 17 16 15 14 13 12 11 SIG ATN CKT RG1 RG0 RG2 LOG LOG +V SIG S RLA R +IN OUT COM LB OUT COM +OUT 1k
V
1k
V
4.7
m
F 100
V
4.7
m
F 100
V
0.1% SIGNAL 0.1% AD641 INPUT SIG ATN ATN ATN ATN SIG –IN LO COM COM IN OPTIONAL BL1 –V BL2 –OUT S ITC TERMINATION 1 2 3 4 5 6 7 8 9 10 RESISTOR NC NC 4.7
V
OPTIONAL –5V OFFSET BALANCE RESISTOR
Figure 24. Connections for a Single AD641 to Verify Basic Performance –10– REV. D Document Outline FEATURES PRODUCT DESCRIPTION PIN CONFIGURATIONS AD641--SPECIFICATIONS ELECTRICAL CHARACTERISTICS THERMAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS ESD CAUTION REVISION HISTORY AD641--TYPICAL DC PERFORMANCE CHARACTERISTICS TYPICAL AC PERFORMANCE CHARACTERISTICS CIRCUIT DESCRIPTION CIRCUIT OPERATION FUNDAMENTALS OF LOGARITHMIC CONVERSION INTERCEPT STABILIZATION CONVERSION RANGE EFFECT OF WAVEFORM ON INTERCEPT LOGARITHMIC CONFORMANCE AND WAVEFORM SIGNAL MAGNITUDE INTERCEPT AND LOGARITHMIC OFFSET OPERATION OF A SINGLE AD641 ACTIVE CURENT-TO-VOLTAGE CONVERSION EFFECT OF FREQUENCY ON CALIBRATION SOURCE RESISTANCE AND INPUT OFFSET USING HIGHER SUPPLY VOLTAGES USING THE ATTENUATOR OPERATION OF CASCADED AD641s ELIMINATING THE EFFECT OF FIRST STAGE OFFSET PRACTICAL APPLICATIONS RSSI APPLICATIONS 250 MHz RSSI CONVERTER WITH 58 dB DYNAMIC RANGE OUTLINE DIMENSIONS ORDERING GUIDE