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AD641AN датащи(PDF) 10 Page - Analog Devices |
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AD641AN датащи(HTML) 10 Page - Analog Devices |
10 / 16 page REV. C AD641 –10– SIGNAL MAGNITUDE The AD641 is a calibrated device. It is, therefore, important to be clear in specifying the signal magnitude under all waveform conditions. For dc or square wave inputs there is, of course, no ambiguity. Bounded periodic signals, such as sinusoids and triwaves, can be specified in terms of their simple amplitude (peak value) or alternatively by their rms value (which is a mea- 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 the AD641 response is a consequence of the input voltage, not power. However, provided that the appropriate value of inter- cept for a specific waveform is observed, rms measures may be used. Random waveforms can only be specified in terms of rms value because their peak value may be unbounded, as is the case for Gaussian noise. These must be treated on a case-by-case basis. The effective intercept given in Table I should be used for Gaussian noise inputs. On the other hand, for bounded signals the amplitude can be expressed either in volts or dBV (decibels relative to 1 V). For example, a sine wave or triwave of 1 mV amplitude can also be defined as an input of –60 dBV, one of 100 mV amplitude as –20 dBV, and so on. RMS value is usually expressed in dBm (decibels above 1 mW) for a specified impedance level. Through- out this data sheet we assume a 50 Ω environment, the customary impedance level for high speed systems, when referring to signal pow- ers in dBm. Bearing in mind the above discussion of the effect of waveform on the intercept calibration of the AD641, it will be apparent that a sine wave at a power of, say, –10 dBm will not produce the same output as a triwave or square wave of the same power. Thus, a sine wave at a power level of –10 dBm has an rms value of 70.7 mV or an amplitude of 100 mV (that is, √2 times as large, the ratio of amplitude to rms value for a sine wave), while a triwave of the same power has an amplitude which is √3 or 1.73 times its rms value, or 122.5 mV. “Intercept” and “Logarithmic Offset” If the signals are expressed in dBV, we can write the output current in a simpler form, as: IOUT = 50 µA (Input dBV – XdBV) Equation (4) where InputdBV is the input voltage amplitude (not rms) in dBV and XdBV is the appropriate value of the intercept (for a given wave- form) in dBV. This form shows more clearly why the intercept is often referred to as the logarithmic offset. For dc or square wave inputs, VX is 1 mV so the numerical value of XdBV is –60, and Equation (4) becomes IOUT = 50 µA (InputdBV + 60) Equation (5) Alternatively, for a sinusoidal input measured in dBm (power in dB above 1 mW in a 50 Ω system) the output can be written IOUT = 50 µA (InputdBm + 44) Equation (6) because the intercept for a sine wave expressed in volts rms is at 1.414 mV (from Table I) or –44 dBm. OPERATION OF A SINGLE AD641 Figure 24 shows the basic connections for a single device, using 100 Ω load resistors. Output A is a negative going voltage with a slope of –100 mV per decade; output B is positive going with a slope of +100 mV per decade. For applications where absolute calibration of the intercept is essential, the main output (from LOG OUT, Pin 14) should be used; the LOG COM output can then be grounded. To evaluate the demodulation response, a simple low pass output filter having a time constant of roughly 500 µs (3 dB corner of 320 Hz) is provided by a 4.7 µF (–20% +80%) ceramic capacitor (Erie type RPE117-Z5U-475-K50V) placed across the load. A DVM may be used to measure the averaged output in verification tests. The voltage compliance at Pins 13 and 14 extends from 0.3 V below ground up to 1 V below +VS. Since the current into Pin 14 is from –0.2 mA at zero signal to +2.3 mA when fully limited (dc input of >300 mV) the output never drops below –230 mV. On the other hand, the current out of Pin 13 ranges from –0.2 mA to +2.3 mA, and if desired, a load resistor of up to 2 k Ω can be used on this output; the slope would then be 2 V per decade. Use of the LOG COM output in this way provides a numerically correct decibel read- ing on a DVM (+100 mV = +1.00 dB). Board layout is very important. The AD641 has both high gain and wide bandwidth; therefore every signal path must be very carefully considered. A high quality ground plane is essential, but it should not be assumed that it behaves as an equipotential plane. Even though the application may only call for modest bandwidth, each of the three differential signal interface pairs (SIG IN, Pins l and 20, SIG OUT, Pins 10 and 11, and LOG, Pins 13 and 14) must have their own “starred” ground points to avoid oscillation at low signal levels (where the gain is highest). OUTPUT A 10 11 DENOTES A SHORT, DIRECT CONNECTION TO THE GROUND PLANE. 16 18 19 20 17 9 8 7 6 10 5 3 2 14 LOG OUT LOG COM SIG +OUT RG2 –VS SIG –OUT AD641 RG0 RG1 CKT COM ATN OUT SIG +IN +VS ITC BL1 ATN IN ATN COM ATN COM ATN LO SIG –IN BL2 1k 1k NC NC 4.7 –5V NC ALL UNMARKED CAPACITORS ARE 0.1 F CERAMIC (SEE TEXT). OUTPUT B 4.7 F RLA 100 0.1% RLB +5V OPTIONAL OFFSET BALANCE RESISTOR OPTIONAL TERMINATION RESISTOR SIGNAL INPUT 12 13 14 15 4.7 F 100 0.1% Figure 24. Connections for a Single AD641 to Verify Basic Performance |
Аналогичный номер детали - AD641AN |
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Аналогичное описание - AD641AN |
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