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AD630S Datasheet(PDF) 7 Page - Analog Devices |
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AD630S Datasheet(HTML) 7 Page - Analog Devices |
7 / 8 page AD630 REV. C –7– MODULATED OUTPUT SIGNAL CARRIER INPUT CM ADJ DIFF ADJ 2.5k AMP A AMP B –V 10k 10k 5k 9 10 COMP 1 15 7 16 14 13 12 2 20 +VS –VS AD630 A B 2.5k 19 18 17 11 8 6 5 10k 4 3 10k MODULATION INPUT Figure 18b. AD630 Configured as a Gain-of-Two Balanced Modulator 10V 5V 5V 20 s MODULATION INPUT CARRIER INPUT OUTPUT SIGNAL Figure 19. Gain-of-Two Balanced Modulator Sample Waveforms BALANCED DEMODULATOR The balanced modulator topology described above will also act as a balanced demodulator if a double sideband suppressed carrier waveform is applied to the signal input and the carrier signal is applied to the reference input. The output under these circum- stances will be the baseband modulation signal. Higher order carrier components will also be present which can be removed with a low-pass filter. Other names for this function are synchro- nous demodulation and phase-sensitive detection. PRECISION PHASE COMPARATOR The balanced modulator topologies of Figures 18a and 18b can also be used as precision phase comparators. In this case, an ac waveform of a particular frequency is applied to the signal input and a waveform of the same frequency is applied to the refer- ence input. The dc level of the output (obtained by low-pass filtering) will be proportional to the signal amplitude and phase difference between the input signals. If the signal amplitude is held constant, then the output can be used as a direct indication of the phase. When these input signals are 90 ° out of phase, they are said to be in quadrature and the AD630 dc output will be zero. PRECISION RECTIFIER-ABSOLUTE VALUE If the input signal is used as its own reference in the balanced modulator topologies, the AD630 will act as a precision recti- fier. The high frequency performance will be superior to that which can be achieved with diode feedback and op amps. There are no diode drops which the op amp must “leap over” with the commutating amplifier. LVDT SIGNAL CONDITIONER Many transducers function by modulating an ac carrier. A Lin- ear Variable Differential Transformer (LVDT) is a transducer of this type. The amplitude of the output signal corresponds to core displacement. Figure 20 shows an accurate synchronous demodulation system which can be used to produce a dc voltage which corresponds to the LVDT core position. The inherent precision and temperature stability of the AD630 reduce de- modulator drift to a second order effect. A B 10k 10k 5k 2.5k 2.5k C 100k D 1 F AD630 2 DEMODULATOR AD544 FOLLOWER B PHASE SHIFTER A E1000 SCHAEVITZ LVDT 2.5kHZ 2V p-p SINUSOIDAL EXCITATION 16 1 14 17 9 10 20 19 12 13 15 Figure 20. LVDT Signal Conditioner AC BRIDGE Bridge circuits which use dc excitation are often plagued by errors caused by thermocouple effects, 1/f noise, dc drifts in the electronics, and line noise pick-up. One way to get around these problems is to excite the bridge with an ac waveform, amplify the bridge output with an ac amplifier, and synchronously de- modulate the resulting signal. The ac phase and amplitude information from the bridge is recovered as a dc signal at the output of the synchronous demodulator. The low frequency system noise, dc drifts, and demodulator noise all get mixed to the carrier frequency and can be removed by means of a low-pass filter. Dynamic response of the bridge must be traded off against the amount of attenuation required to adequately suppress these residual carrier components in the selection of the filter. Figure 21 is an example of an ac bridge system with the AD630 used as a synchronous demodulator. The oscilloscope photo- graph shows the results of a 0.05% bridge imbalance caused by the 1 Meg resistor in parallel with one leg of the bridge. The top trace represents the bridge excitation, the upper-middle trace is the amplified bridge output, the lower-middle trace is the out- put of the synchronous demodulator and the bottom trace is the filtered dc system output. This system can easily resolve a 0.5 ppm change in bridge im- pedance. Such a change will produce a 3.2 mV change in the low-pass filtered dc output, well above the RTO drifts and noise. 1M 1k 1k 1k 1k B A B A 5k 10k 10k 2.5 k 2.5 k C D 5k 5k 5k FILTER 2 F 20 2 13 12 AD524 GAIN 1000 1 AD630 2 DEMODULATOR 1kHz BRIDGE EXCITATION PHASE SHIFTER 2 F 2 F 16 15 17 14 9 10 Figure 21. AC Bridge System |
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