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5962-8980701RA Datasheet(PDF) 9 Page - Analog Devices |
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5962-8980701RA Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page ![]() REV. E AD630 –9– 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 circumstances will be the baseband modulation signal. Higher order carrier components that can be removed with a low-pass filter will also be present. Other names for this function are synchro- nous demodulation and phase-sensitive detection. PRECISION PHASE COMPARATOR The balanced modulator topologies of Figures 9a and 9b 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, 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 that the op amp must “leap over” with the commutating amplifier. LVDT SIGNAL CONDITIONER Many transducers function by modulating an ac carrier. A linear variable differential transformer (LVDT) is a transducer of this type. The amplitude of the output signal corresponds to core displacement. Figure 11 shows an accurate synchronous demodulation system which can be used to produce a dc voltage that corresponds to the LVDT core position. The inherent precision and temperature stability of the AD630 reduce demodulator 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 11. LVDT Signal Conditioner AC BRIDGE Bridge circuits that 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 demodulate 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 12 is an example of an ac bridge system with the AD630 used as a synchronous demodulator. The bridge is excited by a 1 V 400 Hz excitation. Trace A in Figure 13 is the amplified bridge signal. Trace B is the output of the synchronous demodu- lator and Trace C is the filtered dc system output. AD8221 REF +IN –IN 49.9 350 350 350 350 CH B– SEL B CH A– SEL A RA RF RINA RINB –VS +15V VOUT +VS COMP AD630AR 12 13 18 RB 14 10 17 16 15 19 20 911 –15V 4.99k 4.99k 4.99k 2 F2 F2 F A BC 1V 400Hz Figure 12. AC Bridge System |
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