AD630
BALANCED DEMODULATOR
AC BRIDGE
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 will also be present which can be removed with a
low-pass filter. Other names for this function are synchronous
demodulation and phase-sensitive detection.
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 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.
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, 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.
Figure 12 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.
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.
This system can easily resolve a 0.5 ppm change in bridge impedance.
Such a change will produce a 3.2 mV change in the low-pass
filtered dc output, well above the RTO drifts and noise.
1kHz
BRIDGE
EXCITATION
AD630
A
؎2 DEMODULATOR
16
LVDT SIGNAL CONDITIONER
AD524
GAIN 1000
15
1k⍀
1k⍀
1k⍀
10k⍀
5k⍀
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
which corresponds to the LVDT core position. The inherent
precision and temperature stability of the AD630 reduce
demodulator drift to a second order effect.
FILTER
5k⍀
A
B
20
2
1k⍀
D
B
2.5
5k⍀ 5k⍀
13
12
k⍀
C
1
2F 2F 2F
1M⍀
17
2.5
k⍀
10k⍀
14
9
PHASE
SHIFTER
10
E1000
AD544
FOLLOWER
SCHAEVITZ
LVDT
AD630
؎2 DEMODULATOR
Figure 12. AC Bridge System
A
5k⍀
B
16
1
15
10k⍀
2.5k⍀
10k⍀
2.5kH
Z
A
B
20
19
C
20V
200s
100k⍀
1F
5V
2V p-p
13
D
14
17
SINUSOIDAL
EXCITATION
BRIDGE EXCITATION
(20V/DIV) (A)
100
90
0V
0V
12
2.5k⍀
AMPLIFIED BRIDGE
OUTPUT (5V/DIV) (B)
9
PHASE
SHIFTER
10
DEMODULATED BRIDGE
OUTPUT (5V/DIV) (C)
10
0V
0V
0%
FILTER OUTPUT (2V/DIV) (D)
Figure 11. LVDT Signal Conditioner
5V
2V
Figure 13. AC Bridge Waveforms
REV. D
–9–
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