Analog Dialogue 49

Low-Power Synchronous Demodulator
Design Considerations
By Brian Harrington
Introduction
“Synchronous Detectors Facilitate Precision, Low-Level
Measurements,” published in the November 2014 issue of
Analog Dialogue, discusses the benefits of using synchronous
demodulation to measure low-level signals in the presence
of relatively high noise levels. This article extends the discussion by looking at some design considerations for sensor
signal conditioning using synchronous demodulation in systems with strict power and cost constraints. When carefully
designed, analog systems are hard to beat for simplicity, low
cost, and low power consumption. This architecture performs
most of the signal processing in the analog domain.
Sensor Excitation
Sensors, which are ubiquitous, are used to measure temperature, light, sound, and a variety of other environmental
parameters. Some sensors act as parameter-dependent voltage
or current sources. Thermocouples, for example, generate
a voltage that is proportional to the temperature difference
between a reference junction and the measurement point.
Most sensors have transfer functions that follow a known relationship with respect to the physical parameter. The transfer
function is often an impedance, where current is the sensor
input and the voltage across the sensor indicates the parameter of interest. Resistive sensors such as load cells, RTDs, and
potentiometers are used to measure strain, temperature, and
angle, respectively. To first order, resistive sensors are frequency independent and have no phase response.
Many sensors require ac excitation signals as their transfer
functions change in frequency and phase. Examples include
inductive proximity sensors and capacitive humidity sensors.
Biometric impedance measurements can yield information
about respiratory rate, pulse rate, hydration, and many other
physiological parameters. In these cases, the magnitude,
phase, or both may be used to determine the value of the
sensed parameter.
In some applications, transducers turn a sample under test
into a sensor. Colorimeters, for example, use an LED to shine
light through a liquid sample under test. The light absorption
of the sample modulates the amount of light detected by a
photodiode to reveal characteristics about the liquid under
test. Blood-oxygen levels can be determined by measuring
the difference in light absorption of red and infrared light in
vascular tissue. Ultrasonic transducers measure gas flow rates
based on Doppler frequency shifts of the ultrasound as it travels through the gas. All of these systems can be implemented
using synchronous demodulation.
Analog Dialogue 49-05, May 2015
Figure 1 shows a synchronous demodulation system for
measuring a sensor’s output signal. An excitation signal,
fx, acts as a carrier that the sensor modulates in amplitude,
phase, or both as a function of the parameter being measured.
The signal may be amplified and filtered before being modulated back down to dc by the phase-sensitive detector (PSD).
An output filter (OF) limits the bandwidth of the signal to the
frequency range of the parameter being measured.
fD
fX
PHYSICAL
PARAMETER
AAF
SENSOR
G
fREF
+1
–1
OF
NOISE
Figure 1. Synchronous demodulation system.
Noise at the sensor output may be due to internal sources
or external coupling. Low-frequency (1/f) noise often limits
performance of the sensor or measurement electronics. Many
sensors are also susceptible to interference from low-frequency
environmental noise. Optical measurements are susceptible to
background light and electromagnetic sensors can be susceptible to radiation from the power supply. Freedom to choose the
excitation frequency to avoid noise sources is a key benefit of
synchronous demodulation.
Choosing an excitation frequency that reduces the effect of
these noise sources is an important way to optimize system
performance. The selected excitation frequency should have a
low noise floor and be far enough away from noise sources so
that modest output filtering can reduce the noise to acceptable
levels. Sensor excitation is often the largest item in the power
budget. If the sensor’s sensitivity vs. frequency is known,
power consumption can be reduced by exciting the sensor
at a frequency where its sensitivity is high.
The Phase-Sensitive Detector
Understanding the requirements of the antialiasing filter
(AAF) and OF requires an understanding of the PSD. Consider a PSD that uses the excitation signal to synchronously
multiply the input signal by +1 and –1. This is equivalent to
multiplying the input signal with a square wave of the same
frequency. Figure 2a shows the time domain waveforms for
the input signal, reference, and PSD output for the case where
the input signal is a square wave with arbitrary phase relative
to the reference.
analog.com/analogdialogue
1
the band-pass filters is determined by the bandwidth of the
low-pass output filter. The PSD output response is the sum of
these band-pass filters, as shown in Figure 3. The part of the
response that appears at dc falls in the pass band of the output
filter. The part of the response that appears at even harmonics
of the reference frequency will be rejected by the output filter.
1.0
SIGNAL
NOISE WINDOWS
When the input and reference are perfectly in phase, the
relative phase is 0°, the switch output is dc, and the PSD
output voltage is +1. As the relative phase increases, the
switch output becomes a square wave at twice the reference
frequency, and the duty cycle and average value diminish
linearly. At a relative phase of 90°, the duty cycle is 50% and
the average value is 0. At a relative phase of 180°, the PSD
output voltage is –1. Figure 2b shows the average output
value of the PSD as the relative phase is swept from 0° to 360°
for square-wave and sine-wave input signals.
2B
REFERENCE
1/3
1/5
0
3fR
PSD OUTPUT
5fR
7fR
FREQUENCY
Figure 3. Signal input spectrum that contributes to PSD output.
At first glance, an infinite sum of harmonics aliasing into the
output filter pass band seems to doom this approach. However,
the impact of the noise aliasing is mitigated because each of the
harmonic terms is scaled by a decreasing factor, and the noise
at the various harmonics adds as the root sum of squares. We
can calculate the noise impact of the harmonic aliasing assuming the noise spectral density of the input signal is constant.
RELATIVE PHASE (REL)
1.0
AVERAGE VALUE OF PSD
fR
AVERAGE VALUE
1/7
0.5
0
Let Vn be the integrated noise in the transmission window
centered at the fundamental frequency. The total rms noise,
VT is
SQUAREWAVE
SINEWAVE
–0.5
VT =
–1.0
0
90
180
Vn2 +
270
VT =
Vn 2 Vn 2
+
+ … = Vn
3
5
360
RELATIVE PHASE (REL)
Figure 2. (a) Time domain waveforms of PSD. (b) Average value
of PSD output as a function of relative phase.
The sine-wave case is less intuitive than the square-wave case,
but it can be calculated by multiplying term by term and separating into sum and difference components as follows:
Vn2 +
Vn 2 Vn 2
+
+ … = Vn
3
5
2
12 + 1 + 1
3
5
2
2
12 + 1 + 1 + …
3
5
Using a handy formula for the sum of a geometric series
that states
n
∑
1
π2
=
= 1.23
(2n + 1)2 8
k=0
the increase in the rms noise due to the harmonic windows is
2
2
2√2VS
1
1
2 ± f1 ±θ )1– …
cos (fR ± fS ± θS) – cos (3fR ± fS ±θS)VT+/ Vn =
cos (5f
1
+
+
+
… = √1.23 = 1.11
R
S
π
3S
5
3
5
1
1
cos (3fR ± fS ±θS) + cos (5fR ± fS ±θS) – …
Thus, the rms noise due to all of the harmonic windows
S ± θS) –
3
5
increases the total noise by only 11% or 1 dB. The output is
VPSD (t) =
As desired, the PSD produces a response proportional to the
cosine of the relative phase of an input signal at the fundamental frequency, but it also produces a response to every
odd harmonic of the signal. With the output filter considered
part of the phase-sensitive detector, the signal transmission
path looks like a series of band-pass filters centered at the
odd harmonics of the reference signal. The bandwidth of
2
still susceptible to disturbers in the pass band of the bandpass filters, and harmonic distortion from either the sensor
or electronics before the PSD will cause errors in the output
signal. If these harmonic distortion terms are unacceptably
large, they can be reduced with an antialiasing filter. The
requirements for the antialiasing and output filters will be
considered in the following design example.
Analog Dialogue 49-05, May 2015
3.3V
E-100 SERIES LVDT
3.3V
R34
ADG794
C24
R33
C4
3.3V
VDD
INP
ADA2200
VOCM
SDA
OUTP
R4
R35
AD7192
INN
OUTN
C25
SCL
ALERT
CLKOUT
RCLK
GND
CLKIN
Figure 4. Simplified LVDT position-sensing circuit.
LVDT Design Example
Figure 4 shows a synchronous demodulation circuit that
extracts position information from a linear variable displacement transformer (LVDT), which is a specially wound
transformer that has a moveable core that is affixed to the
position to be measured. An excitation signal is applied to
the primary. The voltage on the secondary changes in proportion to the position of the core.
There are many types of LVDTs and ways of extracting position from them. This circuit uses the LVDT in 4-wire mode.
The two LVDT secondary outputs are connected such that
the voltages oppose each other, performing a subtraction.
When the LVDT core is in its null position, the voltages on the
secondaries are equal, and the difference voltage across the
windings is zero. As the core moves from the null position,
the difference voltage across the secondary windings increases.
The sign of the LVDT output voltage changes based on direction. The LVDT chosen for this example measures a ±2.5-mm,
full-scale core displacement. The voltage transfer function is
0.25, which means that the differential output will be 250 mV
per volt applied to the primary when the core is displaced
2.5 mm from the center.
Integrated Synchronous Demodulator
The ADA2200 integrated synchronous demodulator uses
a unique charge-sharing technology to perform discrete
time-signal processing in the analog domain. Its signal path
consists of an input buffer, an FIR decimation filter that
performs antialiasing filtering, a programmable IIR filter, a
phase-sensitive detector, and a differential output buffer. Its
clock-generation functions synchronize the excitation signal
to a system clock. Programmable features are configured
through an SPI-compatible interface.
VDD
ADA2200
INP
8
INN
LPF
fMOD
CLKIN
XOUT
÷2m
fSI
CLOCK
GEN
SYNCO
÷8
fSO
GND
90°
SPI
BOOT FROM
EEPROM (I2C)
RST
BOOT
Figure 5. ADA2200 synchronous demodulator.
Analog Dialogue 49-05, May 2015
The RC network between the LVDT output and the ADA2200
input provides low-pass filtering of the LVDT output signal
and also produces the relative phase shift required to maximize the demodulator’s output signal. Recall from Figure 2b
how the maximum PSD output occurs at relative phase shifts
of 0° or 180°. The ADA2200 has a 90° phase control that
enables ±90° relative phase offsets to be used as well.
Signal energy present at odd multiples of the demodulation frequency will appear in the pass band of the output filter. The FIR
decimation filter implements antialiasing filtering, providing a
minimum of 50 dB attenuation at these frequencies.
The IIR filter can provide additional filtering or gain if
required. Since the IIR filter precedes the phase-sensitive
detector, its phase response will affect the PSD signal output
bandwidth. This must be taken into account in the design of
the filter response.
OUTN
Output Filter
VOCM
The pass band of the output filter should be selected to match
the bandwidth of the parameter being measured, but limit
the broadband noise of the system. The output low-pass filter
must also reject output spurs that are created at the even multiples of the PSD.
VCM
RCLK/SDO
CONTROL
REGISTERS
Antialias Filtering
OUTP
PROGRAM
FILTER
÷2n+1
The 4.92-MHz clock generated by the AD7192 24-bit Σ-Δ ADC
is used as the master clock. The ADA2200 generates all of the
internal signals it needs for clocking the filters and PSD, as
well as generating the excitation signal on the RCLK pin. It
divides the master clock by 1024 to generate a 4.8-kHz signal
to control the CMOS switch. The CMOS switch converts the
low-noise 3.3-V source into a square-wave excitation signal
to the LVDT. The 3.3-V supply used for the excitation source
is also used as the ADC reference, so any drift in the voltage
source will not degrade the measurement accuracy. At fullscale displacement, the LVDT will output a 1.6-V peak-to-peak
output voltage.
SCLK/SCL
SDIO/SDA
CS/A0
This circuit uses the LPF inherent in the AD7192 Σ-Δ ADC. It
can be programmed to have a sinc3 or sinc4 response, with
transfer function zeroes at multiples of the output data rate.
3
Figure 6 shows the sinc3 transfer function normalized to the
ADC output data rate.
Setting the output data rate of the ADC equal to the demodulation frequency rejects the spurs at the PSD output. The
ADC’s programmable output data rate acts as a selectable
bandwidth output filter. The available output data rates (fDATA)
are 4.8 kHz/n, where 1 ≤ n ≤ 1023. Thus, the ADC averages
the demodulator output over n demodulation clock periods
for each output data value. Because the master clock and the
ADC clock are synchronous, the transfer function zeroes of
the ADC’s output filter will fall directly on every harmonic of
the modulation frequency and all of the output spurs will be
rejected for any value of n.
0
If the output noise from the ADA2200 were frequency independent, the expected effective number of bits would increase
by one bit for every 4× decrease in the output data rate. The
ENOB doesn’t increase as much at the lower output data rates
due to the ADA2200 output driver’s 1/f noise, which begins to
dominate the noise floor at the lower output data rates.
Linearity
The linearity was measured by first performing a two-point
calibration at core displacements of ±2.0 mm. From these
measurements, the slope and offset were determined to establish a best straight line fit. Next, measurements were taken at
core displacements across the ±2.5-mm full-scale range. The
measured data was subtracted from the straight line data to
determine the linearity error.
–10
0.10
–20
ERROR %
0.05
–40
0
–50
–60
(%)
FILTER GAIN (dB)
–30
–70
–80
–0.05
–0.10
–90
–100
–0.15
0
fDATA
2fDATA
3fDATA
FREQUENCY (Hz)
–0.20
–0.20
Figure 6. AD7192 sinc3 filter transfer function.
–0.10
–0.05
0
0.05
0.10
0.15
0.20
DISPLACEMENT (mm)
The programmable output data rate enables a straightforward
trade-off between noise, and bandwidth/settling time. The
output filter has noise bandwidth of 0.3 × fDATA; 3-dB frequency of 0.272 × fDATA; and settling time of 3/fDATA.
The ADC digital filter has a 3-dB bandwidth of about 1.3 kHz
at the highest output data rate of 4.8 kHz. The RC filter
between the demodulator and the ADC is relatively flat up
to that frequency to minimize the bandwidth required by
the ADC. In systems with lower maximum data rates, the
RC filter corner frequency can be reduced proportionally.
Noise Performance
The output noise of the circuit is a function of the ADC output
data rate. Table 1 shows the effective number of bits in the digitized data vs. the ADC sample rate assuming a 2.5-V full-scale
output voltage. The noise performance is independent of the
LVDT core position.
Table 1. Noise Performance vs. Bandwidth
ADC Data
Rate (SPS)
4800
Output Bandwidth
(Hz)
1300
ENOB
(rms)
13.8
ENOB
(p-p)
11.3
1200
300
75
325
80
20
14.9
15.8
16.2
12.3
13.2
13.5
4
–0.15
Figure 7. Position linearity error vs. LVDT core displacement.
The E Series LVDT used for the circuit evaluation specifies
±0.5% linearity over the ±2.5-mm displacement range. The
circuit performance exceeds the LVDT specifications.
Power Consumption
The circuit consumes a total of 10.2 mW, including 6.6 mW for
driving the LVDT and 3.6 mW for the remainder of the circuit.
The circuit SNR could be improved by increasing the LVDT
excitation signal at the cost of increased power consumption.
Alternatively, the power consumption can be reduced by lowering the LVDT excitation signal and using a low-power, dual
op amp to amplify the LVDT output signal to preserve the
circuit’s SNR.
Conclusion
Many sensor signal conditioning challenges share characteristics that can be addressed with synchronous demodulation.
Systems with excitation frequencies below 1 MHz and
dynamic range requirements in the 80-dB-to-100-dB range
can be addressed with low-cost, low-power analog circuits
requiring minimal digital post processing. Understanding
the operation of the phase-sensitive detector and the likely
noise characteristics at the output of the sensor are key to
determining the system filter requirements.
Analog Dialogue 49-05, May 2015
References
Meade, M.L. Lock-In Amplifiers: Principles and Applications.
Peter Peregrinus Ltd., 1983.
UG-787: Software-Programmable Evaluation Board for the
ADA2200 Synchronous Demodulator.
Precision Modulators/Demodulators.
Sensor Signal Conditioning with Synchronous Demodulation.
UG-702: Evaluation Board for the ADA2200 Synchronous
Demodulator.
Brian Harrington
Brian Harrington [[email protected]] is an applications
engineer with ADI’s Analog Garage division in Cambridge, MA. Brian joined
ADI in 1994 and has worked in a variety of applications roles where he has
supported customers designing in ADSL modem chipsets, high-speed DACS,
MxFEs, and other ICs.
Analog Dialogue 49-05, May 2015
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