Modern sensors detect a multitude of
real-world analog attributes—temperature,
force, pressure, humidity,
flow, and power, just for starters. In
turn, they typically output some level
of voltage, current, charge, or resistive
analog signal, or a purely digital signal, in proportion
to their respective environmental stimuli. Some sensors
operate autonomously; others need power supplied,
typically in the form of a voltage or current source.
Many times, unique signal conditioning is needed or
incorporated to provide a useful electrical output. Here,
we look at some state-of-the-art techniques for sensor
signal conditioning used in modern analog electronics.
As the need for highly precise operational
amplifiers continues to grow, the
that continuously correct for offset
error—have become increasingly popular.
Many leading amplifier manufacturers
use “zero drift” to refer to any continuously
whether it is an auto-zero or a chopper-stabilized topology, observes Kevin
Tretter, principal product marketing
engineer at Microchip Technology Inc.
Typically, chopper amplifiers are better
suited for dc or low-frequency applications,
whereas auto-zero amplifiers are
suitable for wider-band applications.
Tretter notes that auto-zero architectures
used for zero-drift signal conditioning
contain a main amplifier, which is
always connected to the input, and secondary
amps that continuously correct
their own offset and apply the offset correction
to the main amplifier. Microchip
Technology has implemented this type
of architecture on the MCP6V01, in
which the offset error of the main amplifier
is corrected 10,000 times/sec, resulting
in what Microchip says are extremely
low offset and offset drift.
A chopper-stabilized architecture
also uses a high-bandwidth main amplifier
that is always connected to the
input, as well as an “auxiliary” amplifier
that uses switches to chop the input
signal and provide offset correction
to the main amplifier. In Microchip’s
MCP6V11 low-power amplifier, for
example, chopping action minimizes
offset and offset-related errors.
Although their internal operation
differs, auto-zero and chopper-stabilized
amplifiers share the same goal: to minimize
offset and offset-related errors. This
results in not only low initial offset but
also low offset drift over time and temperature,
superior common-mode and
power-supply rejection, and elimination
of 1/f (frequency-dependent) noise.
Reza Moghimi, an applications engineering
manager with Analog Devices
Inc, notes that many real-world sensors
produce low output voltages at
low frequencies that require a signal-conditioning
circuit with high gain and
accurate—close to dc—performance.
Applications for such sensors include
precision electronic scales, load-cell
and bridge transducers, interfaces for thermocouple/thermopile sensors, and
precision medical instrumentation.
The offset voltage, offset-voltage
drift, and 1/f noise of nonprecision
amplifiers used for signal conditioning
of these sensors cause errors that
require hardware or software calibration.
Moghimi offers examples of
high-precision signal conditioning in
which zero-drift amplifiers—designed to
achieve ultralow offset voltage and drift,
high open-loop gain, high power-supply
rejection, high common-mode rejection,
and no 1/f noise—benefit designers
by eliminating the need for calibration.
The circuit in Figure 1 uses the
AD7791, a low-power buffered 24-bit
sigma-delta ADC, along with external
ADA4528-x zero-drift amplifiers, in
a single-supply precision weigh-scale
application. The circuit, built and tested
by ADI and described in Reference 1,
yields 15.3-bit noise-free code resolution
for a load cell with a full-scale output of
10 mV and maintains good performance
over the full output data range, from 9.5
Hz to 120 Hz.
Figure 1 This low-power, zero-drift sensor-amplifier-converter circuit uses the AD7791 24-bit sigma-delta ADC and external ADA4528-x zero-drift amplifiers (courtesy Analog Devices).
The differential amplifier in the circuit
comprises two low-noise, zero-drift
ADA4528 amplifiers with 5.6 nV/
of voltage noise density at 1 kHz, 0.3-μV offset voltage; 0.002 μV/°C offset-voltage
drift; and 158 dB and 150 dB
of common-mode and power-supply
rejection, respectively. Circuit gain is
equal to 1+2R1/RG, and the lowpass filters
implemented by placing capacitors
C1 and C2 in parallel with resistors R1
and R2 limit the noise bandwidth to
4.3 Hz, restricting the amount of noise
entering the sigma-delta ADC. C5, R3,
and R4 form a differential filter with a
cutoff frequency of 8 Hz to limit noise
further. C3 and C4 in conjunction with
R3 and R4 form common-mode filters
with a cutoff frequency of 159 Hz.
Another example of high-precision,
low-power signal conditioning is the
electrocardiogram circuit shown in
Figure 2 and described in Reference 2.
The ECG circuit must operate with a
differential dc offset because of the half-cell
potential of the electrodes. The tolerance
for this overvoltage is typically
±300 mV, but in some situations it can
be 1V or more. The downward trend of
supply voltages in ECG circuits and the
presence of this larger half-cell potential
limit the gain that can be applied in the
first stage of signal conditioning.
Figure 2 The AD8607 dual micropower instrumentation amplifier is used for integration, buffering,
and level shifting in this zero-drift signal-conditioning circuit for
an ECG application (courtesy Analog Devices).
The AD8237 architecture solves this
problem by connecting a low-frequency
inverting integrator from the output to
the REF pin that only has to swing as
far as the dc offset, instead of the dc offset
multiplied by the gain. Because the
amplifier applies gain to the integrator
output, large gains can be applied at the
amplification stage, and the precision
requirements of the rest of the system
can be reduced. Noise and offset error
from devices after this amplification in
the signal path contribute less to the
overall accuracy. The AD8607 dual
micropower instrumentation amp, with
115 μA of supply current, is used for
integration, buffering, and level shifting.
Proper decoupling is not shown.
The zero-drift, rail-to-rail input and
output instrumentation amplifier can
operate with a minimum supply voltage
of 1.8V, gain drift of 0.5 ppm/°C, and
offset drift of 0.2 μV/°C. Two external
resistors set gain range from 1 to 1000.
The AD8607 can fully amplify signals
with common-mode voltage at or up to
300 mV beyond its supplies.