Specifications

Sensors
Pressure sensors and weigh scales (force sensing)
www.digikey.com/maxim-industrial 61
load cell) to produce a differential
output voltage in response to
pressure or weight.
Engineers can design a sensor module
that meets the unique requirements
of diverse force-sensing applications.
A successful design will include the
suitable sensing element for the
physical property and an appropri-
ately designed signal chain.
Complete signal-chain
solutions
The sensor signal chain must
handle extremely small signals in
the presence of noise. Accurately
measuring changes in the output
voltage from a resistive transducer
requires circuitry that provides the
following electrical functions with
precision: excitation, amplification,
filtering, and acquisition. Some
solutions may also require the use of
digital-signal processing (DSP) tech-
niques for signal manipulation,
error compensation, digital gain,
and user programmability.
Excitation
Accurate and stable voltage or current
sources with low-temperature drift
are generally used for sensor excita-
tion. The sensor output is ratiometric
(usually expressed in mV/V) to the
excitation source. Consequently,
the design typically has a common
reference for both the analog-to-
digital converter (ADC) and the
excitation circuitry, or it uses the
excitation voltage as the reference
for the ADC. Additional ADC channels
can be used to measure the excita-
tion voltage accurately.
Transducer/bridge
This part of the signal chain consists
of the strain-gauge transducers
arranged in a load cell (Wheatstone
bridge format), as briefly explained in
the overview section above.
Amplification and level transla-
tion—the analog front-end (AFE)
In some designs the transducer’s
output-voltage range will be very
small, with the required resolution
reaching the nano-volt range. In
such cases, the transducer’s output
signal must be amplified before it
is applied to the ADC’s inputs. To
prevent this amplification step from
introducing errors, low-noise ampli-
fiers (LNAs) with extremely low offset
voltage (V
OS
) and low-temperature
and offset drifts must be selected.
A drawback of Wheatstone bridges
is that the common-mode voltage
is much larger than the signal of
interest. This means that the LNAs
must also have excellent common-
mode rejection ratios (CMRRs),
generally greater than 100dB. When
single-ended ADCs are used, addi-
tional circuitry is required to remove
large common-mode voltages before
acquisition. Additionally, since the
signal bandwidth is low, the 1/f noise
of the amplifiers can introduce errors.
Chopper-stabilized amplifiers are,
therefore, often used. Some of these
stringent amplifier requirements can
be avoided by using a small portion
of the full-scale range of a very-high-
resolution ADC.
Acquisition—the ADC
When choosing the ADC, look
at specifications like noise-free
range or effective resolution
which indicate how well an ADC
can distinguish a fixed input level.
Alternate terms might be noise-
free counts or codes inside the
range. Most high-accuracy ADC
data sheets show these specifica-
tions as a table of peak-to-peak
noise or RMS noise versus speed;
sometimes the specifications
are shown graphically as noise
histogram plots.
Other ADC considerations include
low-offset error, low-temperature
drift, and good linearity. For certain
low-power applications, speed
versus power is another important
criterion.
Filtering
The bandwidth of the transducer
signal is generally small and the
sensitivity to noise is high. It is,
therefore, useful to limit the signal
bandwidth by filtering to reduce
the total noise. Using a sigma-delta
ADC can simplify the noise-filtering
requirement because of the inherent
oversampling in that architecture
Digital Signal Processing (DSP)—
the digital domain
Besides the analog signal processing,
the captured signals are further
processed in the digital domain
for signal extraction and noise
reduction. It is common to find
focused algorithms that cater to
particular applications and their
nuances. There are also generic
techniques, such as offset and gain
correction, linearization, digital
filtering, and temperature- (and
other dependent factors) based
compensation that are usually
applied in the digital domain.
Signal conditioning/
integrated solutions
In some integrated solutions, all
required functional blocks are
integrated into a single IC commonly
called a sensor signal conditioner. A
signal conditioner is an application-
specific IC (ASIC) that performs
compensation, amplification, and
calibration of the input signal,
normally over a range of tempera-
tures. Depending on the sophistica-
tion of the signal conditioner, the
ASIC integrates some or all of the
following blocks: sensor excitation
circuitry, digital-to-analog converter
(DAC), programmable gain amplifier
(PGA), analog-to-digital converter
(ADC), memory, multiplexer (MUX),