The cost-competitive climate of the consumer,
medical, industrial, and
automotive markets has perpetuated a marked rise in sensor applications, prompting greater use of "deeply embedded"
functions. As a result, the user hardly ever or even never interfaces with
the embedded function, which is very
tightly coupled with the environment
around it.
In many of these applications, high-level code reconfigurability is less important than lower costs. A full-blown microcontroller unit (MCU) sometimes can be considered overkill. This is what
Sensor Platforms had in mind when it
developed its SSP1401 multichannel
sensor interface IC.
The device targets products with deeply embedded functions that require the signal conditioning of different sensor signals. Designers of these products typically don't have the time or budget to develop a sensor-interface product.
"The SSP1401 provides the technologies that both reduce
the cost and time of sensor-application development and
improve sensor performance," says George Hsu, Sensor Platforms' chief technology officer.
READY FOR ANYTHING
The SSP1401's six user-configurable I/O channels can drive (up to 63 mA) and measure most
types of sensor elements, including resistive, capacitive, inductive, piezoresistive, and pulse-count devices (). Falling
under that umbrella are resistive, capacitive, and inductive
accelerometers; capacitive pressure sensors; resistive and
inductive magnetometers; and inductive linear variable differential transformer (LVDT) and velocity sensors.
In addition, the IC handles resistive temperature sensors,
capacitive viscometer sensors, capacitive and resistive liquid-level sensors, capacitive dielectric-measurement sensors,
capacitive touch sensors, and resistive conductivity measurement sensors. Sensors oscillating from 1 Hz to 1 MHz are compatible with the SSP1401's input.
This kind of sensor "fusion" can substantially simplify the sensor-interface
circuitry. For example, a single SSP1401 can interface with two
capacitive pressure sensors, two
inductive magnetic-resonance
sensors, and two resistance-temperature detectors (RTDs).
Many other combinations are
possible.
Fully integrated, the
SSP1401 only requires
three discrete components besides the sensor element. It operates
from a low-voltage (1.6 to 3.6 V) source and dissipates low operating current (less than 0.5 mA,
depending on the sensor
load and the sampling
rate). In sleep mode, it dissipates less than 200 nA. The
chip's dynamic range is 24 bits, and its effective number of
bits rating is 13.
Furthermore, the SSP1401 operates from –40°C to 125°C.
Its signal-to-noise ratio is rated at more than 13 bits. A 20MHz internal clock allows for precise smaller measurements,
and there's support for a 15- to 25-MHz external clock. An
optional count-up and subtract-down capability supports
back-to-front sensor measurements.
A CLEVER INPUT DESIGN
The SSP1401 truly enables
fusion sensing applications. "Given the SSP1401's uniquely flexible frequency-mode interface technology, it is a natural fit for
multi-sensor applications," says Kevin A. Shaw, director of business development at Sensor Platforms.
"For example, think of it driving a pair of magneto-resistive
(MR) magnetic sensors for implementing an electronic compass function on a wristwatch while at the same time being
able to drive a capacitive pressure sensor to provide altitude
information," he continues. "This is a particularly interesting
example since watch-based applications also benefit tremendously from the SSP1401's low operating voltage and power
performance as well."
The interface IC's frequency-based
analog front end is combined with an
efficient, flexible, and deeply embedded
state machine that brings high levels of
control and operating configurability.
Also, it doesn't require costly general-purpose MCUs and their attendant system overheads.
RELAXATION OSCILLATOR
The
input circuit uses a relaxation oscillator
incorporating LR or RC networks. It
directly converts sensor signal values
into digital form without the usual need
for an analog-to-digital converter (ADC),
cost-effectively delivering greater flexibility and performance (). A six-input multiplexer circuit feeds sensor
signals to a sensor comparator, which
in turn feeds the signals to a 16-bit
period counter.
Two adjustable input digital-to-analog
converters (DACs)—a high DAC and a low
DAC—allow for independent adjustment
of high- and low-voltage swings. As a
result, bias adjustments can be made on
each sensor and the setting of a trigger
point. Adjustable measurement times
can be short, depending on the power
budget. Short measurements mean less
power consumption, while longer measurements create better resolution.
All communications between the
sensor and the SSP1401 are performed via a digital serial peripheral
interface (SPI) bus. To measure a given sensor, a request is sent via the
SPI bus, and the interface chip's state
machine and analog block are turned
on. The sensor signal is measured, the
results are stored in a register for later
retrieval, and all non-SPI functions are
returned to the sleep mode. Measurement completion is indicated by both a station register and a data ready
(DRDY) pin, which can function as an
interrupt trigger.
A HARDWARE DEV KIT
The
SSP1401's development kit features a
simple touchscreen demo, which shows
the chip's capabilities and simplifies
hardware development ().
The kit also contains a USB interface
communications board, replete with an
ARM7 processor housing 256 kbytes of
memory code space, as well as a data-acquisition board with breakout ports. Its
sensor board boasts six touchscreen
capacitive buttons and an Eclipse-based
integrated development environment
(IDE) with data-acquisition and control
system analysis tools.
The SSP1401 comes in bare die
form or in a 5- by 5-mm, 28-lead plastic
microlead-frame/quad flat no-lead
(MLF/QFN) package that's compatible
with the European Union's Restrictions
on Hazardous Substances (). It
costs $2.24 each in 1000-unit lots
(packaged part). Sample quantities will
be available in the third quarter. Production quantities are scheduled for
the fourth quarter.
Sensor Platforms
www.sensorplatforms.com