28 April 2017
AG3/AP3 series: amplified, purely analogue MEMS
pressure sensors with extremely good signal-to-noise
ratio (SNR):
a revival of high precision, purely analogue
technology
The AG3 and AP3 series pressure sensors were originally developed for highly accurate measurement of blood pressure. Now it has
been shown that the sensor with the extremely low-noise analogue output signal also has important advantages for other uses, for
example, medical technology applications such as dialysis or thrombosis treatment.
Standard with limits: digital signal processing
Most comparable products on the market use an ASIC1 from a well-known manufacturer for signal conditioning. In this concept,
the analogue signal is converted into a digital one before it is further processed in a digital signal processor (DSP) – a principle that
has become the quasi-standard among MEMS pressure sensors.
In the analogue version of this concept, the digital signal is then converted back from digital to analogue; in the digital version, it is
emitted directly via an SPI or I2C interface. This concept is completely justified and is the right solution for many applications. Signal
processing in the DSP block of the ASIC opens up powerful possibilities, although these are again defined or limited by the
computing power.
But if the customer's requirements exceed those of these mainstream applications, the concept reaches its limits. The solution
either satisfies only part of the specification or it becomes extremely costly. Fujikura is now able to fill this gap with the AG3/AP3
sensors, which take a completely different approach to their rival products in the sensor manufacturing business (Fig. 1).
Figure 1: Fully analogue concept without digital signal processing in the signal path (AG3/AP3 series)
1
ASIC: application-specific integrated circuit
Back to the roots: purely analogue signal conditioning and curve linearisation
Digitalisation is not always better. Some tried-and-true concepts still have their charm and offer clear advantages. With the
AG3/AP3 series, the key word is ‘fully analogue signal conditioning’: Fujikura’s ASIC does away with the AD/DA conversion
altogether and thus digital signal processing. Not only is the signal path purely analogue, so too are the temperature and curve
linearisation, and an outstanding signal quality is achieved by using the latest analogue technology. In contrast to the digital
processing pressure signal, there are no stages at the output, which means that the full resolution can be further processed.
Freedom and price benefits through external AD conversion
The AG3/AP3 series in combination with an external AD conversion offers clear advantages in many applications. The extremely
low-noise, infinitely variable analogue signal is the basis for a high resolution of the output voltage (up to 16 bits), but it can also be
scanned very quickly (up to 120 µs). The resulting performance also depends, of course, on the AD converter used and thus the
customer’s design. With this configuration – pressure sensor with amplified analogue output and external AD conversion – the
customer plays a key role in determining the costs of the overall solution (quality of the AD conversion) and can dimension and
optimise it according to their requirements.
Some Pewatron customers also require true synchronous scanning of several (up to five) pressure sensors, which is not exactly
feasible with the digital pressure sensors typically found on the market. Although a quasi-synchronous read-out can, in theory, be
carried out over several I2C/SPI buses, it is not exact; true, equidistant and synchronous scanning is not possible. To accomplish
this, the various clocks of the digital pressure sensors or their digital blocks would have to be synchronised with each other, which
is not feasible with the above-mentioned sensors. This is not the case with the fully analogue concept with subsequent AC
conversion, which the customer can define and design according to their requirements.
The market has responded favourably to this principle of the AG3/AP3 series, and Fujikura’s bold actions – bucking the trend and
betting on a fully analogue concept – has already paid off. With the AG3/AP3 sensors, Fujikura is able to set itself apart from other
well-known sensor manufacturers and offer the best signal-to-noise/cost ratio on the pressure sensor market.
Figure 2: AG3 (SMD) and AP3( THT) casing