The Importance of Interfacing By Thomas Froehlich, Principal Design Engineer with ams.

For any OEM design team which uses, or plans to use, a MEMS device in an end product, this article shows how features of the interface between the MEMS device and a processor or microcontroller affect the accuracy, precision and stability of the sensor output.



The signals generated by a MEMS sensor need to be routed to a processor or microcontroller before they become useful to the host system. But MEMS devices are produced in a fundamentally different technology from that of solid-state electronic integrated circuits such as microcontrollers or applications processors, and there is a wide mismatch between the characteristics of each.



This means there is a requirement for an interface which can bridge the two worlds, an interface which is optimised for the high bias-voltage, small-signal world of MEMS, but which can also supply a clean, stable output which is usable in the world of digital electronics.



Just how different a MEMS device is from a digital electronic device is clear from a description of the main electrical characteristics of each (see Figure 1). A MEMS sensor typically requires a high bias voltage (5-100V), and produces signal voltages of just a few µV. By contrast, a microcontroller operates on typical digital logic voltages of 0-3.3V, and, with an onboard ADC, can process analogue inputs in the same voltage range.



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Figure 1: Interface IC bridges between the µV MEMS device and the digital logic levels of a microcontroller or processor



Again, MEMS devices produce capacitance changes of tiny magnitudes: changes of less than a few attofarads (1 attoF = 10-18F) need to be detected. The accuracy and stability of the device’s outputs are also highly sensitive to variations in the supply voltage and temperature, and sensitivity shows marked variance from sample to sample due to fluctuations in the MEMS manufacturing process.



In a microcontroller or processor, on the other hand, no direct measurement of capacitance is normally possible and complex calibration to offset for manufacturing variances imposes an undesirable overhead on the processor.



Clearly, then, there is a functional requirement to bridge between the MEMS device and a controller or processor. This, however, is a challenging design to implement. The interface must provide: low-noise amplification of very small signal inputs, alongside the ability to generate clean, stable high-voltage control signals; capacitive sensing front end with sensitivity at the level of attoFarads; very high input impedance and very low input capacitance; sensor calibration; a robust output signal which is immune to the noise and interference coupling typically found in highly integrated devices such as mobile phones; and tolerance for a wide supply voltage range in a power supply which is itself subject to noise and interference.



The last two features are desirable because they make the system designer’s job easier and quicker. They eliminate the need for a dedicated power supply for the MEMS device and save the repeated debugging cycles commonly required in sensor systems that are subject to noise and interference.



The challenge in creating such an interface comes, in part, from the requirement for very low noise levels on the input side, to protect the integrity of the MEMS device’s very small signals, together with high voltages on the output side. And because MEMS devices are typically used in applications which are space-constrained, the designer ideally wants all interface functions bundled into a single chip, with no supporting components required (this also eases the PCB layout task).



And of course, all of this must come at the lowest possible cost, which means implementing the device in CMOS, rather than a more expensive technology that could more readily be tuned to provide high analogue circuit performance.



The penalty for compromising



Because it is so difficult to combine all of the required characteristics in a single IC, it is tempting to compromise on one or more of the specifications of the interface device. But in every case, this will either harm the performance of the MEMS sensor or degrade the output. For instance, the optimal supply voltage to the MEMS device, up to 100V, will generate the largest possible signal output without damaging the device’s internal structures. An interface which fails to supply a high voltage will reduce the magnitude of the signal output, magnifying the effect of noise in the amplification stage.



Not only must the power supply provide a high voltage, it must carry very low levels of noise. For instance, power supplies in mobile phones are exposed to multiple sources of noise which can easily be coupled to the MEMS device’s supply, and even small amounts of noise can badly degrade output quality when the output is at the level of just a few µV. The effect of noise is just as dramatic in the signal amplification stage for the same reason.



Finally, any compromise in the specifications for input impedance and input capacitance at the interface to the MEMS sensor’s output will cause excessive loading on the sensor, reducing the magnitude of the output which, as above, magnifies the effect of noise in the amplification stage.



It is clear, then, that the effect of compromising the specifications of the interface is to impair the quality (accuracy and resolution) of the output. Looking at the MEMS sensor+interface IC package as a ‘black box’, it would be easy to assume that differences between one MEMS-based module and another are derived from differences in the quality and performance of the MEMS sensor itself. In fact, two modules containing exactly the same MEMS sensor could provide dramatically different outputs if they were paired with two different interface ICs. It follows, then, that OEM design engineers should carefully evaluate potential MEMS sensor modules, and not assume that the quality of the MEMS sensor is the sole determinant of the module’s performance.



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Figure 2: A typical MEMS microphone module. The MEMS sensor is in the square package to the left, and the interface IC is shown connected to it by bond wires.



An optimised interface



It is, then, a challenging exercise in analogue electronics design and manufacturing to produce a MEMS interface IC which optimises the performance of the sensor to which it is paired. The scale of the challenge is best illustrated by reference to a real-world example of a MEMS sensor: MEMS microphones are today widely used in mobile phones and other end-products, because they offer performance, stability and reliability advantages over traditional electret condenser microphones (see Figure 2).



A MEMS microphone is a typical instance of a MEMS sensor in its electrical characteristics. A single-chip interface IC to a MEMS microphone must implement the functions shown in Figure 3. In the tight physical confines of a mobile phone, it is essential that the interface integrates all the required functions, with no external components used.



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Figure 3: A block diagram of an interface IC between a MEMS microphone and the handset’s baseband processor



Successful implementation of such a device requires a combination of advanced analogue circuit design techniques and a fabrication process optimised for low noise and high voltage. In an interface IC designed and manufactured by austriamicrosystems AG to partner a MEMS microphone transducer, innovative designs have achieved extremely low levels of noise in the power supply and the signal amplifier.



For instance, voltage regulation is provided with a Low Drop-Out regulator (LDO), a device type which provides good decoupling of supply and digital noise from the small-signal analogue circuit and which produces a precise input voltage for the charge pump. But the power efficiency of conventional LDO designs is poor, an external capacitor on each output is typically required for good stability, and charge pump ripple introduces excessive noise into the sensor supply.



austriamicrosystems’ IC uses a Flipped Voltage Follower technique, however, to implement a capacitor-less, multiple-output LDO which offers low power consumption, provides a precisely regulated charge pump supply and occupies a small area on the die.



Innovation is also apparent in a patented low-noise amplifier design, which again requires no external capacitor, occupies a small die area and includes low- and high-frequency noise filters.



Alongside the circuit design, austriamicrosystems’ HV (High Voltage) CMOS fabrication process also supports low noise and up to 120V analogue circuit operation. CMOS is normally used for manufacturing digital circuits, but at austriamicrosystems’ fab in Graz, Austria the proprietary 0.35µm HV CMOS process has been optimised for high-performance analogue, while preserving the low-cost advantages that CMOS confers.



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Figure 4: An interface IC in a MEMS microphone module (functional characteristics)



Together, the combination of circuit design and manufacturing process produce an IC that interfaces directly to both the MEMS microphone and a baseband processor, with no external components required, and is capable of providing:



-<5µV noise in the analogue circuit

-a low 130µA current draw

-<100fF input capacitance

-~10TΩ input impedance

-A clean, stable 5-20V low-noise bias voltage independent of the external supply to the IC



The performance of the microphone module is strongly affected by the supply and amplification characteristics of the austriamicrosystems interface IC. The module offers a very low figure for total harmonic distortion (below 0.5% at 100dBSPL (Sound Pressure Level) and below 5% at 115dBSPL) and a signal-to-noise ratio of 62dB(A) at 94dBSPL.



This specific example of the MEMS microphone module is representative of the situation facing all MEMS sensor specifiers: a combination of low noise, high voltage, high impedance and low capacitance are required to optimise the performance of the sensor, and this is produced through advanced circuit design implemented in an optimised fabrication process. These characteristics, as much as the quality of the sensor itself, determine the quality of the sensor input to the system processor or microcontroller.



At the same time, the interface IC helps system designers to integrate the microphone quickly and easily by providing both inputs (a stable supply voltage) and outputs which are immune to the noise and interference commonly found in environments such as mobile phones.