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. 2017 Jun 7;17(6):1312.
doi: 10.3390/s17061312.

A Capacitance-To-Digital Converter for MEMS Sensors for Smart Applications

Javier Pérez Sanjurjo  1 , Enrique Prefasi  2 , Cesare Buffa  3 , Richard Gaggl  4
Affiliations

A Capacitance-To-Digital Converter for MEMS Sensors for Smart Applications

Javier Pérez Sanjurjo et al. Sensors (Basel). .

Abstract

The use of MEMS sensors has been increasing in recent years. To cover all the applications, many different readout circuits are needed. To reduce the cost and time to market, a generic capacitance-to-digital converter (CDC) seems to be the logical next step. This work presents a configurable CDC designed for capacitive MEMS sensors. The sensor is built with a bridge of MEMS, where some of them function with pressure. Then, the capacitive to digital conversion is realized using two steps. First, a switched-capacitor (SC) preamplifier is used to make the capacitive to voltage (C-V) conversion. Second, a self-oscillated noise-shaping integrating dual-slope (DS) converter is used to digitize this magnitude. The proposed converter uses time instead of amplitude resolution to generate a multibit digital output stream. In addition it performs noise shaping of the quantization error to reduce measurement time. This article shows the effectiveness of this method by measurements performed on a prototype, designed and fabricated using standard 0.13 µm CMOS technology. Experimental measurements show that the CDC achieves a resolution of 17 bits, with an effective area of 0.317 mm², which means a pressure resolution of 1 Pa, while consuming 146 µA from a 1.5 V power supply.

Keywords: CDC; MEMS; capacitive sensors; dual-slope; low power; pressure sensor.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
(a) Sketch of a typical differential capacitive sensing cell for a MEMS structure. Stators A and B are anchored to the chip substrate and they form a differential capacitor with rotor; (b) Diagram with forcing acting on a suspended mass.
Figure 2
Figure 2
Block schematic of the MEMS sensor and CDC.
Figure 3
Figure 3
Modulation and chopping scheme.
Figure 4
Figure 4
(A) Standard Dual Slope; (B) Standard noise-shaping Integrating Dual-Slope.
Figure 5
Figure 5
(A) Self-Oscillated noise-shaping Integrating Dual-Slope scheme; (B) Time diagram of the CDC.
Figure 6
Figure 6
Block schematic of the reconfigurable digital filter.
Figure 7
Figure 7
Die photo of the MEMS sensor and the CDC bonded together in a package with a hole for pressure control.
Figure 8
Figure 8
Test-chip welded and working connected to the test PCB and pressure controller.
Figure 9
Figure 9
Use of configurable capacitors in offset and gain in the Preamplifier: (a) single ended outputs of the Preamplifier with range of offset capacitors; (b) single ended outputs of the Preamplifier with range of gain capacitors.
Figure 10
Figure 10
Time diagram of the signals that proves the behavior of the noise-shaping Integrating Dual-Slope measured from the test-chip.
Figure 11
Figure 11
FFT of the CDC for an input pressure of 1050 hPa.
Figure 12
Figure 12
Digital output code of the CDC vs. input pressure in differential measurements analysis.
Figure 13
Figure 13
RAW data out of the transfer characteristics measured in the prototype for different temperatures.
Figure 14
Figure 14
Error of resolution in the MEMS depending on the pressure for each temperature.
Figure 15
Figure 15
Compensated transfer characteristics after temperature, offset and gain error for different temperatures.
Figure 16
Figure 16
State-of-the-art plot. Figure of merit (FoM) vs. resolution.
See this image and copyright information in PMC

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