A sigma–delta interface ASIC for force-feedback micromachined capacitive accelerometer

Abstract

A single-loop fourth-order sigma-delta (RD) interface circuit for micromachined accelerometer is presented in this study. Two additional electronic integrators are cascaded with the micromachine sensing element to form a fourth-order loop filter to eliminate quantization noise. A precise model for the overall system is set up based on nonlinear model of 1-bit quantizer. Three main noise sources affecting the overall system resolution of a RD accelerometer: mechanical noise, electronic noise and quantization noise are analyzed in more detail. A switchedcapacitor charge integrator and correlated double sampling are applied to reduce input-referred electronic noise. The ASIC is fabricated in 0.5 lm two-metal two-poly n-well CMOS process, and test results show that the noise density floors of the open-loop and closed-loop modes are 12 and 80 lg/Hz 1/2 , respectively, the sensitivity is 1.25 V/g, the full measurement range can be achieved from -2 to ?2 g, and the power dissipation is 40 mW.

A low noise sigma-delta microaccelerometer interface circuit

“…4(b) shows the silicon accelerometer wire-bonded to the interface chip. The sensing element (4:4 mm  4:4 mm) is fabricated in bulk silicon technology and the mechanical noise is 1.4 µg/Hz 1=2 , and the specifications are presented in [4]. The sensor system is powered by the Agilent E3631.…”

“…The static capacitance is 18.92 pF, and the initial sensing gap distance d 0 is 3.5 µm. The resonant frequency w 0 of the sensor is 1628 Hz, and the mechanical noise is 1.4 µg/Hz 1=2 [4].…”

A high-order sigma-delta accelerometer interface circuit

“…During the sampling phase (S1 = high, S2 = low), the CDS capacitor C C will save the 1/f noise and offset of the operational amplifier, and in the integration phase (S1 = low, S2 = high), C C will cancel out the offset and lowfrequency noise [6]. The output of the SC C-V convertor circuit is written as [7]:…”

“…The charge amplifier is followed by a sample and hold circuit to pick up the voltage signal. In [7], a passive filter, which operates as a lead compensator, is used to minimize the power consumption and ensure fast setting of the stage. However, a heavy compensation factor is selected for a high-Q sensor element, and the lowfrequency gain of the passive filter decreases, which will reduce the loop gain of the system.…”

“…Stability is difficult to achieve for a high-order AEÁ electromechanical modulator due to the increased number of poles in the loop filter [7]. The increased poles lower the phase margin at higher frequency and lead to an unstable system.…”