MEMS-based electrically isolated analog-to-digital converter

Details MEMS-based electrically isolated analog-to-digital converter MEMS-based electrically isolated analog-to-digital converter

2001-10-18

2003-07-15

JeanPierre, Peguy (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Analog to digital conversion

C438S052000, C341S143000

Reexamination Certificate

active

06593870

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
FIELD OF THE INVENTION
The present invention relates to electrical isolators and in particular to electrical isolators that employ microelectromechanical system (MEMS) devices.
BACKGROUND OF THE INVENTION
Electrical isolators are used to provide electrical isolation between circuit elements for the purposes of voltage level shifting, electrical noise reduction, and high voltage and current protection. Circuit elements can be considered electrically isolated if there is no path in which a direct current (DC) can flow between them. Isolation of this kind can be obtained by capacitive or inductive coupling. In capacitive coupling, an electrical input signal is applied to one plate of a capacitor to transmit an electrostatic signal across an insulating dielectric to a second plate at which an output signal is developed. In inductive coupling, an electrical input signal is applied to a first coil to transmit an electromagnetic field across an insulating gap to a second coil, which generates the isolated output signal.
Both such isolators essentially block steady state or DC electrical signals. Such isolators, although simple, block the communication of signals that have significant low frequency components. Further, these isolators can introduce significant frequency dependent attenuation and phase distortion in the transmitted signal. These features make such isolators unsuitable for many types of signals including many types of high-speed digital communications.
In addition, it is sometimes desirable to provide high voltage (>2 kV) isolation between two different portions of a system, while maintaining a communication path between these two portions. This is often true in industrial control applications where it is desirable to isolate the sensor/actuator portions from the control portions of the overall system. It is also applicable to medical instrumentation systems, where it is desirable to isolate the patient from the voltages and currents within the instrumentation.
The isolation of digital signals is frequently provided by optical isolators. In an optical isolator, an input signal drives a light source, typically a light emitting diode (LED) positioned to transmit its light to a photodiode or phototransistor through an insulating but transparent separator. Such a system will readily transmit a binary signal of arbitrary frequency without the distortion and attenuation introduced by capacitors and inductors. The optical isolator further provides an inherent signal limiting in the output through saturation of the light receiver, and signal thresholding in the input, by virtue of the intrinsic LED forward bias voltage.
Nevertheless, optical isolators have some disadvantages. They require a relatively expensive gallium arsenide (GaAs) substrate that is incompatible with other types of integrated circuitry and thus optical isolators often require separate packaging and assembly from the circuits they are protecting. The characteristics of the LED and photodetector can be difficult to control during fabrication, increasing the costs if unit-to-unit variation cannot be tolerated. The power requirements of the LED may require signal conditioning of the input signal before an optical isolator can be used, imposing yet an additional cost. While the forward bias voltage of the LED provides an inherent noise thresholding, the threshold generally cannot be adjusted but is fixed by chemical properties of the LED materials. Accordingly, if different thresholds are required, additional signal conditioning may be needed. Additionally, the LED is a diode and thus limits the input signal to a single polarity unless multiple LEDs are used.
Further, optical isolators are not well suited for the isolation of analog signals. Optical isolators can only operate to isolate analog signals in one of two ways. One of these is to operate the LED of the optical isolator in its linear range, so that the output signal of the optical isolator accurately reflects the input signal. Maintaining the operation of the LED in its linear range is difficult to do consistently (as is the calibration required to determine what is the LED's linear range). The second way of isolating analog signals is to digitize the analog signal and transmit the digitized bits with multiple optical isolators. Multiple isolators, however, are expensive and bulky and the need to preprocess the input analog signal requires a significant amount of electronics.
Other technologies also exist or are being developed that can be employed to isolate digital and analog signals. For example, U.S. patent application Ser. No. 09/788,928 filed on Feb. 20, 2001, which is hereby incorporated by reference, discloses a mechanical isolator that is manufactured using MEMS techniques and suitable for transmitting digital signals. Similarly, U.S. patent application Ser. No. 09/804,817 filed on Mar. 13, 2001, also hereby incorporated by reference, discloses a MEMS isolator suitable for transmitting analog signals. In each case, the isolator uses a specially fabricated microscopic beam supported on a substrate and whose ends are insulated from each other. One end of the beam is connected to a microscopic actuator, which receives an input signal to move the beam against a biasing force provided by a biasing device. The other end of the beam is attached to a sensor detecting movement of the beam. For the digital isolator, the biasing force is constant, and beam movement occurs only when the input signal is sufficient to overcome the biasing force.
Although such MEMS devices can provide signal isolation, the devices by themselves cannot be implemented as isolated analog-to-digital converters (isolated-ADCs). To the extent such MEMS devices are employed as isolated analog-to-analog converters, the output of the devices can be converted into digital format by the addition of a conventional analog-to-digital conversion circuit, thus producing isolated-ADCs. However, the addition of this circuit adds to the expense of the MEMS devices. Further, while it is possible to design converters that operate open-loop, closed-loop converters are preferable in order to maintain desired linear operation of the converters over a relatively large range of possible input signals. Consequently, designing isolated-ADCs using conventional MEMS devices that are employed as isolated analog-to-analog converters not only requires that conventional analog-to-digital conversion circuits be provided at the output of the MEMS devices, but also requires feedback circuitry such as a proportional-integral control circuit. Because the feedback circuitry, conventional analog-to-digital conversion circuitry and MEMS devices are typically physically located on different microchips, the costs associated with designing and constructing isolated-ADCs by way of these conventional devices is further increased.
Therefore, it would be desirable if a new isolated-ADC that employed a MEMS device was developed, where the new isolated-ADC employed simpler, less costly and more easily manufactured circuitry.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a microelectromechanical system (MEMS) circuit in which the MEMS device forms part of a sigma-delta converter. The converter provides stability, a simplified design, and a digitized output such that the circuit acts as an isolated analog-to-digital converter (isolated-ADC) with a digital output that can be used for later computerized processing.
Generally, an analog signal that is input to the MEMS device is converted into a force applied to a beam within the MEMS device. Also applied to the beam is a feedback signal. The combined forces upon the beam move the beam with respect to a sensor, which outputs a signal indicative of the position of the beam. The signal is compared with a reference value representative of a reference position of the beam, and the result of the comparison is provided as a digital signal that is used to generate the output signal as well as the feedback signal.
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