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 rlcZONE Products for the week of November 19, 2007
Microbridge Says...
Microbridge Incorporates Four eTC Rejustors into a New Bridge Configuration
Single Passive Chip is Ideal Signal Calibration and Temperature Compensation Solution Corrects Offset for Wheatstone Bridge Sensor Applications
The leader in innovative analog calibration technology, Microbridge Technologies, Inc. introduced the MBW-303 Wheatstone Bridge Offset Conditioning Network in conjunction with the MEMs Industry Group's (MIG) Congress, L'Auberge Del Mar Resort & Spa, San Diego Calif.
The Conditioning Network is comprised of four eTC Rejustors (re-adjustable resistors), the world's first fully analog, passive electronic Temperature Compensation (eTC) dividers, configured in a bridge. Designed for Wheatstone bridge sensors in automotive, healthcare, consumer, and instrumentation markets, this single chip solution compensates for offset errors and offset drift in a Wheatstone bridge, solving the problem at the source.
The MBW-303 simplifies the design process allowing sensor offset calibration and temperature compensation after final assembly, which negates all cumulative errors associated with the assembly processes, such as manufacturing tolerance errors, error introduced by stress, etc. Dynamic adjustment provides cost and labor savings and the networks' re-adjustability reduces re-work.
"We recognized that many of our rejustor devices have been targeted by customers for sensor calibration and compensation applications," said Nick Tasker, vice president, Business Development at Microbridge. "This device represents the next step in device integration for these applications. There is a need for higher and higher precision and integration. If precision cannot be maintained over the entire operating temperature range of the sensor, it really isn't precise. The MBW 303 maintains offset precision over the full operating temperature range."
"According to MEMS analyst group WTC (Wicht Technologie Consulting), MEMS in applications like consumer electronics, telecom and bio/medical will continue to grow to over $3.3 Billion, in products such as microphones, inertial sensors, microfludics and mobile phones," said Karen Lightman, Managing Director, MEMS Industry Group. "MEMS Executive Congress addresses this burgeoning market from a commercialization standpoint. It provides a unique forum in which both MEMS manufacturers and integrators will discuss the latest industry innovations from their respective points-of-view. Such a fruitful exchange will propagate even greater innovation in MEMS technology, with end users reaping the biggest rewards."
The MBW-303 enables adjustment and verification to be performed in a single temperature cycle. It can operate in temperatures ranging from -40degrees C to +150. In addition, it has a small footprint for high-density applications, doesn't require power or memory to hold calibration, and doesn't require an external temperature sensor. The MBW-303 enables precision electrical in-circuit adjustment for ohmic and temperature correcting offset errors. The adjustment process is isolated from the circuit, allowing true, passive in-circuit calibration and compensation.
The MBW-303 device is eligible for the rejustor Design Challenge contest running through December 31, 2007. Further information and registration forms are available online at http://www.rejustorchallenge.com
EN-Genius Says…
Every now and again a practical development comes along that alters the way things have been done for a hundred years. This low-cost surface-mount passive component from Microbridge Technologies, Inc. fits that description.
 Every engineering student, electronics technician, and design engineer knows and loves the classic differential Wheatstone bridge, whether it's comprised of a single sensing element with three balancing resistors, or it’s a full-bridge with four sensing elements. The Microbridge Technologies MBW-303 alters the picture. I view it as a departure and an enhancement to traditional Wheatstone bridge configurations. A glance at the schematic shows the internal bridge connections of an MBW-303.
The problem this device addresses is that manufacturing variations in typical sensing elements lead to offset errors. In some applications, Wheatstone bridge sensors develop low signals-of-interest in the presence of high offset signals. When used with a digitizer, the input-range of the digitizer circuit input-amp and/or ADC must be large enough to accept the signal along with the offset.
 That's where the totally passive MBW-303 comes in. It essentially removes offset (and any offset drift), so that an amplifier's input range can be reduced, letting the amplifier be optimized to focus on the signal of interest. Naturally, this will boost the performance of an amplification chain, and it can cut the SNR (signal-to-noise ratio) of a system as well. By the way, the inherent noise in a wideband MBW-303 is comparable to that of a standard thin-film resistor.
Microbridge's latest Rejustor is an integrated calibration and temperature compensation product, too. Combining these virtues should make the MBW-303 appealing to anyone who needs to design a stable bridge. What's more, Microbridge wares can let you achieve one-step calibration and passive adjustment, which can save time (hence money) when a circuit goes into production. The technique makes a circuit adaptive and adjustable, and does that in-circuit to boot.
Inside the Package
If you could look inside one of these micro-resistor packages, you’d see that Rejustor bridges are electrically adjustable CMOS-fabbed MEMS (micro-electro-mechanical systems). Made using a very repeatable process, Rejustor resistance values can be adjusted at the factory to high precision. Rejustors, with the same electrical behavior as precision fixed resistors, also have the added non-trivial benefit that their resistance can be in-circuit adjusted. This is way cool, as the kiddies say.
Temperature instability is typically viewed as a disadvantage, but in Microbridge's case it's used to get precision adjustment during the device’s manufacture. By thermally isolating portions of resistive films in the MEMS-making process, and by providing localized and electrically-controlled heat during manufacturing, controlled material properties such as room-temperature resistivity and given tempcos are achieved.
Individual resistance elements can also be matched to an application. For example, in the adjustment of a piezoresistive bridge-based sensor, it's advantageous for a resistor to have a large negative TCR (temperature coefficient of resistance), and have the TCR become more positive as it's trimmed down. MBW-303s can be set up to support that.
Microbridge 2-terminal passive Rejustors can also eliminate the need for extra temperature sensors in some circuits. Indeed, the 2-terminal devices can act as temperature sensors, as well as adjustment devices.
 Applications for predecessor 2-terminal Rejustors could include establishing op amp buffer stage gain and offset to ensure dynamic range. An amplifier offset (and TC-offset) could be compensated for, in the analog domain, right at the source. No processor lookup-tables or data converters would be needed, and continuous-valued adjustment would eliminate quantization noise. Pot wiper resistance and mechanically-generated wiper noise would be sidestepped as well.
As I mentioned earlier, whether we're discussing Microbridge's MBW-303 bridge or the company's simpler 2-terminal products, adjustment can also be accomplished electronically after a circuit board is assembled. This advantage also means you might get away with letting surrounding parameter variations accumulate. You could then use a passive Rejustor to null out the cumulative error budget, and perhaps do that most effectively during final test. These kinds of adjustments could also be carried out at low voltage and low current before and/or after packaging.
Adjustable Rejustor micro-resistors are also non-volatile and they're re-adjustable many times, bi-directionally, to very high precision (0.1% to 0.002%). So, discard the notion of laser trimming, and send for some MBW-303 samples. I'd be interested to know where you find application for them, and what kind of results you achieve.
The MBW-303 Bridge is sampling in QFN-16 and will be priced at $2.67 in 1000-piece lots.
Data Sheet
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