THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

XLR8R PART DEUX

by Tom Cantrell

Start ý dV/dt ý Hot Air ý Spec Review ý Time... ý and Temperture • Family Ties • Sources and PDF

SPEC REVIEW

Attention usually centers on the sensitivity of the device, and for many applications, the more the better. For instance, an accelerometer can make for a decent inclinometer, taking advantage of the simple mathematics relating tilt to the force of gravity. But, it requires fine measurements, especially as tilt approaches 0ý (i.e., 0 g). Thus, a highly sensitive (1 or 2 g) unit is the call, and both MEMSIC and Analog Devices can oblige.

However, donýt forget human nature. Is your nifty gadget ever going to get dropped? Fumble-fingered need not apply? Thanks to "no moving parts," MEMSIC units can tolerate a whopping 50,000 g of abuse, 50 times that of the more fragile Analog Devices units.

On the other hand, maybe power consumption is your bag. Because hot air calls for a heater, it shouldnýt be a surprise that MEMSICýs approach calls for more juice. With MEMSIC at 4.5 mA versus 0.6 mA for Analog Devices, neither can be considered a power hog, but the difference would really add up over time.

Furthermore, thanks to speedy turn-on time, an Analog Devices-based design, particularly those with low frequency (i.e., low sampling rate) requirements, can take advantage of the option of powering the unit down between samples. For example, for a 2-ms cycle (powerup, sample, power down) at 100 samples per second, the duty cycle is only one-fifth (i.e., chip powered up 200 ms per second), reducing power consumption by a factor of five.

By contrast, anyone with a furnace or fireplace knows heating up air takes awhile. The turn-on time for MEMSIC units is a leisurely 200 ms, ruling out the power cycling option for all but the lowest frequency applications.

Slower applications may be a realistic concession for MEMSIC units because thermal inertia ultimately limits the overall frequency response to 45 Hz. This can be boosted somewhat (e.g., to 160 Hz) with the addition of external chips that vary gain with frequency. Nevertheless, itýs still a small fraction of the 6-kHz bandwidth spec from Analog Devices.

PREVIOUS • NEXT

Circuit Cellar provides up-to-date information for engineers. Visit www.circuitcellar.com for more information and additional articles.
For subscription information, call (860) 875-2199, subscribe@circuitcellar.com or subscribe online. ýCircuit Cellar, the Magazine for Computer Applications. Posted with permission.