Conditioning Sensor Signals
by Bryon Moyer
November 30, 2011 at 2:11 PM
Some time back, ZMDI made an announcement about a sensor conditioner they had released. A couple things gave me cause for pause as I looked it over. First was the description of a one-pass calibration process as being unique. The other was the fact that a major component of the advanced sensors you may see presented at conferences, examples of which we covered in a sensor article series earlier this year, is the associated circuitry required to turn a raw sensor output into a reliable, usable signal. I.e., conditioning the signal on the same chip as the sensor.
So I checked in with ZMDI to get their thoughts on both of these topics.
With respect to calibration, all sensors require it, worldly imperfections being what they are. Calibration involves measuring the response of the sensor and then applying corrections that are stored in the sensor; each unit has to be individually calibrated. The question is how you do it.
Some apparently correct using analog techniques; some, including ZMDI, use digital. Some – most of the analog ones in particular – use a multi-pass calibration process to set all of the various parameters because there may be coupling between them, so you need to set some values before measuring and setting others. So you do one measurement pass to acquire one set of values and set a correction. Then you do another pass and set a different parameter. Etc.
The one-pass approach measures all necessary data in one pass, and then offline software – e.g., in a PC – can calculate all of the corrections and program them into the sensor’s EEPROM. This is inherently a faster process than multi-pass.
As far as integrating the conditioner with the sensor is concerned, ZMDI agrees that, in principle, this can certainly be done and would be “a reasonable and mutually beneficial advancement,” although no ongoing projects at ZMDI were identified. They indicated that the kinds of sensors best suited to a combined solution are, of course, those that involve MEMS processes that integrate nicely with CMOS. Those include, in particular, piezo-electric sensors measuring things like pressure and strain as well as those that measure inertia – vibration and acceleration.