With the rapid expansion of available sensor elements driven by the growth of MEMS (microelectromechanical systems) sensors, the considerations of sensor interface design become ever more important. The design engineer needs to understand both the sensor as well as the application in order to make the proper design tradeoffs in this already tricky art of analog front-end design. The challenge is further compounded with the trend toward MEMS technologies and their inherently smaller signals. This article attempts to cover some of the basics of sensor interface design and gives a cursory overview of the challenges and trade-offs of the possible approaches.
It's Not Just a Resistor
Fundamentally, every sensor can be modeled as a simpler component, albeit a component with a value that changes over time. Usually this means we can treat them as either a simple passive impedance, such as a resistance, capacitance, or inductance, or as an active source, such as a current or voltage source.
As these values change with time, we need to be able to convert that change into a time-varying voltage. Furthermore, we need to maintain the linearity of the sensor while we do this. Hopefully, the sensor of choice is linear to begin with, but even if it's not, we don't want to make it any worse. Parasitic components in the measurement topology can have huge effects on the linearity.
Impedance and Bridging
The choice of the analog front end depends on many factors, but the first is the impedance of the sensor. To effectively measure a sensor, the front end must not "load" the sensor. It's common to think we can just plug a sensor into an ADC (analog-to-digital converter), but what if the sensor is a small-value capacitor, with a large equivalent resistance (i.e. the magnitude of the impedance at the frequency of interest)? ADCs (or voltage pre-amps, scopes, meters, etc.) have large input impedances themselves and, being in parallel, will load the sensor, reduce the sensitivity, and impair the linearity. Figure 1 shows elementary circuit topologies for measuring large and small impedance components.
One of the problems with these "single-ended" topologies is that they can have huge dynamic range. It's easy to mistakenly drive the voltage (in Figure 1(a), for example) to the supply rails with the wrong choice of drive current. It would be nice to balance the output voltage in between the rails.
The next decision is to choose whether to use a bridge topology, the simplest of which is a half-bridge, shown in Figure 2 (a). Here we use a pair of similarly valued components in series, forming a voltage divider:
The current through them is common so the voltage between them will be at the midpoint of the supply. Thus, small changes in Zs will cause small changes in the mid-point voltage. The same is true for the dual case where we measure currents, as shown in Figure 2(b).
The benefit of this method is the balancing of the voltage (or current) between the limits. The detriment is that large swings in the sensor impedance introduce non-linearities because the midpoint voltage of the voltage divider will approach the rails asymptotically, not linearly, for large changes in Zs.
Parasites and Noise
Every component has "parasitic" terms. A real wire has resistance (usually small) and capacitance (sometimes not small); inductors have series resistance, and voltage sources have non-zero output impedance. While most of the time we can ignore these parasitics, especially in moderate frequency digital systems, they often become the limiting factor for sensor measurements.
Embedded Technology Magazine
This article first appeared in the January, 2007 issue of Embedded Technology Magazine.