[Design View / Design Solution]
Zero-Drift IA Takes The Strain Out Of Sensor Measurements
Instrumentation amplifiers can abet many sensor applications, from ratiometric bridges to low-side current sensing.
Sensor measurements typically translate physical phenomena of interest into electronic-circuit parameters such as resistance and capacitance, which can then be read with a bridge circuit. Bridge circuits produce an output voltage or current signal that is ratiometric with respect to temperature and powersupply voltages, thereby enabling the measurement system to be immune to these variables. Sensor examples can include:
Thermistors for temperature sensing
Resistive/capacitive strain gauges for pressure sensing
Magnetoresistive sensors for direction/position sensing
Sensors that produce a signal voltage or current directly don’t require a bridge circuit to transform the physical variables. Examples include thermocouples, ECG-based medical instrumentation, and voltage across the currentsense resistor in a power-monitoring circuit.
Today’s sensor applications range from consumer electronics (thermometers, pressure scales, GPS systems, etc.), to automotive (fuel sensors, knock sensors, brake-line sensors, window pinch control), to industrial and medical instrumentation (valve-position sensing, temperature-based system calibration, and ECG). Their operating environment is rich in EMI noise, power-supply harmonics, ground-loop currents, and ESD spikes, while the signals of interest to be extracted are relatively small.
Thus, the analog-sensor interface becomes non-trivial and must maintain exacting specifications while rejecting these environmental phenomena. For commercial success, it must also deliver low cost, small size, and (for battery-operated meters) low supply current.
TO AMP OR NOT TO AMP System designers like to keep analog chains short in the hope of improving the signal’s immunity to external noise phenomena. (Digital circuitry is generally immune to noise, but not always.) Lengthy analog chains in the past tackled a given signal-processing task in sequential stages.
One stage, for example, provided differential gain without common-mode rejection, and another provided common- mode rejection without differential gain, etc. Dual and high-voltage supply rails also helped relax the signalto- noise constraints on analog circuits. The requirements for shorter analog chains and single-supply, low-voltage, analog power-supply rails have forced the evolution of innovative architectures to meet these challenges.
One decision that arises early in a system design is whether or not the analog-to-digital converter (ADC) and sensor can interface directly. Such direct connections offer an advantage in some applications.
High-resistance ratiometric bridges, for instance, can use the rudimentary internal reference present in many ADCs, and some modern ADCs contain a high-impedance buffer or PGA that can be used to isolate the sensor signal from loading and from current spikes caused by the ADC’s sampling circuitry.
On the other hand, a substantial case can be made for using an instrumentation amplifier (IA) to interface the sensor to an ADC:
Amplifying small analog signals at their source improves the overall signal-to-noise ratio in some applications, especially if the sensor is not close to the ADC.
Many high-performance ADCs lack high-impedance inputs and must therefore be driven by an amplifier of low source impedance to get the full benefit of their specifications. Without an intermediate amplifier for such configurations, aberrations like input current spikes and mismatched source resistances can introduce gain errors.
An external amplifier allows the user to optimize the signal conditioning (filtering) for an application.
The best semiconductor process for fabricating an ADC isn’t necessarily the best one for fabricating amplifiers.
The gain offered by an IA makes for an easier interface between sensor and ADC, both by easing the system design constraints and by lowering the overall system cost. For example, a much higher-resolution and expensive ADC would be required to read an un-gained sensor signal than that required for an amplified sensor signal.
LOW OFFSET A BIG ASSET School textbooks are great at describing the ideal world. All of the unknowns in an equation can be derived, and all problems have an answer listed in the back. The real world, on the other hand, is best described by long hours in the lab trying to get analog circuits to work, often when program milestones are just around the corner.
Various sources of dc error are encountered when using IAs to read sensor signals. Perhaps the most critical of these is the effect of input offset voltage. In fact, every other source of dc error is modeled in terms of the input offset voltage: dc CMRR represents the change of dc input offset voltage with input common-mode voltage, and dc PSRR represents the change of dc input offset voltage with variation in powersupply voltage.
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