3. Signal conditioning Sensors are very rarely directly connected to the registering instruments, as the signal may be too weak, incompatible or noisy. Sensor signal has to be conditioned. Many sensors are of the resistance type, where the magnitude of resistance can vary from ohms to hundreds of kΩ. Example: thermistor: resistance range 100 Ω – 10 MΩ In many cases the changes of resistance are small (platinum RTDs have TCR ca. 0.385%/oC, strain gauges change its resistance below 1% over the entire operating range). Measuremt of small resistance changes is therefore critical.

Sensitivity measurements The useful signal for the voltage divider , figure below, is equal Variation of the useful signal caused by the change in resistance (sensor resolution) is given by Maksimum resolution is obtained for RL = RS . Voltage divider formed by the sensor with resistance RS connected in series with a load resistor RL. Useful signal as a function of RS/RL. For a wide RS range the good resolution is obtained (max. for RS = RL). For high RS/RL one obtains small resolution. The circuit frequently used in indicators of the threshold value crossover.

Sensor with resistance RS in a voltage feedback circuit of inverting amplifier. Load resistor in a voltage feedback circuit of inverting amplifier. In this configuration called potentiometric (a current through sensor is independent of RS) resolution does not depend on RS and its value can be controlled by selecting RL. A constant voltage VZ exists on RS (potentiostatic measurement). Output voltage is a product of RL and a varying current VZ/RS. Measurement resolution depends on RS.

Current mirror (Wilson circuit) as a current source. Sensor powered from a current source Voltage follower polarized with a Zener diode controls the transistor, which generates current independent of RS. Output currrent iout flowing through controlled transistor T1 is equal to the input current iin, which depends od voltage V1 and resistance R1. Transistor T2 can multiply this current several times. Resolution is independent of RS.

Ratiometric circuits Circuits of this type are applied only in the case when a source of error is of multiplicative nature (power supply instability, variations of ambient temperature, humidity, pressure, aging effects) but not additive (thermal noise). It requires essentially the use of two sensors, one active and the other playing the role of a reference sensor (shielded from the stimulus or insensitive to it). An analog divider circuit gives an output voltage V0 proprtional to the ratio of two input voltages, which is independent of both the supply voltage and amplification of the amplifier. Division can be also realised digitally.

Typical example of a ratiometric circuit is a sensor in a bridge circuit with conversion of analogue signal in ADC converter. Both bridge and converter supply voltages come from the same source. Variation of the supply voltage does not influence the output signal. For accurate measurements we do not neeed power supply of a high stability. In a ratiometric circuit the output code DOUT on a converter output is a digital representation of the ratio of converter input signal AIN and the reference signal VREF , then supply voltage instabilities do not influence the output signal. In the system as in figure on the left, additional source of reference voltage REF independent of VDD is used and the circuit is no more ratiometric. Above solutions are developed in the case of large variations of AIN.

Bridge measurement circuits Typical example is a resistance bridge with investigated sensor in one arm (piezoresistive gauge, thermistor). Resistances can be also replaced by capacitors or inductors. The output voltage for the Wheatstone bridge (as in the figure) is equal: Balance condition (VO = 0): Maximum sensitivity (δVO /VC) is obtained for R1 = R2 and R3 = RS In general the output voltage is a nonlinear function of disbalance ΔR = RS – R

It is hard to obtain both required amplification and a high CMRR. A Wheatstone bridge can operate both in a balanced configuration (at the output there exists an error amplifier which through the feedback restores the balance) or in a disbalanced configuration which is used more frequently. Nonlinear output is corrected digitally in the software. Op amp at the output of a disbalanced bridge. Both RF and polarization current influence the balance condition. It is hard to obtain both required amplification and a high CMRR. Better approach is the use at the output an instrumentation amplifier. The gain is set with a single resistor RG, which does not unbalance the bridge. Very good common mode rejection is obtained.

Op amp in resistive bridge with linearization of the output. Op amp in a bridge with floating voltage source VC. Output amplifier produces a forced null , by adding a voltage in series with the variable arm. That voltage is qual in magnitude and oppositee in polarity to the incremental voltage across the varying element and is linear with ΔR. Output is linear for restricted changes of Rx.

Instrumentation amplifier It is a kind of differential amplifier with gain control Input circuit is separated from the feedback resistors. The amplifier can be built in a monolithic form. Input impedance is of order 109 Ω or higher. Input signals of microvolt order are amplified with damping summation signal of volts order (high CMRR coefficient in a range 70 - 100 dB), what is important in particular for 50 Hz frequency.

Charge amplifier Very special class of amplifiers with extremely low bias current. Used in the case of high impedance sensors, as piezoelectric, capacitive, pyroelectric and other generating very low charges or currents. Belong to AC amplifiers with cut-off frequencies: upper f2 = 1/(2πR2C2) and lower f1 = 1/(2πR1C1) .

Sensor standardization Two-wire interface IEEE 1451.4 Class 1 with constant current powered sensor. TEDS - Transducer Electronic Data Sheets

Multiwire interface IEEE 1451.4 Class 2 z with shared wire.

Smart module block diagram of a gas sensor using IEEE 1451 Smart module block diagram of a gas sensor using IEEE 1451.4 Classs 2 standard with embedded TEDS.