L6/7 - Interface Electronics

Question 1

Give 5 common sources of sensor error.

Answer 1

1. Calibration
   Defines initial offset and sensitivity variation between different sensors
2. Static
   Measurement accuracy of a constant (static) pressure: includes non-linearity, hysterisis and repeatability.
3. Dynamic
   Includes frequency response, rise time, phase shifts, overshoot, settling time, etc.
4. Temperature
   Includes variations in all sensor characteristics with temperature, eg, offset & sensitivity.
5. Stability
   Sensors ability to maintain the same transfer function over a certain time period

Question 2

How do Bridge Structures eliminate temperature error from resistive circuits?

Answer 2

In a Full Bridge Structure / Whetstone Bridge the output voltage is proportional to the difference in resistance between two pairs of elements.
Since temperature affects each element uniformly, the overall effect on the output signal is minimized. The voltage output remains a function of the physical phenomenon being measured (like strain) rather than temperature changes.

Question 3

What values of R1, R2, R3, R4 give the maximim sensitivity in a Full Bridge Structure?

Answer 3

R1=R2=R3=R4

Question 4

What is the Piezoelectric effect?

Answer 4

When charge is generated in crystalline materials
upon application of mechanical stress

Question 5

What is the opposite of the Piezoelectic effect?

Answer 5

Electrostiction where application of electric charge causes deformation in a crystalline structure.

Question 6

True or False
In Piezoelectic materials the Piezoelectric effect depends only on the material properties?

Answer 6

False
The effect depends on the direction of cut in the crystal (Miller Index).

Question 7

How can we get precise MEMS resistors?

Answer 7

Resistors can be trimmed! Laser trimming and other post processing techniques.

Question 8

Define FSO

Answer 8

Full Scale Output (FSO) refers to the maximum output signal produced by a device when it is subjected to its maximum input measurement.

Question 9

Define sensetivity (for a sensor)

Answer 9

Sensitivity is defined as the change in output voltage per unit change of input per volt of bias.
Often refered to in terms of FSO. This being FSO divided by measurand change to generate FSO per volt of bias voltage.

Question 10

Define TCS

Answer 10

Temperature Coefficient of Sensitivity.
How the sensors sensetivity (ΔOutput/ΔInput) is affected by temperature.

Question 11

For a Full Bridge Structure in Constant current mode
FSO ∝ ?

Answer 11

Bridge Resistance

Question 12

A Full Bridge Structure in Constant voltage mode
FSO ∝ ?

Answer 12

Sensor sensetivity

Eg. pressure sensetivity

Question 13

Piezoelectric Resistors have what sort of relationship to temperature?
How does this affect a Full Bridge Structure Pressure Sensor in Constant Voltage mode?

Answer 13

Resistance is usually proportional to temperature (Positive TCR) and this is true of Piezoresistors.
For a temp increase, all bridge resistances increase and pressure sensetivity decreases.

Question 14

Is it better to operate Full Bridge Structures in Constant Voltage or Constant Current mode?

Answer 14

In Constant Current mode temperature compensation is simpler.
Constant Voltage mode however provides better accuracy as bridge resistance variations do not affect
the output due to the balanced nature of the bridge.

Question 15

In a Bridge Structure how is temperature compensation achieved?

Answer 15

Insertion of a resistor either in parallel to the bridge (constant current mode)
or in series (constant voltage mode) to compensate for FSO temperature dependent errors.
The insertion of diodes in series with the bridge in constant voltage mode will also compensate for TCS as they have a -ve temperature coefficient.

Question 16

What is a DCASP?

Answer 16

(DCASP) - Digitally controlled Analogue Signal Processor
A Bridge Structure Sensor with analogue interface circuits that are programmable i.e. (gain and offset) with additional control of the sensor’s supply source to correct and linearise sensor errors.
In this architecture, the sensor is excited by a constant current - the temperature dependent bridge voltage is digitized by an A/D converter.
The digital value controls the address of two locations in non-volatile memory were offset and FSO adjustment values are stored. These values were converted back to the analogue domain and fed into the programmable amplifier.

Question 17

What is SNR?

Answer 17

Signal to Noise Ratio
Expressed as either a ratio A:B or in dB using SNR=20log(S/N)

Question 18

What is quantisation error?

Answer 18

Quantization error is the difference between the analog signal and the closest available digital value at each sampling instant from the A/D converter. Quantization error also introduces noise, called quantization noise, to the sample signal. The higher the resolution of the A/D converter, the lower the quantization error and the smaller the quantization noise.

Question 19

What is ADC resolution?

Answer 19

The ADC accuracy expressed in bits.

The minimum resolvable value = Vmax / ADC resolution

Question 20

How do you convert Celcius to Kelvin?

Answer 20

Kelvin = Celsius + 273.15

it might be convinient to assume C ~ 27 or 300K if its not stated

Question 21

How do you convert from RMS to peak to peak value?

Answer 21

P-P Value = RMS Value × 2√2