5.11.
A
V
=
V δ
5.11
( )
out
4 ref
Where A is the gain of the IA andδ is the change in the resistance of the sensor cor-
responding to some physical action. Here only, δ is being amplified.
Activity 5.4
Capacitance to Voltage
Activity 5.4.1 Motivation
The electrical property of capacitance is a physical principle behind many of the
sensors because it is a property which varies directly proportionally to the distance
between the metal plates.
Capacitors can be used as sensors which can detect the presence of an object between
their plates. This is because capacitors are sensitive to the material that resides between
their metal plates. Thus, this principle can be used as a detector to determine when
someone enters a space. In the case of the piezoelectric sensor, we use the fact that
the voltage of a charged capacitor will vary inversely proportional to its capacitance.
The output voltage is amplified to a usable level by an op-amp circuit.
Activity 5.4.2 Circuits
This activity briefly describes how capacitance can be measured. It should be noted
that capacitance can be measured in the same ways for measuring resistance i.e by
using a voltage divider or a bridge circuit, See Fig. 5. 9 and Fig. 5.10. Instead of using
African Virtual University
resistors, capacitors are used. However, one critical difference is that V must be a
ref
sinusoidal signal since the capacitor blocks DC.
Activity 5.5
Data Acquisition
In this section you will learn about the functions of the different sections of data ac-
quistion. Fig. 5.11 shows the steps into which Data acquisition can be divided. Each
step of the data acquistion process: Anti-aliasing; sample/Hold ; and Quantization
are described as follows.
Figure 5.11 Data Acquisition
Activity 5.5.1 Anti-aliasing
The essential requirement is that all signals must be bandlimited to less than half the
sampling rate of the sampling system. For broad spectrum signals, an analog lowpass
filter must be placed before the data acquisition system. The minimum attenuation
of this filter at the aliasing frequency should be at least:
A in = 20log 3∗ 2B
(
)
5.12
( )
m
Where B is the number of bits of the ADC. This formula is derived from the fact that
there is a minimum noise level inherent in the sampling process and there is no need
to attenuate the sensor signal more than to below this noise level.
Task 5.3 Further reading and Note making
(a) Use Compulsory Reading 5 and other references to write short notes on
- the problems with the Anti-aliasing Filter:
- how the problems can be solved.
African Virtual University
Activity 5.5.2 Sample and Hold
In this section you will learn that:
The purpose of the sample and hold circuitry is to take a snapshot of the sensor signal
and hold the value. This happens once every sample period when the switch connects
the capacitor to the signal conditioning circuit. During this period, the capacitor holds
the voltage value measured until a new sample is acquired. Amidst all these the ADC
must have a stable signal in order to accurately perform a conversion. Fig. 5.13
is an equivalent circuit for the sample and hold circuit. Many times, the sample and
hold circuitry is incorporated into the same integrated circuit package.
C
Figure 5.13 Equivalent Circuit for a Sample and Hold
However, a Sample and Hold circuit has problems which are attributed to: Finite
Aperture Time; Signal Feedthrough; and Signal Droop.
Activity 5.5
Analog to Digital Conversion
In this section you will learn that:
(i) The purpose of the analog to digital is to quantize the input signal from the
sample and hold circuit to 2B discrete levels - where B is the number of bits
of the analog to digital converter (ADC).
(ii) The input voltage can range from 0 to V (or
to +
for a bipolar
ref
−Vref
+Vref
ADC). What this means is that the voltage reference of the ADC is used to
set the range of conversion of the ADC.
(iii) For a monopolar ADC, a 0 V input will cause the converter to output all ze-
ros.
(iv) If the input to the ADC is equal to or larger thanV then the converter will
ref
output all ones.
(v) For inputs between these two voltage levels, the ADC will output binary
numbers corresponding to the signal level.
(vi) For a bipolar ADC, the minimum input is
and not 0 V.
−Vref
