Basic Facts about Data Acquisition (DAQ) and Process Control

The goal of most electronic systems is to measure or control some physical quantity. The system will have to acquire data from the environment, process this data and record it. As a control system, it will also have to interact with the environment.

The flow of information in a typical data acquisition (DAQ) can be outlined as follows:

1. The input transducers measure a variable/some property of the environment.
2. The output from the transducers is conditioned (amplified, filtered, etc.).
3. The conditioned analog signal is digitized using an analog to digital converter (ADC).
4. The digital information is acquired, processed and recorded by the computer.
5. The digital control systems are converted to analog signals using a digital to analog converter (DAC).
6. The analog signals are conditioned e.g. amplified, filtered, etc. aptly for an output transducer e.g. a motor.
7. The output transducer interacts with the environment.

Transducers

The electrical systems are able to respond to voltage and current signals in the electrical domain i.e. amplitude, frequency, phase and time constant.

An input transducer is needed to convert a signal from its domain (non-electrical) into the electrical domain.

A transducer may generate an electrical signal by varying one of the following: voltage, current, resistance, capacitance, self-inductance, mutual inductance, transistor gain, etc. The most basic transducers respond to temperature, electromagnetic radiation intensity, force, displacement or chemical concentration. Provided these transducers are coupled to the time domain, they can be used to measure any physical or chemical quantity.

Examples of input transducers are:

• Photo-diode
• Phototube
• Piezoelectric crystal
• Radio antenna
• Mechanical switch
• Strain gauge
• Thermocouple
• Hall effect device

The output transducers convert signals from the electrical domain to the domain that can be perceived by one of the five human senses. A considerable amount of power is normally needed to convert information from the electrical domain.

Examples of output transducers are:

• Electric motor
• Light-emitting diode
• Cathode ray tube
• Radio frequency transmitter
• Loudspeaker

Signal Conditioning Circuits

Signal conditioning actually occurs in the interface between the transducers and the electrical circuit. A low-level signal amplifier and a low–pass filter are some of the common signal conditioners that appear after the input transducers. The output signal is normally conditioned by a low-pass filter and some type of power amplifier.

Switch Debouncing

When mechanical switches are opened or closed, there are brief current oscillations due to mechanical bouncing or electrical arcing. This phenomenon is known as switch bounce as illustrated below:

A single closing of a switch can result in multiple voltage transitions that usually occur within a few milliseconds.

The sequential logic circuit shown in Figure (b) below can provide an output that is free from multiple transitions associated with switch bounce.

As the switch breaks contact B, signal bounce occurs on the B line. There is a small delay as the switch moves from contact B to A, and then the signal bounce occurs on the A line as the contact is established with A. But as a result of the feedback and logic, the output Q experiences only a single transition from low to high hence the output is bounce free.

Operational Amplifiers for Gain, Offset and Function Modification

An operational amplifier can be employed to perform the following functions as a means of signal conditioning:

• Increase the amplitude of the signal.
• Filter the signal.
• Decrease the signal output impedance.
• Provide a variable gain and offset control [this provides a useful way of calibrating a transducer’s output signal].

The dynamic range of a signal from an input transducer may be too large to process through the data acquisition system for example the analogue to digital converter (ADC) is often the limiting factor. You can use a linear amplifier and choose to overflow or reduce the overall gain. The latter approach will cause a loss in precision. Another method is to use a nonlinear amplifier with a logarithmic gain, V_OUT = log (V_IN).

Sample and Hold Amplifiers

The objective of the sample and hold amplifier is to free an analog at the instant the HOLD command is issued and make the analog voltage available for an extended period. The sample and hold circuit shown below consists of a voltage-holding capacitor and a voltage follower with switch closed.

V_{out (t –t sampled)} = V_{in (t sample)}

Where t sampled is the time when the switch was last opened. Often, an op amp buffer is also used on the V_in side of the switch to minimize current drawn from the input voltage source V_in.

The type of capacitor for this application is the key factor to consider. A low-leakage capacitor such as polystyrene or polypropylene type is a good choice. An electrolytic capacitor would be a poor choice because of its high leakage. This leakage would cause the output voltage value to drop during the ‘’hold’’ period.

Gated charge to voltage amplifier

The gated charge to voltage amplifier is designed specifically as an integrating amplifier to measure the area under a narrow pulse.  Its capacitor must be discharged before a new sample is taken. If the initial charge on the capacitor is zero, then the output voltage from the amplifier follows the gate signal.

This sampling amplifier is usually used with pulsed signals when the area under the signal is of primary interest. The entire signal is integrated and the output is insensitive to the details of the signal shape.

If the signal pulse rides on a relatively constant but non-zero offset voltage, the effect of the offset can be determined by generating a gate when no signal pulse is present. The resulting output voltage is known as a pedestal and can be subtracted from the data signal at a later point in the system.

Comparator

A comparator is used to determine whether one, signal is greater than the other. It is a single bit analog to digital converter. The comparator is very similar to an operational amplifier but has the digital true/false output.

The comparator is an example of an op amp circuit where there is no negative feedback and the circuit exhibits gain. The result is that the op amp saturates. Saturation implies that the output remains at is most positive or most negative output value.

Oscillators

Constant frequency square wave oscillators provide a repetitive signal that can be used as timing reference for various logic or control functions.

Application to Interval Timers

With increasing system complexity, the need may arise for several repetitive timing signals with different periods. If each timing signal is obtained from a separate oscillator, the signals will have a random and variable phase relationship. They will be asynchronous and may lead to hitches. A better technique is to use one high frequency oscillator with a short period and from it, derive all longer period signals. If the longer period is a multiple of two of the clock period, a counter with flip-flops can be used. If the period is not a multiple of two of the clock period a divide-N counter can be used.

Analogue to Digital Conversion

The analogue to digital converter (ADC) is used to convert an analogue voltage to a digital number.

Analogue to digital converters are designed based on a number of different principles i.e.

• Successive approximation
• Flash (Parallel encoding)
• Single slope
• Dual slope integration

Successive Approximation ADC

The successive approximation ADC is the most commonly used design because it is relatively fast and cheap.

The analogue output of a high speed digital to analogue converter (DAC) is compared against the analogue input signal. The digital result of the comparison is used to control the contents of a digital buffer that both drives the DAC and provides the digital output word.

Description of the Operation of Successive Approximation ADC

Using Figure (d) above as our reference, the operation of the ADC is as follows: When the start signal is applied, the sample and hold (S & H) amplifier latches the analogue input, then the control unit begins an iterative process, where the digital value is approximated, converted to an analogue value with D/A converter and compared to the analogue input with the comparator. When the D/A converter output is equal the analogue input, the end signal is set by the control unit, and the correct digital output is available at the output.

The successive approximation ADC uses fast control logic which requires only n comparisons for an n-bit binary result.

Parallel Encoding ADC (Flash ADC)

The parallel encoding or flash ADC provides the fastest operation at the expense of high component count and high cost.

The resistor network sets discrete thresholds for a number of comparisons. All comparators with thresholds above the input signal go false while those below go true. Then the digital encoding logic converts the result to a digital number.

Dual Slope ADC

The limitations associated with DAC in a successive approximation ADC can be avoided by using the analog method of charging a capacitor with a constant current, the time required to charge the capacitor from zero to the voltage of the input signal becomes the digital output. When charged by a constant current the voltage on a capacitor is a linear function of time and this characteristic can be used to connect the analogue input voltage to the time a determined by a digital counter.

Digital to analogue Conversion

The process of converting a number held in a digital register to an analogue voltage or current is achieved with a digital to analogue converter (DAC). The DAC works as interface between a computer and an output transducer.

The simplest type of D/A converter is a resistor ladder network connected to an inverting summer op amp circuit.

This specific converter is a 4 bit R-2R resistor ladder network which requires only two precise resistance values R and 2R. The digital input to the DAC is a 4 bit binary number represented by bits b₀, b₁, b₂, b₃, where b₀ is the least significant bit (LSB) and b₃ is the most significant bit (MSB).  Each bit in the circuit controls a switch between ground and the inverting input of the op amp.

DAC Limitations

Common DAC limitations are:

• An anomalous step size between adjacent binary numbers.
• Non-monotonic behaviour.
• Zero output.