Analog to Digital Multiplexing

Multiplexing before A/D conversion with single S/H circuit

The multi-channel DAS has a single A/D converter preceded a multiplexer
In analog to digital conversion, it is convenient to multiplex analog inputs rather than the digital output. There are three ways f analog to digital multiplexing as discussed below.

1).Multiplexing before A/D conversion with single S/H circuit.

2.) Mutliplexing before A/D conversion with individual S/H circuit.

3). Multiplexing after A/D conversion.

The individual analog signals are applied directly or after amplification and/or signal conditioning, whenever necessary to the multiplexer. These are further converted to digital signals by the use of A/D converters sequentially. When the conversion is complete, the status line from converter causes the sample/hold to return to the sample mode. Acquires the signal of the next channel on completion of acquisition either immediately or upon command, the S/H is switched to hold mode, a conversion begins again and multiplexer selects next channel. This method is relatively slower than systems S/H outputs or even A/D converter outputs are multiplexed, t has the advantage of low cost due to sharing of a majority of a systems.

Multiplexing before A/D conversion with individual S/H

When a large number of channels are to be monitored at same time but at moderate speed, the technique of multiplexing outputs of S/H are particularly attractive. The simultaneous sampled system multiplexer. An individual S/H is assigned to each channel and is updated synchronously by a timing circuit. The S/H outputs connected to an A/D converter through a multiplexer, resulting in a sequential readout of the outputs.

Multiplexing after A/D conversion

The block diagram of the multichannel DAS using digital multiplexing .In this each analog input signal is given to an individual sample and hold circuit and A/D converter. This type of DAS is used in industrial data acquisition systems, where many strain gauges, thermocouples and LVDT are distributed over large plant area. The outputs of A/D converters are given to the digital multiplexer through the processor and buffer circuits.

Digital to Analog Multiplexing

In a data acquisition system, it is often necessary to combine, or multiplex a number of analog signals into a single digital channel or conversely a single digital channel into a number of analog channels: Both digital signals and analog voltage can be multiplexed. So, there are two ways of multiplexing: digital to analog multiplexing and analog to digital multiplexing.
In digital to analog ‘conversion a very common application of multiplexing is found in computer technology, where digital information arriving sequentialy from the computer is distributed to a number of analog devices, such as an oscilloscope, a pen recorder, an analog tape recorder and so on. There are two ways to accomplish multiplexing. The first method uses a separate D/A converter for each channel. The second method uses one single D/A converter together with a set of analog multiplexing switches and sample-and hold circuits on each analog channel. Information arriving sequentially from the computer is distributed to a number of analog devices, such as an oscilloscope, a pen recorder, an analog tape recorder and so on. There are two ways to accomplish multiplexing. The first method uses a separate D/A converter for each channel. The second method uses one single D/A converter together with a set of analog multiplexing switches and sample-and hold circuits on each analog channel

The digital information is applied simultaneously to all channels, and channel selection is made by gating clock pulse to the appropriate output channels. One D/A converter is required per channel, so that the initial cost may be
 

Some what higher than the second system, but the advantage is that the analog information is available at the DAC output for an indefinite period of time. The second method uses only one D/A converter and is therefore slightly lower in initial cost. The multiple sample-and-hold technique however requires that the signal on the sample-and-hold circuits be renewed at periodic intervals.

Frequency to Voltage converter

A frequency to voltage converter provides an analog voltage proportional to the frequency of the input signal. The circuit is often a precision charge dispenser, wherein a capacitor is charged to a predetermined level and the stored charge is discharged into an integrator or low pass circuit for every cycle of the input waveform. In this the monostable multivibrator together with the precision switch (inverted t switch) that follows it, generates a pulse of precise amplitude VR and precise period feeding into an averaging network. The final output is a DC voltage proportional to the average of the input frequency.

Voltage to Frequency Converter | Analog Devices

This converter consists of an integrator that feeds a comp which in turn drives a one shot multivibrator. An electronic switch discharges the integrator via a current source. A voltage to frequency converter using op-amp.

The waveform associated with the voltage to frequency converter .The input voltage Vin causes the integrator output to ramp in the negative direction. If integrator output starts from some positive voltage, the output reach zero voltage and the one shot multivibrator will provide an output pulse. Using this zero voltage of the integrator as the starting. For the investigation of the V/F converter, the integrator output be ramp n the positive direction for a time equal to t2 this

Level converters / Signal converters In a Signal Conditioning System

The output of the system transducer might be in one form (i.e., change in voltage or resistance change) while the readout device might require the signal in another form (eg: 4 to 20 mA). This requires some form of signal conditioning which will suitably alter the signal to make it compatible for interaction with subsequent system elements.
The level/signal conversion either changes the output signal of the transducer from one voltage or current level to another or from one form to another. This is frequently done for compatibility with either the transmission medium in use or the levels required by the instruments being used. In instrumentation systems, signals are frequently transmitted as a current level rather than a voltage level. Using a current level technique eliminates the effects of voltage drop. Consequently such a system requires the transducer output signals to be converted to the proper current level at the sending end (V to I converter) and back to the compatible form (I to V converter) with the instruments at the receiving end.

Voltage to current converter

A voltage to current converter for a 4 20 mA current loop. Two voltage controlled current sources are in the converter. One current source senses the power supply current for the amplifier plus the current from the voltage-controlled

current source, I2 and sets the sum to equal 4 mA. The second voltage-controlled current I1 provides a variable current as a function of the transducer voltage as provided by the instrumentation amplifier. This second voltage-controlled current source provides from’ 0 to 16 mA for a total current from 4 to 20 mA.


Current to voltage converter 

Photo diode and photo multiplexer tubes provide output current proportional to the light flux, but independent of the load impedance. An op-amp which works as current to voltage converter. Current source with shunt source resistance R is applied at the inverting terminal of the op-amp. Virtual ground at the input makes the current flow through the feedback resist R resulting in the output voltage Vo=IsRf Often a capacitor C is placed in shunt with R to reduce the high frequency noise.

Filter & Linearization In signal conditioning system

A network designed to attenuate certain frequencies but pass others without attenuation is called a filter. In measurement system the transducers often does not measure the physical parameter precisely. The information present is not in standard form, h the need for further filtering and analysis. Filters can be designed to reject signals over specific desired frequency ranges like low 1 filters, high pass filters, notch filters etc.

The filter circuits can be implemented by using only resistors, capacitors and inductors called passive filters or using active devices such as transistors, op-amps called active filters. The passive low pass filter. (LPF), high pass filter (H PF) and their respective frequency response curves.

For LPF at low frequencies the capacitive reactance is very high and the capacitor circuit can be considered as an open circuit. Under this condition, the output equals the input or voltage gain is equal to unity. At very high frequencies, he capacitive reactance is very low and the output voltage v i small as compared to the input voltage V Hence as the frequency is increased the gain falls and drops off gradually. In high pass filter at low frequencies the capacitive reactance is high, the output is minimum and the gain is small. When frequency is high, the capacitive reactance is small; the output equals the input and. the gain approaches unity. Hence this circuit passes high frequencies while rejects low frequencies.

The cut-off frequency for LPF or, HPF is given by

Linearization


    In many cases, the proportionality that exists between the input variable t& the transducer and its output signal, is non-linear. The readouts or recorders f systems ate generally designed to respond to signals which were assumed to be ‘linear, so the actual nonlinearities cause errors in the measured data. To reduce these errors, the out of the transducer can be linearized, before it is passed into instruments and recorders. This can be done either with analog circuitry or by a computer. In analog linearization, the signal is passed through a circ that has a response which is the inverse of the transducer. Example, if the transducer has an exponential response, its output signal might be passed through an amplifier circuit that h logarithmic response, producing an output that is the linear of the measured variable. Alternatively, the signal could be dig after which a digital computer could be used to generate logarithm of the input signal.

Instrumentation Amplifier

The operational amplifier is mostly used for arnplification, as it has very high gain and wide bandwidth. The low level signal outputs of electrical transducers often need to be amplified before further processing. This is done by the use of instrumentation amplifier. The important features of instrumentation amplifiers are

1. Selectable gain with high gain accuracy and gain linearity

2. Differential input capability with high gain CMRR.

3. High stability of gain with low tempera co-efficient.

4. Low DC offset and drifts errors referred to input.

5. Low output impedance.

    The instrumentation amplifier is often a package consisting of op-amps wired up with accurate and stable resistive feedback to give a desired gain. The instrumentation amplifier can be used directly to amplify the signals by a fixed factor because of its closed-loop configuration.

The gain accuracy, gain stability and drift performance are normally specified by the vendors, hence there is no effort required in choosing the input and feedback configuration. Depending on the configuration employed, the instrumentation amplifier will often yield a high CMRR even when the source impedance exceeds 1 MQ and when the impedance is unbalanced for a large value of variations (1 to 10 K  In actual applications, an instrumentation amplifier is a specific combination of a suitable DC operational- amplifier wired up, with feedback.

The schematic diagram of an instrumentation amplifier constructed with DC op-amp and resistive network

Many of the input specifications of op-amps are directly to determine the input specifications of instrumentation amplifiers.

     Assume

The input amplifiers A1 and A2 act as the input buffers with unity gain for common mode signal ec and with gain of (1+2R2 /R1 ) for differential signals. The high input impedance is ensured by the non-inverting configuration in which they operate. The common- mode rejection is achieved by the following stage which is connected as a differential amplifier.

The optimum common-mode rejection can be obtained by adjusting R6 or R7 ensuring that R5 /R4 = R7/R6 The amplifier A3 can also be made to have some nominal gain for the whole amplifier by an appropriate selection of R4 R5 R6 and R7

    To reduce the pickup of noise voltages in the connections between the transducer and the amplifier, the leads to the transducer are kept as short as possible and the amplified signal is transmitted to the required distance. There are situations where the low level transducer must be transmitted through some length of wires. Noise currents are introduced in these lines. There are methods of shielding the connecting wires from external signal pickup. One effective method is called guarding; A signal source connected to a differential amplifier through wires that are in close proximity to a 220V. Power lead, which capacitive couples power line frequency voltages into the signal amplifier. If the amplifier is perfect, the voltage induced in each lead is the same and the capacitance and resistance paths to the ground are identical. Thus the power line interference is connected to the ground with identical currents from each side of the line.

If the leakage resistance or capacitance is different for c relative the currents t instrument ground are from only one side of the differential line. By the addition of a shield which is connected to one side of the signal and to the instrument case, the capacitive coupled signal from the power frequency line are coupled to the shield and are safely conducted to the instrument case and to the ground. The combination of the instrument case, its ground connection and the shield extending to the signal source represents a complete Shield around the entire measuring system.

    There are situations where noise environment is so severe that conventional amplifiers cannot survive the signal levels encountered. In these situations, an isolation amplifier is used to prevent the high noise signals from being conducted to the data acquisition equipment.

Isolation amplifier

The transducer is connected in a rather conventional fashion to an Instrumentation amplifier. The output of this amplifier is fed to a balanced modulator which provides a bipolar square wave with amplitude Proportional to the signal level. The high frequency square wave is called the carrier. The modulated square wave being an AC signal with no DC level can be coupled through a transformer to a balanced demodulator. The square-Wave signal generator transformer coupled to serve as the carrier for the demodulator which removes the carrier and restores the input level. After small amount of filtering, the output of the isolation amplifier is an accurate representation of the input voltage.

Signal Conditioning System

The signal conditioning equipment is required to perform line processes such as amplification, attenuation, integration differentiation, addition or subtraction They are also require non-linear processes such as modulation, demodulation, s filtering, clipping and clamping, squaring and linearising or  multiplication by another function. These functions require selection of components and faithful reproduction of the final c for the presentation stage.

    The signal conditioning of a data acquisition equipment is in  many cases an excitation and amplification system for p transducer or an amplification system for active transducer. In both cases, the transducer’s output is brought to the required level it useful for conversion, processing, indicating and recording.

Block diagram of a signal conditioning system

The transducer is connected to one arm of the bridge and the signal is transferred to an instrumentation amplifier after calibration. Instrumentation amplifier is followed by a filter, which is used to eliminate noise from the signal.