Connecting Analog

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  Copyright © 2007 DynamicSignals LLC, 900 N. State Street, Lockport, Illinois 60441-22001 Analog Front-End Most data acquisition systems involve obtaining data from various transducers that produce analog signals. Often, the signals from the transducers are low level and requirevarious kinds of signal conditioning. Also, the transducers are frequently located somedistance from the data acquisition front-end. Ensuring proper connections to the analogsignal of interest and ground are essential to obtaining accurate measurements. 4.1 Proper Connections to the Sensors  Measuring Data and Not Noise   4.1.1 Differential vs. Single-ended Input Channels  Two very important concepts that affect the performance of a data acquisition system are single-ended  and  differential. A single-ended input channel, as shown in Figure 4.1(a),completes the circuit from a sensor to the data acquisition (DAQ) system input circuit viaa signal wire and a return wire. The return wire is usually the cable shield and is generallyconnected to the DAQ system's circuit common—which is generally connected to ground. In an ideal world, this should not present a problem. Unfortunately, the world is filled withmany noise sources that can interfere with data. The problem with single-ended inputcircuits is that the cable shield is part of the signal path—  and any noise voltage developed across the shield adds full-force to the signal!  Figure 4.1: A single-ended data acquisition input channelFigure 4.1(b) shows a sensor that is connected to ground and is wired to a grounded   Copyright © 2007 DynamicSignals LLC, 900 N. State Street, Lockport, Illinois 60441-22002 single-ended input. If there is 100 millivolt ac potential difference between the sensor's ground and the DAQ system's ground, then all of the 100 millivolt noise will besuperimposed on the signal. If the sensor is a thermocouple, the noise is likely larger than thedc signal voltage. Even if the sensor has no connections to ground, the data acquisitionsystem must be carefully designed so all connection points are very close to the same potential, or errors will be introduced to the received signal of the various channels. Also, if the input wiring is close to sources of electrical noise, interference may be coupled into thesignal path.A differential-input channel—often called a balanced input   —is generally connected toa sensor as shown in Figure 4.2(a). A shielded twisted pair  cable is most often used for differential operation. The signal is received by an instrumentation amplifier. The primary characteristic of an instrumentation amplifier is that it delivers a signal at itsoutput that is proportional to the difference  between the voltage on its + input and its - input.A real-world example of a differential-input channel is shown in Figure 4.2(b). Notethat, in this case, the signal circuit is completed  without any signal passing through theshield. If a noise voltage is impressed across the shield because of a ground potentialdifference, the effect on the signal will be greatly attenuated. The characteristic of thecable can cause interference to creep into a differential system with long cable runs. Theshielded cable must contain a twisted pair. Some single-pair cables have both conductorsin the same plane instead of having the pair of wires twisted. If this cable is near a strongsource of electrical noise, one signal wire will be nearer to the noise than the other, and thenoise will not cancel as well. Also, some two-pair cables with separate shields do notcontain pairs that are twisted. Systems using such cables can exhibit unacceptablecoupling—called crosstalk—between the channels, even though each pair is shielded.Figure 4.2: A differential data acquisition input channel  Copyright © 2007 DynamicSignals LLC, 900 N. State Street, Lockport, Illinois 60441-22003 4.1.2 Common Mode and Normal Mode Common mode rejection ratio—often abbreviated CMRR—describes the effect that un-wanted noise between the signal conductors and ground has on the desired input signal. Itis called  common mode because the unwanted signal (often caused by power line noise) isimpressed across both conductors of a differential pair and—in the ideal case—is cancelled out by the balanced system. CMRR is generally measured as shown in Figure 4.3. The testvoltage is impressed across both conductors of the differential input. Often this test uses a1000-ohm resistor in one leg to represent the fact that the real transducer source may not be perfectly balanced. CMRR is usually expressed in decibels (dB). In this case the dBvalue represents a logarithmic voltage ratio between the output signal caused by a common-mode voltage and that from a normal-mode voltage. A normal-mode voltage is that impressed across the input conductors. For single-ended systems with the shield conductor connected toground at the instrumentation front-end, the ratio between common mode and normal modeis 1:1—  no common-mode rejection.  Figure 4.3: Measuring Common Mode Rejection RatioEach 20 dB represents an increase in the common mode rejection ratio by a factor of 10. Therefore a CMRR of 80 dB represents an attenuation of common-mode noise equal to10,000 to 1. Another important parameter here is the maximum linear input swing on theinput circuit. This value is often ±10 volts. With a CMRR of 80 dB, a 10 volt RMS (RootMean Square) common-mode signal should have an effect on the input signal of 2millivolts (20/10,000). However, if this is a sine wave, the voltage will reach positiveand negative peaks of about 14 volts. This will likely be outside the linear range of theinstrumentation amplifier and will result in substantial feed-through during portions of eachcycle.As just discussed, the value of CMRR determines how much common-mode noise getsconverted to normal-mode signal. Some amount of noise may also enter the system asnormal-mode signal, caused by noise pickup at the transducer, etc. For whatever cause,once an unwanted signal becomes a normal-mode voltage, it can be eliminated only byfiltering. This is usually accomplished by low-pass filtering. For slowly changing signals,such as thermocouples, this is often accomplished by one- or two-pole passive (resistor-capacitor) filters connected directly to the input pair before any electronics. The cutoff frequency for these filters is often in the range of 2 to 10 Hertz to provide good normal-modeattenuation to power line frequency and its harmonics.  Copyright © 2007 DynamicSignals LLC, 900 N. State Street, Lockport, Illinois 60441-22004 For fast-changing signals, any attempt to attenuate 50 or 60 Hz power line frequencieswould prevent these changes from being monitored by the data acquisition system. In thiscase, sufficient precautions—such as proper grounding and good CMRR—must be taken to prevent noise from becoming normal-mode in the first place. 4.1.3 Isolation As indicated earlier, most instrumentation front-ends require that each conductor of adifferential input—and the signal conductor of a single-ended input—remain within about±10 volts of the input common ground. This is not satisfactory for input signals with largecommon-mode voltage present. An example of this is the measurement from a current shuntthat is several hundred volts above ground. An isolated input circuit is appropriate for suchan application. This isolation can be built into the data acquisition system or consist of anisolation block in front of the DAQ system. In either case, the most common practice is to usean isolation amplifier for this purpose. An isolation amplifier usually uses a dc-to-dc-converter-powered floating amplifier that produces a high-frequency signal whose dutycycle is proportional to the input voltage. This high-frequency signal is then coupled acrossan isolation barrier and filtered. The resulting signal is proportional to the input voltage and iscoupled via the output amplifier. An isolated input circuit monitoring a floating shunt isshown in Figure 4.4. The advantage of such an input circuit is that it can accommodatehigh common-mode voltages. Isolated input channels result in a significant increase in per-channel cost. Frequency response is usually limited to about 30 kilohertz. Isolated inputcircuits are usually specified only when a non-isolated approach cannot be used.Figure 4.4: An isolated data acquisition input circuit 4.1.4 Grounding—Bad Ground Loops vs. Good Ground Loops The subject of good grounding practices usually causes more arguments than any other aspects of high-performance data acquisition. Also, it is generally felt that ground loopsshould be avoided. The purpose of this section is to de-mystify the issue of grounding and show that there are bad   AND good    ground loops!A good wiring practice for a voltage-input channel is shown in Figure 4.5(a). For thiscase, the sensor is not grounded and the cable shield is connected to the midpoint of thesensor as well as to the ground connection at the data acquisition system. This shield connection also meets the requirement of most instrumentation front-ends—  there must be
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