Chapter 6

Chapter 6

Chapter 6

General Amplification

General Amplification

55

Chapter 6 General Amplification

General Amplification

Data acquisition systems use many amplification circuits to accomplish their tasks. The versatility of operational amplifiers makes them the universal analog building block for signal conditioning. In this chapter, some uses of amplifiers in data acquisition equipment are discussed.

Data Acquisition Front Ends

Data acquisition systems differ from single channel measurement instruments because they are capable of measuring multiple channels. A voltmeter with a selector switch to allow measurement of multiple voltages is a very simple data acquisition system. The high degree of operator interaction required limits the performance of this simple system, but the function is the same.

Analog inputs

Mux

IA

ADC

Figure 6.01 illustrates a simple data acquisition system which consists of a switching network (MUX), an instrumentation amplifier (IA) and an analog-to-digital converter (ADC). These three sections allow measurement of multiple voltage channels using a single A/D. The individual circuit blocks each have unique capabilities and limitations which together define the performance of the system.

Digital data

Fig. 6.01: Simple data acquisition system

Switching Networks

The switching network in an automated data acquisition system can be either relay or solid-state, depending primarily on channel-to-channel switching speed and isolation requirements. In high-speed systems, defined as those switching channels more than

200 times per second, solid state switching is used. Solid state switching networks are configured with analog switches or multiplexers and are capable of switching channels over 1,000,000 times per second. Some drawbacks to solid state switches are input voltage limitations, on-resistance, charge injection, and crosstalk.

Input Voltage Limitations. Input voltage limitations of analog switches are generally defined by the connected power supply voltages, usually plus and minus 15V measured with respect to analog common. There are also “protected” devices which can with-

56

Chapter 6

stand somewhat higher voltages, up to 30-50 volts. The voltage limitations of analog switches and multiplexers also depends on whether or not the devices are powered at the time the input voltages are applied. As a general rule, it is better to have the analog data acquisition system powered before connecting to live measurement voltages.

On-resistance. All switches have on-resistance. An ideal switch, which is closely approximated by a mechanical switch or relay contact, has zero resistance when the switch is closed. A reed relay contact can be less than 10 milli-ohms. A good analog switch can have on-resistance of 10-100 Ohms, while an analog multiplexer can have 100-2500 Ohms of on-resistance per channel. The on resistance adds directly to the signal source impedance and can affect measurement accuracy.

Charge injection. Charge injection is an undesirable characteristic of analog switching devices, which couples a small portion of the input gate drive voltage to the analog input and output signals.

From the standpoint of the measurement, this glitch in the output signal can cause error and will be measured on the input signal if the source impedance is not low. There are circuit design techniques which can be used to minimize the effects of charge injection, but the most effective method is to keep source impedance as low as possible.

Drive signal

Mux output

Fig. 6.02: Effect of charge injection on a mux output

Channel-to-channel Crosstalk. Channel-to-channel crosstalk is another non-ideal characteristic of analog switching networks, especially IC multiplexers. Crosstalk occurs if the voltage applied to any one channel affects the accuracy of the reading from another channel. Conditions are especially optimum for crosstalk when signals of relatively high magnitude and high frequency are being measured, such as 4V to 5V signals connected to channel one while 100 mV signals are on an adjacent channel. Multiplexing at high frequencies also exacerbates crosstalk because of capacitive coupling between switch channels. Again, the best defense from crosstalk is keeping source impedances low.

Instrumentation Amplifiers

The block following the switching network in a data acquisition system is an amplification block. This stage uses an amplifier configuration called an instrumentation amplifier and has several important functions. These functions include rejection of commonmode voltages, signal voltage amplification, minimizing the effect of multiplexer on resistance, and properly driving the ADC input.

57

General Amplification

Chapter 6 General Amplification

V s

V signal

V common-mode

+

V out

= A v

V signal

Common mode voltage is the voltage measured relative to analog common which is common to both input voltages. The two input voltages could be

4.1V and 4.2V, for example; the common mode voltage is 4.1V and the differential voltage between the two is

0.1V. Ideally the instrumentation amplifier (IA) ignores the common mode voltage and amplifies the difference between the two inputs. The degree to which the amplifier rejects common

Fig 6.03: Typical instrumentation amplifier

mode voltages is given by a parameter called common-mode rejection ratio

(CMRR). The CMRR is expressed in decibels (dB’s); a ratio of 10 is 20 dB and a ratio of 100 is 40 dB. Each successive ratio of

10 adds 20 dB. If an instrumentation amplifier with a CMMR of 80 dB is measuring a 20 mV signal in the presence of a 4V common mode voltage, the error due to the common mode voltage would be 0.4 mV, which is still 2% of the signal. The ability of an IA to reject “high” common mode voltages is sometimes misinterpreted as an ability to reject high voltages. What is really being rejected is a voltage level having a high magnitude relative to a small signal voltage, such as 4V relative to 20 mV.

Signal voltage ranges are frequently smaller than the range of the system ADC. For example, a 0-100 mV signal is much smaller than the 0-5V range of a typical ADC.

To achieve the maximum practical resolution for the measurement, a gain of 50 is needed. Instrumentation amplifiers are capable of gains in this range. In fact, instrumentation amplifiers are capable of gains ranging from unity to well over 10,000 in some specific applications. In multiplexed systems, the gains of IA’s are usually restricted to the range of 1-1,000.

There is a special class of instrumentation amplifiers with programmable gain which can switch between fixed gain levels at sufficiently high speeds to allow different gains for different input signals delivered by the input switching system. These amplifiers are called programmable gain instrumentation amplifiers (PGIA). The same digital control circuitry which selects the input channel can also select a gain range allowing a high degree of flexibility in the system.

58

Chapter 6 General Amplification

The non-ideal on resistance of analog switches will add to the source impedance of any signal source to create measurement error. This effect is minimized by the extremely high input impedance of the IA. The input stage of an IA consists of two voltage followers, which have the highest input impedance of any common amplifier configuration.

The high impedance and extremely low bias current drawn from the input signal create a minimal voltage drop across the analog switch sections resulting in a more accurate signal reaching the input to the IA.

The output of the instrumentation amplifier has a low impedance which is ideal for driving the ADC input. The typical ADC does not have high or constant input impedance. It is important for the preceding stage to provide a signal with the lowest impedance practical.

Instrumentation amplifiers have some limitations including offset voltage, gain error, limited bandwidth, and settling time. The offset voltage and gain error can be calibrated out as part of the measurement, but the bandwidth and settling time are parameters which limit the frequencies of amplified signals and limit the frequency at which the input switching system can switch channels between signals. A series of steady DC voltages applied to an instrumentation amplifier in rapid succession creates a difficult composite signal to amplify. The settling time of the amplifier is the time necessary for the output to reach final amplitude to within some small error (often 0.01% ) after the signal is applied to the input. In a system scanning inputs at 100 kHz, the total time spent reading each channel is 10

µ

S. If analog-to-digital conversion requires 8

µ

S, settling time of the input amplitude to the required accuracy must be less than 2

µ

S.

Although it has been stated that offset voltage and gain error can be calibrated out, it is important to understand the magnitude of the problem to allow good decisions as to when these error corrections are necessary. An IA with an offset voltage of 0.5 mV and a gain of X2 measuring a 2V signal will only incur an error of 1 mV in 4V on the output which is 0.025%. An offset of 0.5 mV and a gain of X50 measuring a 100 mV signal will incur an error of 25 mV in 5 volts or 0.5%. Gain error is similar. A stage gain error of

0.25% will have a greater overall effect as gain increases resulting in larger errors at higher gains and minimal errors at unity gain. System software can generally handle known calibration constants with mx + b routines but some measurements are not critical enough to justify the effort.

59

Chapter 6 General Amplification

Analog-to-Digital Converters

The analog to digital converter stage is the last link in the chain between the analog domain and the digitized signal path. In any sampled data system, such as a multiplexed data acquisition system, there is a functional need for a sample-hold stage preceding the ADC. The ADC cannot correctly digitize a time varying voltage to the full resolution of the ADC unless the voltage changes very slowly. Some ADCs have internal sample-holds or use architecture which emulates the function of the sample-hold stage.

For the purpose of this discussion, it is assumed that the ADC block includes a suitable sample-hold circuit to stabilize the input signal during the conversion period.

The primary parameters of ADCs in data acquisition systems are resolution and speed.

Data acquisition ADCs typically run at speeds from 20 kHz to 1 MHz and have resolutions of 12- or 16-bits. ADC’s have one of two types of input voltage range, unipolar or bipolar. The unipolar type range typically runs between 0 volts and some positive or negative voltage such as 5V. The bipolar type range will typically run from a negative voltage to a positive voltage of the same magnitude. Many data acquisition systems have the capability of reading bipolar or unipolar voltages to the full resolution of the

ADC, which necessitates some sort of level shifting stage to allow bipolar signals to use unipolar ADC inputs and vice versa. A typical 12-bit, 100-kHz ADC has an input range of -5V to +5V and a full scale count of 4,096. Zero volts corresponds to nominally 2,048 counts. If the range of 10 volts is divided by the number 4,096, the LSB (least significant bit) magnitude of 2.44 mV is obtained. A 16-bit converter with the same range and

65,536 counts has an LSB value of 153

µ

V.

Signal Conditioning Amplification

Many real world sensors have very low signal levels. These levels are too low for direct application to multiplexed data acquisition system inputs with relatively low gains. Two common examples are thermocouples and strain-gage bridges which both deliver full scale outputs of less than 50 mV. There are two broad classes of signal conditioning circuits, multiplexed and dedicated. In multiplexed conditioners, multiple input sources are sequentially switched through one amplifier signal path allowing for an economical cost per channel with a somewhat reduced performance. The more expensive dedicated type have amplifier stages for each signal path and multiplex the high-level signals only.

Most amplifier stages are applications of one of two basic configurations, inverting or noninverting. See figure 6.04 for an illustration of each. Each configuration has a simple equation which provides the idealized circuit gain as a function of the input and feedback resistors. There are also special cases of each configuration which make up the rest of the fundamental building blocks, namely the unity gain follower and the difference amplifier.

60

Chapter 6 General Amplification

R

2

+

R

1

+

Inverting

Gain = -

R

2

R

1

R

1

Gain =

R

2

+1

R

1

R

2

Non-Inverting

Fig. 6.04: Inverting and non-inverting amp

Source Impedance &

Multiplexing

MUX

R

Multiplexing and a high source impedance do not mix well. The reason that low source impedance is always important in a multiplexed system can be explained with a simple RC circuit, see figure 6.05. Multiplexers have a small parasitic capacitance to analog common on all signal inputs as well as out-

R source

R on

+

AMP puts. These small capacitance values, when combined with source resistance and fast sampling time, affect measurement accuracy. A simple RC equiva-

Fig. 6.05: Simple RC circuit

lent circuit consists of a DC voltage source with a series resistance, a switch, and a capacitor. When the switch closes at t = 0, the voltage source charges the capacitor through the resistance. When charging 100 pF through 10 kOhm, the RC time constant is 1

µ

S. In a 10

µ

S time interval of which 2

µ

S is available for settling time, the capacitor will only charge to 86% of the value of the signal, introducing a major error.

If the same calculations are done with a 1,000 Ohm resistor, the capacitor will easily charge to an accurate value in 20 time constants.

Voltage Measurement Errors

One must consider several factors when making voltage measurements. This section will discuss a few of these, including: input and source impedance, differential voltage measurements, and isolation considerations.

61

Chapter 6 General Amplification

R s

V sig

Transducer

R i

V

ADC

=

R i

R s

+ R i

V sig

Fig. 6.06: Input and source impedances

Input and Source Impedances

Figure 6.06 depicts how system input impedance and the transducer’s source impedance combine to form a voltage divider, which reduces the voltage being read by the ADC. The input impedance of most input channels is 1M

Ohm or greater, so it is usually not a problem if the source impedance is low. However, some transducers (piezoelectric transducers, for example) have high source impedances and should be used with a special chargesensitive amplifier. In addition, multiplexing can greatly reduce a data acquisition system’s effective input impedance. See Chapter 3 for detailed discussion.

Attenuators & Multiplexing

V

IN

R

A

R

B

+

V

OUT

There is a basic voltage range of 5V to

10V associated with most data acquisition inputs. Voltages over 10V are generally not readable without attenuation. Simple resistive dividers can easily attenuate a voltage, which is too high for a 5V input, but there are two drawbacks complicating this simple

V

OUT

=

R

B

R

A

+ R

B

V

IN scheme. Voltage dividers have a substantially lower impedance to the source than direct analog inputs, and, they have very high output impedance

Fig. 6.07: Buffered attenuator

to the multiplexer input. Consider this example, a 10:1 divider reading 50V.

If a 900 KOhm and 100 KOhm resistor are chosen, to provide a reasonable 1 MOhm load to the source, the impedance seen by the analog input is a disagreeable 90.0 KOhm which will never allow an accurate multiplexed reading. If the values are both downsized by a factor of 100 to allow the output impedance to be lower than 1,000 Ohms, the input impedance seen by the measured source is 10 KOhm. Most voltage measurement applications could not tolerate a 2K Ohm/volt instrument. Hence the conclusion: simple attenuation is not practical with multiplexed inputs.

62

Chapter 6 General Amplification

Attenuators & Buffers

The answer to the voltage divider problem is unity gain buffering of the voltage divider output. A dedicated unity gain buffer does not have a source impedance problem with 90 KOhms. The output impedance is very low to the multiplexed analog input as well.

Inputs

IA

LPF Buffer

Mux

To ADC

IA LPF Buffer

Low-Pass Filters

Low-pass filters attenuate higher frequencies to varying degrees depending on the number of stages and the magnitude of the high frequency

Fig. 6.08: Filtered/buffered data acquisition system

relative to the corner frequency. There is no need for high bandwidth in an amplifier stage if the signal of interest is of a much lower frequency. In fact it is a basic intent of this design to eliminate excessive bandwidth in all circuits in order to limit noise. One of the major benefits of individual signal conditioning stages for low-level sensors, as opposed to multiplexed stages, is the opportunity to include low-pass filtering on a per-channel basis in the signal path. In a multiplexed circuit (an amplifier which is being shared by multiple low level DC signals), the main signal path generally cannot lowpass filter due to the fast settling time necessary in multiplexed systems.

R

The best place for low pass filtering is in the individual signal path prior to buffering and multiplexing. For very small signals, amplifying with an instrumentation amplifier prior to filtering allows an active low pass filter to operate at levels of optimum signal-tonoise ratio. Figure 6.10 on the following page illustrates a typical amplifierfilter-mux configuration.

C

+

Fig. 6.09: Simple RC filter with buffer

To Mux

63

Chapter 6 General Amplification

Ch 1

Ch 2

Ch 3

Ch 4

IA

IA

IA

IA

Switched bias resistors

(per channel)

Low-pass filter per channel

Low-pass filter per channel

Low-pass filter per channel

Low-pass filter per channel

Mux To A/D

Channel address lines

2

Fig. 6.10: Typical amplifier-filter-mux configuration

Measuring High Voltages

It was shown earlier that measurement of voltages above 5-10V with data acquisition inputs requires attenuation and that attenuation requires buffering. A typical data acquisition system would require a signal conditioning stage, which provides attenuation and buffering, in order to accommodate high-voltage inputs.

For higher voltage measurements a fully differential attenuator, calibrated to match the buffered differential inputs of a data acquisition system would be required. With this type of setup, differential voltages as high as 2000 volts peak-to-peak are measurable. Figure 6.11

illustrates this type of system.

High-voltage adapter

10 M

50 k

10 M

50 k

5 M

5 M

Data acquisition system

A

B

Channel 1

C

For systems with a true differential input, neither the input’s low or high end are connected to ground. In this case, attenuating an incoming signal requires two matched resistor dividers; one on signal-high and one on signallow. The low end of each divider is grounded. For example, to provide

Fig. 6.11: Typical data acquisition system with a highvoltage adapter

64

Chapter 6 General Amplification

200:1 attenuation without compromising the high input impedance, an accessory device is required. In the case of our typical system, the two matched buffer amplifiers (2 op amps in the same package), A and B, on the front end of the data acquisition system draw nearly identical bias current through their inputs. If matched voltage dividers are placed on the high and low signal inputs, the bias current through the dividers will cause an identical voltage offset on the output of the op amps A and B. Since these two outputs are identical, they can be thought of as a common mode voltage that the differential amplifier, C, rejects. See

Figure 6.11.

i

Total i a i b i b

Data acquisition system

A

B

C

Fig. 6.12: Single ended attenuation of a differential input

This scheme attenuates both the high and low signal, allowing both ends to be hundreds of volts off ground. For example, the high input can be 110 VAC while the low end has a 50 VDC offset. After attenuation, the peak voltage of the high signal is 0.77V

and the low signal is 0.25V, which is well within the readable range of the data acquisition system’s differential inputs.

If a single ended measurement is attempted by attenuating only the high signal and pulling the low signal to ground, two problems are encountered. First, if the data acquisition system ground and the signal ground are dissimilar, tying them together creates a ground loop and produces error in the reading. The second error component results from the bias currents i b

flowing through the two op amps, A and B. The bias current produces a voltage drop through the divider on the high input, but there is no matching drop through the low input. The difference is presented to the differential amplifier, C, producing an offset on its output. See Figure 6.12.

65

Chapter 6 General Amplification

Single-Ended & Differential Measurements

Data acquisition systems have provisions for single-ended and differential input connections. The essential difference between the two is the choice of the analog common connection. Single-ended multichannel measurements require that all voltages be referenced to the same common node, which will result in measurement errors unless the common point is very carefully chosen; sometimes there is no acceptable common point. Differential connections cancel or ignore common mode voltages and allow measurement of the difference between the two connected points. When given the choice, a differential measurement is always better. The rejected common mode voltages can be steady DC levels or noise spikes.

S

1

+

Diff

S

1

-S

2

The best reason to choose single-ended measurements is for a higher channel count that is available in some devices.

Most data acquisition products allow you to double the number of channels in a differential system by selecting single-ended operation.

S

2

Noise Reduction in Differential Mode

Fig. 6.13: Voltages common to both lines are rejected by the differential amplifier

There is a widely misunderstood aspect of differential measurements of floating sources. A common example is a flashlight battery connected to a differential input pair. The measurement cannot be made unless an additional connection is made to the analog common of the analog input device. This connection is frequently made with a

10K-100K resistor. The resistor provides a bias current return path for the two inputs which are each connected to high impedance voltage follower stages on the input of an instrumentation amplifier. If this bias current return path is not established, the amplifier will saturate and cannot measure the voltage difference between the inputs. Some data acquisition inputs provide switched resistors to analog common for this purpose.

The reason these bias resistors are switched is that there are situations in which they are not needed. The situation in which the bias resistors are not needed is where a DC path already exists for bias current to flow from the amplifier inputs to analog common.

This usually occurs when the device under test is already referenced to analog common.

66

Chapter 6 General Amplification

Differential Voltage Measurements

In a single-ended voltage measurement, the voltage on the high input lead is measured with respect to analog common. Differential measurement utilizes a differential amplifier to measure the difference in voltage between the two signal leads. Differential measurements provide greater noise rejection; furthermore, some measurements, most notably strain-gage measurements, require measurement of the voltage difference between two signal leads.

Thermocouple

Differential amplifier

Figure 6.14 depicts a differential amplifier configured for a thermocouple measurement. Note the 10 KOhm resistor between the low side and

Rb=10k

Ω ground. Even though the amplifier measures the voltage difference between the two inputs, a path to ground is required on at least one input of the amplifier. This resistor provides the path for the amplifier bias

Fig. 6.14: Differential thermocouple measurement

currents. If not present or too high in value, the amplifier bias currents will cause the inputs to approach one of the power supply rails and exceed the common mode range of the amplifier. In most applications, the common mode range is typically

±

10V. The common-mode voltage produced by the bias currents is V cm

=2i b

R b

To determine the minimum value for R in the case of a completely floating input, R may be a short circuit on one side only. Otherwise, under the worst case conditions, the voltage error in percent of voltage applied due to source resistance and

R is:

E(%)=100 R

S

/2R b

. This applies to R for the balanced configurations. Typical values for R are 10 KOhm to 10 MOhm. If both equations cannot be satisfied, the source resistance is too high to operate within specifications.

Some amplifier inputs have built-in termination resistors for differential measurements, but very high impedance inputs require the user to supply a termination resistor. Introducing a termination resistor into the configuration dramatically lowers the input impedance in non-isolated systems and may load the circuits being measured.

67

Chapter 6 General Amplification

V exc

Excitation

100k

100k

+

Consider the strain gage in Figure 6.15.

If the excitation voltage is referenced to the same common as the differential amplifier, termination resistors are not required because the strain gage itself provides a current path to ground for the instrumentation amplifier.

However, if the excitation voltage is floating or referenced to a separate common, at least one termination resistor is required. A balanced termination with 100 KOhm resistance can also be used.

Fig. 6.15: Differential strain gage measurement with switchable termination resistors

Common mode rejection ratio (CMRR) is an important specification for instruments making differential measurements. A large CMRR is necessary to accurately measure a small differential signal on a large common mode voltage. The CMRR specifies the maximum effect of the common mode voltage on a differential measurement.

An amplifier’s CMRR is typically specified in decibels (dB); a ratio of 10 results in 20 dB, and each additional factor of 10 results in an additional 20 dB. For example, 100 dB corresponds to 10 5 , so an instrument with a CMRR of 100 dB measuring a signal on a common mode voltage of 1V can have an error as large as 0.01 mV. If the common mode voltage is larger, or if greater accuracy is desired, an amplifier with a CMRR of at least 120 dB would be required. See Chapter 5 for detailed discussion.

CH0 H

CH0 L

CH1 H

CH1 L

CH2 H

CH2 L

Motor

Loads

Shunts

Good for a low signal reference

Bad for a low signal reference

Analog

Common

Mux Board

DBK12,13, or 14

Fig. 6.16: Ground referenced differential measurement

Measuring High DC

Currents with Shunts

It is frequently necessary to measure high currents with data acquisition instruments. Generally, DC currents are measured by monitoring the voltage drop across a calibrated shunt which has a 50 mV or 100 mV drop at full rated current. Figure 6.16 shows a differential measurement of three shunts which have been strategically placed in the common sides of three motor

68

Chapter 6 General Amplification

circuits to allow inexpensive monitoring with a non-isolated, low voltage input. It is not always possible to locate the shunts in higher voltage circuits in the electrically convenient common leg. In these instances, isolated analog inputs are the recommended solution.

Measuring AC Currents with Current Transformers

High AC currents can be measured with shunts also, but the inherent danger of direct connection to AC lines is undesirable. Isolation from the AC line voltages is provided by current transformers (CT) along with a ratio reduction of the input current.

A 500:5 CT for example, has a 100:1

C.T.

ratio

100:1

I = 500A rms

0.01

V out

= 50mV rms ratio and will have a secondary current of 5 amperes when 500 amperes is flowing in the primary. If a low value load resistor such as 0.01 Ohm

Fig. 6.17: Measuring high AC currents with current transformers

is connected to the output of a current transformer, the full load secondary current will produce a 50 mVACrms, which can be easily read with an analog input. Although the output voltage seems low, higher resistance values will overload the CT and reduce the accuracy of the measurement. If a CT has a VA rating of 2 VA, the maximum usable load resistance including lead wire is only 0.08 Ohms. It is also necessary to remember that an open-circuited CT with primary current flowing can generate over 1,000 volts at lethal current levels.

Signal Connection Methods

Measurement of low level signals depends heavily on proper wiring between the signal sources and the data acquisition device. The amplifier stages in signal conditioning equipment are not able to determine the difference between the signal and inadvertently added noise voltage due to improper wiring practices. Whenever low signal levels

(less than 1V) are being measured, twisted pairs or shielded twisted pairs provide the best protection against unintentional pickup of noise voltages. If a shield is used, it should only be grounded at one end, preferably at the signal source.

69

Chapter 6 General Amplification

70

Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement