Basics covering analog signals, PLC analog input modules, transducers/transmitters and the wiring of input transducers/transmitters to the PLC analog input module. Single ended and differential wiring are also discussed.

1. Analog Control

2. Supplemental material
Chapter 12 & 13

3. 3
 Discrete signals only have two states; ON
or OFF.
 Analog signals have an infinite number of
states. It can be totally ON, totally OFF or
anything in-between.
 PLC’s/PAC’s use analog I/O modules in
applications where the field device signals
are continuously varying.
Overview

4. Analog Signal
Continuous Signal
Time
MeasuredorCommandSignal

5. Analog I/O Devices
 Flow meters
 Humidity
transducers
 Load cells
 Potentiometers
 Pressure sensors
 Temperature
sensors
 Vibration
transducers
 I/P valves
 Motor controllers
 Fans/Blowers
 Heaters
 Chart recorders
 Actuators
 Analog meters

6. Analog Input ProcessProcess
Transducer
Transmitter ADC
0000000000000000
15 0
Physical
Signal
Low level
Voltage or
Current
Voltage or current
compatible with
analog input module
Analog
Input
Module
Toprocessor
Computer or PLC or PAC
Storage register

7. 7
Most common
 4 to 20mA
 0 to 10Vdc
 ±10Vdc
Less common
 0 to 1Vdc
 0 to 5Vdc
 1 to 5Vdc
 ±5Vdc
Analog Input Rates

8. Analog Output
Process
DAC
Analog
Output
Module
0000000000000000
11 0
Transducer
From
Processor
Storage register
in processor
Voltage or
current output
Analog
Signal
From water
reservoir
To water
supply
I/P Valve
Pump
Transmitter

9. 9
Most common
 4 to 20mA
 0 to 10Vdc
 ±10Vdc
Less common
 10 to 50mA
 0 to 5Vdc
 ±2.5Vdc
 ±5Vdc
Analog Output Rates

10. 10
 Analog signals are encoded by
representing them in a binary word, as a
binary number.
 The binary number corresponds to the
value of the analog signal at a given
instant in time.
Analog Signals

11. 11
 Analog input signals are represented in
digital format through the process of
sampling the continuous analog waveform
at regular intervals of time and then
performing an analog to digital
conversion.
 If the sample rate is twice the rate of the
analog signal change, the signal can be
exactly reproduced. (Actually, analog
signals can never be perfectly produced.
There will always be some error).
Sampling

12. 12
 The process of assigning a discrete binary
number to each sample introduces an
error known as quantization error.
 Quantization is an unavoidable error
resulting from the difference between the
actual value of the analog sample and the
nearest value encoded by one of the
binary numbers.
Sampling

13. Sampling
http://www.videomaker.com/article/14524/

14. Sampling
http://www.music-production-school.com/pro_tools_software_article.shtml

15. 15
 Analog to Digital Converter (ADC)
 ADC’s are used to interface analog input
signals with digital circuits, PLC’s, PAC’s or
computers.
 ADC’s are essentially a quantizing process,
whereby an analog signal is represented
by discrete states. These states can be
assigned appropriate codes such as
straight binary, BCD, gray code or binary
two’s compliment.
Analog to Digital
Conversion

16. 16
 Assume:
 A straight binary count.
 3-bit output
 Input of: 0Vdc to 1Vdc
 1 𝑏𝑖𝑡 𝐿𝑆𝐵 =
𝐹.𝑆.
2 𝑛
 Where:
 F.S. = Full scale input of the ADC
 n = number of output bits.
3-Bit ACD
D2 D1 D0
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1

17. 17
 From the previous example and table the
following should be noted:
 The 3-bit ADC has 2n output states where ‘n’ is
the number of output bits.
 The value of input voltage required to change
the output by 1-LSB is obtained by:
3-Bit ADC
1 𝑏𝑖𝑡 𝐿𝑆𝐵 =
𝐹.𝑆.
2 𝑛 =
1 𝑉
23 𝐵𝑖𝑡𝑠
=
1 𝑉
8 𝐵𝑖𝑡𝑠
=
1
8
𝑉/𝐵𝑖𝑡 ±
1
2
𝐿𝑆𝐵

18. 3-bit ADCDigitalBinaryOutputCount
Ideally QuantizedAnalog Input Volage
3-Bit ADC Analog Voltage Input vs. Digital Bianry Output Count
000
100
011
010
001
111
110
101
1/8 7/83/45/81/23/81/40 1.0 F.S.
1 LSB
NominalQuantized
Value ½ LSB
Binary

19. 3-Bit ADC
Input vs. Output
Analog Input
for a 1VDC F.S.
ADC
Analog Input
for a 10VDC
F.S. ADC
Binary Digital
Output
0 0.00 000
1/8 1.25 001
1/4 2.50 010
3/8 3.75 011
1/2 5.00 100
5/8 6.25 101
3/4 7.50 110
7/8 8.75 111

20. 20
 Analog input values shown represent the center
point of the analog values for each output word,
with transition points ±½ LSB from the center
points. The quantizing error or uncertainty is thus
½ LSB.
 The MSB (1002) output corresponds to ½ V on
the input which is half of the F.S. input range.
The largest output word (1112) corresponds to
7/8 V and F.S.
3-Bit ADC
𝑉𝑜 𝑚𝑎𝑥𝑜𝑢𝑡 = 𝐹. 𝑆 − 1 𝐿𝑆𝐵
or
1 𝑉𝑑𝑐 −
1
8
𝑉𝑑𝑐 =
7
8
𝑉𝑑𝑐

21. 21
 An ADC requires a certain length of time,
called conversion time, to change an
analog signal into the corresponding
digital signal.
 If the analog signal changes during the
conversion time the converter output may
be in error. To prevent this a sample and
hold circuit is used.
Sampling Concepts

22. Simplified Sample
and Hold (FYI)
Input
Voltage
A1 A2
Electronic
Switch
Switch
Driver
C
Output
Voltage
Common
Sample
Control

23. 23
 Conversion Time
 The total time required to completely convert
an analog input current or voltage to a digital
output. The time is affected by the propagation
delay in the various circuits. It may be
specified as a number of conversions per
second.
 This is one of the most important specifications
to be considered when selecting an ADC.
Converters having more output bits usually
require more time.
ADC Characteristics

24. 24
 Resolution
 The amount of input voltage required to
increment the output by one LSB. The
resolution is determined by the number of
output bits. It is specified in terms of the
number of output bits or as one part per
number of output states. As an example: the
resolution of an 8-bit converter is specified as
either 8-bits or 1 out of 256
1
28 =
1
256
 Example:
 The resolution of our 3-bit ADC is 1 out of 8
1
23 =
1
8
ADC Characteristics

25. 25
 Digital to Analog Converter (DAC)
 Digital systems can be used to transmit analog
signals. This is accomplished by using a DAC
whereby a given digital signal is transformed
into its equivalent analog signal.
 Three categories
 Current output
 Voltage output
 Multiplying type
Digital to Analog
Converter (DAC)

26. 26
 The DAC process can be viewed as finding
the equivalent weight of and object (less
then one unit) with weights in
geometrically proportional units, such as
1/8, 1/4 and 1/2. By using these weights in
various combinations, 8 different
measurements ranging from zero units to
7/8 units can be obtained.
Concept & Transfer
Function

27. 27
 Assume:
 A straight binary count.
 3-Bit output
 Output of: 0Vdc to 1Vdc
 Where:
 F.S. = Full scale input of
DAC
 n = Number of output
bits.
3-Bit DAC
D2 D1 D0
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1

28. 28
 By the previous example and table the
following should be noted:
 The 3-Bit DAC has eight 2 𝑛 = 8 possible input
combinations 𝑛 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑝𝑢𝑡 𝑏𝑖𝑡𝑠 , ranging
from 0002 𝑡𝑜 1112.
 If the F.S. analog voltage is 1Vdc, the smallest
unit 𝐿𝑆𝐵 001 is equivalent to
1
8
𝑉. No voltage or
step smaller can be identified by this DAC
(resolution).
3-Bit DAC

29. 3-Bit DAC Output
Voltage vs. Binary
3-Bit DAC Output Voltage vs. Binary Count
0
1/8
1/4
3/8
1/2
5/8
3/4
7/8
1
000 001 010 011 100 101 110 111
Binary Input Count
OutputVoltageChange

30. 3-Bit DAC Input vs.
OUtput
Binary Digital
Input
Analog Output
for a 1VDC F.S.
DAC
Analog Output
for a 10VDC
F.S. DAC
000 0 0.00
001 1/8 1.25
010 1/4 2.50
011 3/8 3.75
100 1/2 5.00
101 5/8 6.25
110 3/4 7.50
111 7/8 8.75

31. 31
 The MSB 𝑏𝑖𝑛𝑎𝑟𝑦 100 has the equivalent
value equal to
1
2
𝑉 𝑜𝑟 50% 𝐹. 𝑆.
 For the maximum input signal 𝑏𝑖𝑛𝑎𝑟𝑦 111 ,
the analog is
7
8
𝑉, which is
1
8
𝑉 less than
the F.S. value. Therefore:
𝑉𝑜 𝑚𝑎𝑥𝑜𝑢𝑡 = 𝐹. 𝑆. −𝐸𝑞𝑢𝑖𝑣. 𝑉 𝑓𝑜𝑟 1 𝐿𝑆𝐵
Where:
𝑉𝑜 𝑚𝑎𝑥𝑜𝑢𝑡 = 𝑡ℎ𝑒 𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑎𝑛𝑎𝑙𝑜𝑔 𝑜𝑢𝑡𝑝𝑢𝑡 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
𝐹. 𝑆. = 𝐹𝑢𝑙𝑙 𝑠𝑐𝑎𝑙𝑒 𝑜𝑢𝑡𝑝𝑢𝑡 𝑟𝑎𝑡𝑖𝑛𝑔 𝑜𝑓 𝑡ℎ𝑒 𝐷𝐴𝐶
3-Bit DAC

32. 32
 Settling Time
 The time required, after a code transition, for
the DAC output to reach its final value within
specified limits, (usually ±1/2 LSB). The
settling time of a voltage output DAC is longer
then that of a current output type due to the
extra circuitry to transform current to voltage.
DAC Specifications

33. 33
 The smallest incremental change in output
voltage/current of a DAC.
In the previous example, for a 3-bit DAC
with an output voltage range of 0Vdc to
1Vdc, the resolution is:
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 =
𝐹. 𝑆.
2 𝑛
or
0.125𝑉𝑑𝑐 =
1 𝑉𝑑𝑐
23
Resolution

34. Transducers/Transmitters
Single-Ended vs Differential
Analog Input Wiring

35. 35
 Transmitters/transducers convert the
signal from an analog input device into a
current or voltage that an analog input
module can accept.
 Transducers convert field device signals to a
voltage. (0 to 10Vdc, -10 to +10Vdc, 0 to
5Vdc, etc.)
 Transmitters convert field device signals to a
current. (4 to 20mA is typical)
 Many times, the two terms are used
without regard to voltage or current.
Transducers/Transmitters

36. Xducer/Xmitter
Examples
 Xducer/Xmiters are
selected based on
the input and
output signals they
accept and produce.
 Some
xducer/xmitters are
universal and can
be programmed for
a variety of input
signal types and a
variety of output
types.

37. 37
 The transmitter/transducer has an analog
input sensor wired to its input and its
output is wired to the input of an analog
input module.
Xmitter/Xducer
Summary
Process
Transducer
Transmitter
Physical
Signal
measured
by analog
input
sensor
Low level
Voltage or
Current
Voltage or current
compatible with
analog input module
To Analog
Input Module

38. 38
 Xducers/xmitters are wired in a circuit
most often called a ‘current loop’ or a
‘voltage loop’.
 The xducer/xmitter might require a power
supply; ‘loop power’.
 Other xducers/xmitters might supply their
own ‘loop power’.
 Always refer to the manufacturers
specifications and wiring diagrams.
Wiring Configuration

39. Wiring Configuration
 These xducers
/ xmitters
require loop
power.
 Xducers /
xmitters that
do not require
loop power will
be wired
directly to the
module.
2- Wire Transducer/Transmitter
3- Wire Transducer/Transmitter
4- Wire Transducer/Transmitter
Xducer/xmitter
Xducer/xmitter
Xducer/xmitter

40. 40
 Analog input modules accept analog
signals.
 These inputs are referred to as channels.
 The inputs can be voltage or current.
 Some input modules require configuration
to set if the module or individual channels
can accept voltage or current. Depending
on the module, configuration can be done:
 Through the programming software.
 With DIP switch settings.
 Input wiring arrangement.
Analog Input Modules

41. 41
 Most analog input modules can be wired
single-ended or differential.
 The methods are usually defined by the
type of input devices and/or the physical
location of the devices.
 Single-ended wiring is usually used in low
noise (electrical noise) environments and
differential is usually used in areas where
there is a lot of Electro-Magnetic
Interference (EMI) (a.k.a. Noise)
Single-Ended vs
Differential

42. 42
 Measurements taken with an analog input
module that are single-ended, take the
voltage or current difference between a
single wire and ground.
 The noise is only on the positive wire and
therefore is measured along with the
output signal from the input sensor.
 In areas where high levels of electrical
noise are present, the noise could cause
measurement errors.
Single-Ended

43. 43
 Measurements taken with an analog input
module that are differential are “floating”,
there is no ground reference.
 The measurement is taken as the voltage
difference between the two wires.
 In areas where high levels of electrical
noise are present, the noise is added to
both wires, in-phase, and can than be
filtered by the common mode rejection of
the module.
Differential

44. Analog Input Wiring
 The diagram
shows single
ended wiring of
several xducers
/ xmitters
wired to an
analog input
module.
Xducer/xmitter
Xducer/xmitter
Xducer/xmitter
Xducer/xmitter

45. Single Ended Wiring

46. Differential Wiring

47. 1756-IF8 Single
Ended Voltage
 Notes:
 All terminals
marked RTN are
connected
internally.
 Terminals
marked iRTN are
not used for
single-ended
voltage wiring.
 Do not connect
more than two
wires to any
single terminal.

48. 1756-IF8 Single
Ended Current
 Notes:
 All terminals marked
RTN are connected
internally.
 For current applications,
all terminals marked
iRTN must be wired to
terminals marked RTN.
 A 249W current loop
resistor is located
between IN-x and iRTN-
x terminals.
 Place additional loop
devices (e.g. strip chart
recorders, etc.) at the A
location in the current
loop.
 Do not connect more
than two wires to any
single terminal.

49. 1756-IF8 Differential
Voltage
 Notes:
 All terminals marked
RTN are connected
internally.
 If multiple (+) or
multiple (-) terminals
are tied together,
connect that tie point to
a RTN terminal to
maintain the module’s
accuracy.
 Terminals marked RTN
or iRTN are not used for
differential voltage
wiring.
 Do not connect more
than two wires to any
single terminal.
 Important:
 When operating in 2-
channel, high speed
mode, only use
channels 0 and 2

50. 1756-IF8 Differential
Current
 Notes:
 All terminals marked RTN
are connected internally.
 A 249W current loop
resistor is located between
IN-x and iRTN-x terminals.
 If multiple (+) or multiple
(-) terminals are tied
together, connect that tie
point to a RTN terminal to
maintain the module’s
accuracy.
 Place additional loop
devices (e.g. strip chart
recorders, etc.) at the A
location in the current loop.
 Do not connect more than
two wires to any single
terminal.
 Important:
 When operating in 2
channel, high speed
mode, only use channels 0
and 2.