Connecting Sensors to Analog Inputs

Introduction

You can connect different sensors to the analog input modules depending on the measuring method; voltage and current sensors, and resistors.

This section contains general information that is generally applicable to all the connection options for sensors described in the sections that follow.

Cables for analog signals

To reduce electrical interference, you should use twisted-pair shielded cables for the analog signals. The shield of the analog signal cables should be grounded at both cable ends.

If there are potential differences between the cable ends, an equipotential bonding current can flow over the shield, which leads to an interference of the analog signals. In such a case, you should ground the shield at one end of the cable only.

Isolated analog input modules

With the isolated analog input modules there is no electrical connection between the reference point of the measuring circuit M_ANA and the M terminal of the CPU.

You must use isolated analog input modules if a potential difference E_ISO can occur between the reference point of the measuring circuit M_ANA and the M terminal of the CPU. By means of a equipotential bonding conductor between the M_ANA terminal and the M terminal of the CPU, make sure that E_ISO does not exceed the permitted value.

Non-isolated analog input modules

With the non-isolated analog input modules, you must establish a connection between the reference point of the measuring circuit M_ANA and the M terminal of the CPU. Establish IM 153. For this purpose, connect the M_ANA terminal with the M terminal of the CPU and IM 153. A potential difference between M_ANA and the M terminal of the CPU and the IM 153 can result in corruption of the analog signal.

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Limited potential difference E_CM

Only a limited potential difference E_CM (common mode voltage) may occur between the measuring lines M– of the input channels and the reference point of the measuring circuit M_ANA. In order to prevent the permissible value from being exceeded, you must take different actions, described below, depending on the potential connection of the sensors.

Abbreviations and mnemonics used in the figures below

The abbreviations and mnemonics used in the figures below have the following meanings:

M +: Measuring lead (positive) M –: Measuring lead (negative)

M_ANA: Reference potential of the analog measuring circuit M: Ground terminal

L +: Terminal for 24 VDC supply voltage

E_CM: Potential difference between inputs and reference potential of the M_ANA measuring circuit

E_ISO: Potential difference between M_ANA and M terminal of CPU

Connecting isolated sensors

The isolated sensors are not connected with the local ground potential (local ground). They can be operated free of potential.

With isolated sensors, potential differences might arise between the different sensors. These potential differences can arise as a result of interference or the local distribution of the sensors.

In order to prevent the permissible value for E_CM from being exceeded when operating in areas with heavy EMC interference, we recommend you to connect M– with M_ANA.

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You can operate the CPU in Grounded mode (refer to the figure below) or Ungrounded mode.

Isolated sensors

M+

M L+

M–

M_ANA M+

M– Backplane

bus ADC

M_internal L+M

Ground bus CPU E_ISO

Logic

Recommended connection

Figure 4-7 Connecting Isolated Sensors to an Isolated AI

Isolated sensors

M+

M L+

M–

M_ANA M+

M– Backplane

bus

M_internal L+

M Ground bus

CPU

ADC Logic

Recommended connection

Figure 4-8 Connecting Isolated Sensors to a Non-Isolated AI

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Note

Do not connect M– to M_ANA when connecting two-wire transmitters for current measurement and resistance-type sensors. This also applies to inputs which are not used.

Non-isolated sensors

The non-isolated sensors are connected with the local ground potential (local ground). When using non-isolated sensors, you must connect M_ANA to the local ground.

Connecting non-isolated sensors

Caused by local conditions or interferences potential differences E_CM (static or dynamic) can occur between the locally distributed individual measuring points. If the potential difference E_CM exceeds the permissible value, you must provide equipotential bonding conductors between the measuring points.

When connecting non-isolated sensors to optically isolated modules, you can operate the CPU in Grounded mode (refer to the figure below) or Ungrounded mode.

Non-isolated sensors

M+

M L+

M–

M_ANA M+

M– Backplane

bus E_CM

ADC

M_internal ML+

Ground bus CPU E_ISO

Logic

Equipotential bonding conductor

Figure 4-9 Connecting Non-Isolated Sensors to an Isolated AI

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When connecting non-isolated sensors to non-isolated modules, you can operate the CPU only in Grounded mode.

Non-isolated sensors

M+

M L+

M–

M_ANA M+

M– Backplane

bus

M_internal L+

M Ground bus

CPU

ADC Logic

E_CM

Equipotential bonding conductor

Figure 4-10 Connecting Non-Isolated Sensors to a Non-Isolated AI

Note

Non-isolated 2-wire transmitters and non-isolated resistance-type sensors may not be used with non-isolated AI!

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