Configuration of 0/4..20 mA differential inputs

Technical background

The internal input electronics of the terminals referred to above have the following characteristic (see Fig. Internal connection diagram for 0/4..20 mA inputs):

• Differential current measurement, i.e. concrete potential reference is primarily not required.
  The system limit applies is the individual terminal EL30xx/EL31xx.
• Current measurement via a 33 Ω shunt per channel, resulting in a maximum voltage drop of 660 mV via the shunt
• Internal resistor configuration with GND point (A) central to the shunt
  The configuration of the resistors is symmetric, such that the potential of (A) is central relative to the voltage drop via the shunt.
• All channels within the terminal have this GND_int potential in common.
• the common GND_int potential (A)
• is connected for 1 and 2 channel terminals to a terminal point and not with GND_PC (power contact).
• is connected for 4 channel terminals with GND_PC
• The center point of the voltage drop over the 33 Ω shunt is referred to common mode point (CMP).
  According to the technical product data, the maximum permitted common mode voltage V_cm refers to the potential between the CMP of a channel and the internal GND or the potential between the CMP of 2 channels within a terminal.
  It must not exceed the specified limit (typically ±10 or ±35 V).

Accordingly, for multi-channel measurements V_cm specifications must be followed.

The block diagram for a 2 channel terminal shows the linked GND points within the terminal (Fig. Internal connection for 0/4..20 mA inputs of a EL3xx2):

For all channels within the terminal V_cm-max must not be exceeded.

Vcm for 0/4..20 mA inputs If Vcm of an analog input channel is exceeded, internal equalizing currents result in erroneous measurements. For 1 and 2 channel terminals the internal GND is therefore fed out to a terminal point, so that the Vcm specification can be met through application-specific configuration of this GND point, even in cases of atypical sensor configuration.

Example 1

The 2-channel EL3012 is connected to 2 sensors, which are supplied with 5 and 24 V. Both current measurements are executed as low-side measurements. This connection type is permitted, because at I_max CMP_ch1 and CMP_ch2 are approx. 330 mV above 0 V, which means that V_cm is always < 0.5 V. The requirement of V_cm < 10 V (applicable to EL30xx) is therefore adhered to.

If the EL30x1/EL30x2 or EL31x1/EL31x2 terminals have no external GND_int connection, the GND_int potential can adjust itself as required (referred to as "floating"). Please note that for this mode reduced measuring accuracy is to be expected.

Example 1a

Accordingly, this also applies if the floating point GND_INT is connected to another potential.

Example 2

The same EL3012 is now again connected with the two 20 mA sensors, although this time with one low-side measurement at 5 V and one high-side measurement at 12 V. This results in significant potential differences V_cm > 10 V (applicable to EL30xx) between the two channels, which is not permitted.

To rectify this, GND_int can in this case be connected externally with an auxiliary potential of 6 V relative to "0 V". The resulting A/GND_int will be in the middle, i.e. approx. 0.3 V or 11.6 V.

Example 3

In the EL3xx4 terminals GND_int is internally connected with the negative power contact. The choice of potential is therefore limited.

The resulting CMP is 23.6 V, i.e. >> 10 V (applicable to EL30xx). The EL30x4/EL31x4 terminals should therefore be configured such that CMP is always less than V_cm,max.

Summary

This results in certain concrete specifications for external connection with 0/4..20 mA sensors:

• We recommended connecting GND_int with a low-impedance potential, because this significantly improves the measuring accuracy of the EL30xx/31xx.
  Please note the instructions relating to the V_cm potential reference.
• The V_cm potential reference must be adhered to between CMP ↔ GND_int and CMP_ch(x) ↔ CMP_ch(y).
  If this cannot be guaranteed, the single-channel version should be used.
• Terminal configuration:
• EL3xx1/EL3xx2: GND_int is connected to terminal point for external connection.
  GND_int should be connected externally such that condition 2 is met.
• EL3xx4: GND is connected with the negative power contact.
  The external connection should be such that condition 2 is met.

If the sensor cable is shielded, the shield should not be connected with the GND_int terminal point but with a dedicated low-impedance shield point.

• If terminal points of several EL30xx/EL31xx terminals are connected with each other, ensure that condition 2 is met.

Connection of GNDint In the EL30x1/EL30x2 and EL31x1/EL31x2 terminals the internal GND, GNDint connection is fed out to terminal contacts. To achieve a precise measurement result GNDint should be connected to a suitable external low-impedance potential, taking account the specifications for Vcm.