How to choose signal separator?

One of the ways to avoid problems due to ground differentials and some types of external interference is to introduce galvanic isolation between the signal source and destination. Devices that serve to this purpose are usually called signal separators though most of them measure the signal, amplify, filter and limit it and only then send it over galvanic barrier and form back to the same or another standardized signal (in which case 'signal converter' would be more descriptive name).

Ideal signal separator would be one that does not distort, even if only by delaying, or limit the signal - neither its amplitude nor its bandwidth - and does not require additional power supply. Unfortunately, the ideal is not possible (and sometimes not even desirable) and one has to take into account not only technical details of particular separator (like input resistance, bandwidth, signal delay, or allowable output load), but also the specific solution of signal separation it represents. Need for additional power supply makes installation planning even more complicated - less so when the supply input is galvanically separated from both signal input and output but the need of it still has to be taken into account.

As mentioned, passing analog signals in industry is based on some standards. Voltage signals may be sent without distortion on relatively short distances. Main standard in this case is voltage range of 0÷10V but signals like 1÷5V or bipolar ones like ±5V may also be found in use. Sending analog signal over distances longer than several meters is dominated by so-called current loop of 4÷20mA. Occasionally, installations may be found where the older standard, based on 0÷20mA range, is in use.

All this makes choosing right separator a nontrivial task and we're well aware that the range of signal separators offered may be confusing. Hopefully, following tables and short descriptions will simplify the task.

Obviously, deciding factor is the type of input and output signal (if not identical), so the following table contains links to blocks of text dedicated to specific signal type combinations.

4÷20mA/4÷20mA 4÷20mA/0÷20mA 4÷20mA/0÷10V
0÷20mA/4÷20mA 0÷20mA/0÷20mA 0÷20mA/0÷10V
0÷10V/(0)4÷20mA 0÷10V/0÷10V I,U/4÷20mA
universal:   0÷20mA, 4÷20mA, 0÷10V, 1÷5V

If one already knows separator designation and needs only to confirm the choice by reading some additional information about it, one of the links collected in next table will provide a shortcut to relevant text. Note that separators have been divided into two groups depending on the side from which separator gets its power supplied (it's not necessarily additional power supply - some separators are supplied from the signal itself). Universal separators have power supply isolated from both input and output.

Signal separators

supplied from: output side

 

input side

I/I

I/U

U/U

U/I

 

I/I

U/U

U/I

T621
T624
T721
T724
T727
T729
T820, T821
T823, T824
T826, T827
T828, T829
T920, T921
T921s
T923, T924
T924p
T924s
T732
T735
T831
T832
T834
T835
T761
T860, 1
T870, 1
T667
T767
T867
T967
T967s
  T863 T864
T874
T862
T872
Universal U,I / U,I - T837, T838
Programmable U,I / 4÷20mA - T1239


4÷20mA/4÷20mA

Lets begin with separators that are closest to ideal signal separator - that is do not require additional power supply connection and are inserted into the signal line in the simplest possible way. Sometimes called passive separators, they function as constant current transformer. Input signal is switched at comparatively high frequency to be passed through a transformer and magnetic field feedback ensures reproduction of input signal at the output. Depending on the direction of signal and energy flow there are two distinct cases. If, as represented by the drawing below, the signal source is also the source of energy then the separator passes most of that energy to output. The small amount of energy used by the separator causes additional voltage drop, Us, that adds to voltage drop on load resistance.

It may sound a bit complicated, but the drawing should make it evident that the signal source simply sees increased (and current-dependent) load. In case of the T924p separator, additional voltage drop of 1.6V at 20mA leads to load resistance increase of 1.6V/20mA=80Ω. (Signal separator T924sp that fits in enclosure of only 6.2mm width and triple separator T924p3 work on the same principle, though with higher voltage drop)
Naturally, one cannot assume that the upper current limit will never be crossed - to allow range overflow signalling one needs to add a margin of 1÷2mA. Additionally, one has to consider the possibility of signal distortion caused by external interference coupled to signal lines - to allow for its active compensation, a couple of volts safety margin is needed. In practical terms, as current signal source is characterized by maximum resistive load (or maximum voltage called compliance voltage), one needs to subtract from it, say, R=(Us+2V)/21mA to get the maximum allowable load resistance on the receiving end (if only compliance voltage, Uc, is given, maximum load resistance may be estimated as (Uc-Us-2V)/21mA).

The other case, when energy flow is reversed in respect to signal flow direction, allows for supplying the signal source (for example a pressure or temperature transmitter). Also in this case, the separator is using part of the energy forcing additional voltage drop, as shown below.

That is how separator T924ps works - mainly like T924p connected in reverse. Switching its input with output provides sought energy flow while the transmitter, compared to previous case, forms active load resistance.

Other 4÷20mA/4÷20mA signal separators from our offer use different methods to achieve signal separation but some of them may also use energy carried by the signal to avoid the need of separate power supply.

Separators T624, T923, T924, and T924s are supplied from output current loop - their outputs form two-wire current sources of variable resistance. Energy for their internal use comes from additional voltage drop (at least 15V for T624, T923 i T924, and 10V for T924s). The drawing below illustrates their functioning in a circuit.

Power supply on the output side may be placed close to the separator - in such case the signal will return through the supply ground line. Due to low input resistance of these separators, one may connect several separator inputs in series thus obtaining several isolated measurement channels.
Separator T624 belongs to so-called economic series. The only difference between T923 and T924 is accuracy class: 0.2 i 0.1, respectively.

Signal separator T924s needs separate explanation. Besides the output part, its input also consumes energy from the signal - leading to small input voltage drop of 1.7V at 20mA. In exchange, there is no internal DC/DC converter inside that would otherwise be needed to deliver energy from output and thus no possibility of introducing noise to signal.
While 'common' separators have constant input resistance of about 50÷60Ω causing voltage drop of 1÷1.2V at 20mA, separator T924s measures input current with a 10Ω resistor and the remaining energy resulting from voltage drop, on the order of few mW, uses to supply its input electronics. The chart below shows input voltage drop dependence on input current.

Additional advantage of T924s separator is small width of its enclosure - only 6.2mm (and low price).

Signal separators designated as T724, T823, and T824 are equipped with additional power supply connector. Due to the number of output connections they are called sometimes tri-wire separators. All may work with loads up to 450Ω. Input resistance reaches 60Ω.

Usually signal receiver also requires supply but this was intentionally omitted in the drawing. Comparing this drawing with drawings containing so-called two-wire separators supplied from output signal, one should be able to notice that the latter may always replace the tri-wire ones.
Separator T724 belongs to so-called economic series. The only difference between T823 and T824 is accuracy class: 0.2 i 0.1, respectively.

Last of the 4÷20mA/4÷20mA signal separators, designated as T863 has separate power supply connection on the input side.

It may work with loads up to 450Ω and has input resistance of c.a. 60Ω. While ordering please specify the accuracy class (0.1 or 0.2) and the input signal range (either 4÷20mA or 0÷20mA).


0÷20mA/0÷20mA

T729 belongs to 'economic' series and is lower-cost version of T829 signal separator. Both are produced in 0.1 accuracy class - when accuracy is not that important one may use the T828 separator with accuracy class of 0.2.

The drawing shows how to connect these separators. Their input resistance amounts to 60Ω and output loads should not exceed 450Ω.


0÷20mA/4÷20mA

Separators supplied from output current loop of 4÷20mA belong to the T600 and T900 series. T621 belongs to 'economic' series (with class 0.1), T920 and T921 differ in accuracy (class 0.2 and 0.1, respectively), and T921s is mounted in a thin (6.2mm width) enclosure. T921s may also be specified for another input range. Outputs of these separators form two-wire current sources of variable resistance. Energy for their internal use comes from additional voltage drop of at least 15V.

Power supply on the output side may be coming from outside of the signal-receiving device (dashed line) and the power supply unit may be located close to separator. In such case the signal will return through the supply ground line.

Three types of tri-wire separators (i.e. possessing separate supply connection) are available: T721, T820 i T821. T721 is a cheaper variant of T821. Both are characterized by the same basic accuracy class of 0.1. Separator T820 has accuracy class of 0.2.

The drawing above shows how to connect these separators. Their input resistance amounts to 60Ω and output loads should not exceed 450Ω.


4÷20mA/0÷20mA

T727 belongs to 'economic' series and is a cheaper version of T827 signal separator. Both are produced in 0.1 accuracy class - when accuracy is of lesser concern one may use the T826 separator with accuracy class of 0.2.

The drawing above shows how to connect these separators. Their input resistance amounts to 60Ω and output loads should not exceed 450Ω.

To the same group of separators one could qualify separator T863 which requires separate power supply on input side. Description of T863 may be found in earlier text, when 4÷20mA/4÷20mA signal separators were discussed.


4÷20mA/0÷10V

To this group of separators belong T732, T831 and T832. T732 is economical version of T832. Both are produced in 0.1 accuracy class while T831 converts the signal with 0.2% accuracy. All are supplied from the output side with nominal voltage of 24V and their output load should not be lower than 1kΩ. Output current is internally limited to 15mA, while voltage to 12V. Input resistance amounts to c.a. 60Ω.

One should take care of supply ground connection - if not connected directly to separator terminal, may lead to supply current flow through part of signal line which will affect the voltage seen by receiver.


0÷20mA/0÷10V

Separators T735, T834, and T835 differ in price (T735 is a cheaper version of T835) and accuracy. T735 and T835 are produced in 0.1 accuracy class while T834 offers 0.2% base accuracy. All are supplied from the output side with nominal voltage of 24V and their output load should not be lower than 1kΩ. Output current is internally limited to 15mA, while voltage to 12V. Input resistance amounts to c.a. 60Ω.

Care should be taken about connections of supply ground - if not connected directly to separator terminal may cause supply current flow through signal line which will affect the voltage seen by receiver.


0÷10V/0÷10V

Within this group of separators one may find devices supplied from the input or from the output side. There are other differences - separators supplied from the output side (assumed to be close to receiver) have higher output resistance, i.e. are supposed to work with high resistance loads. Separators supplied from input side, on the other hand, have stronger outputs, which may work with loads as low as 1kΩ.

Separators T761, T860, and T861 are supplied from the output side and may be prepared for almost any input range within -500 and 500V. T761 is an economic version of T861 separator - both in accuracy class of 0.1, while T860 is produced in accuracy class of 0.2. T870 i T871 are separators of wide bandwidth - up to 10kHz.

As in any case where voltage signals are handled, of great importance is supply ground connection. To avoid supply current flow through signal lines, connection of supply ground should be made closest to the separator.

Separators T864 and T874 are supplied from the input side. Either may be ordered with one of the available accuracy classes: 0.1 or 0.2. Separator T874 is characterized by wide (up to 5kHz) bandwidth. Input resistance of these separators is higher than 50MΩ. Output voltage is internally limited to 12V, while output current to 10mA. Following diagram illustrates how separators function in circuits.


0÷10V/(0)4÷20mA

Differences between separators of this group allow to pick the one fitting particular application. T667, T967, and T967s are supplied from the output 4÷20mA current loop. Others require external power supply (nominally 24V=) connected either from the output side (T767, T867), or from the input side (T862, T872). All separators of this group may be ordered with other input voltage ranges, like 1÷5V.
One of two accuracy classes, 0.1 or 0.2, may be chosen for T867, T967, T862, and T872 separators. Others (T667, T767 i T967s) are now produced only in 0.1 class. Separator T967s fits narrow, 6.2mm, enclosure.

Lets start with separators supplied from the 4÷20mA current loop. Their input resistance is close to 60Ω, and maximal output load resistance is given by expression R=(24V-15V)/20mA leading to 450Ω. Naturally, one may apply higher allowed loop supply (up to 35V) thus increasing upper limit of load resistance.

Separators T767, T867 are supplied from the output side with input resistance of 1MΩ (resistance increased to 10MΩ may be requested). Output load should not be higher than 450Ω.

Separators supplied from the input side (T862, T872) may be ordered with either 0÷20mA or 4÷20mA input range and with 0.1 or 0.2 accuracy class. As in any other case where voltage signals are concerned, supply ground connection is of great importance. To avoid supply current flow through signal lines, connection of supply ground should be made closest to the separator.

Separator T872 is characterized by wide signal bandwidth of 5kHz.


Universal signal separators

Separators that can accept and source various standard analog signals, like 0÷20mA, 4÷20mA, 0÷10V, or 1÷5V, were named universal. Isolated power supply input also adds to their universality allowing to leave the supply floating or to ground it on either the input or output side.

So far two types of universal separators have been introduced to our offer: T837 and T838. Though similar (both performing digital signal processing), they differ slightly in their possibilities and inner workings. Both allow to choose input and output signal type with 4-pole DIP switch accessible under the top cover. High accuracy (0.05%) is preserved for any input/output signal combination as all ranges are factory-calibrated. Together with low temperature coefficient (typ ±30ppm/ºC), low noise, and high output resolution (0.5µA or 250µV), it guarantees precise signal conversion in wide range of ambient temperatures.

Simple visualization of input signal level (color of front-mounted LED changes smoothly from green to red with input signal) should prove useful during installation and for subsequent maintenance.

Separator T838 is equipped with additional input allowing to connect 4÷20mA transmitters supplied from current loop (as passive current source). Another advantage of this separator over T837 is faster reaction to input signal changes (<100ms after 10-90% signal change) and smooth signal variation within specified bandwidth (5Hz) making the separator indistinguishable from fully analog ones of similar parameters.

At the cost of narrowing power supply voltage range (24V=, -10/+20%) it was possible to optimize internal DC-DC converter to achieve very low power consumption compared to similar products with isolated power supply.


I,U/4÷20mA

Among quite wide assortment of offered signal separators there is a programmable one, designated as T1239. Its main purpose of separating standard analog signals and converting them to 4÷20mA is supplemented by possibility of elastic modification of transfer characteristic and smart filtering of input signal.

Input signal may be chosen from either -11÷11V or -25÷25mA range. This and all other parameters are configured by a free Windows software available on our site. One may define almost any (monotonous) transfer function by declaring a table of corresponding input/output values or assigning power series coefficients.

Energy needed to supply this separator comes from the output current loop. Minimal voltage drop at output equals 15V.






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