Product Announcement from Knick USA

The Art of Measuring-Image

The available product lines include innovative instruments for measurement and control. A great deal of emphasis is placed on EMC and explosion protection, where the company has made a name for itself.Knick has been certified to ISO 9001 since 1993. The transmission properties required by a device for signal transmission are determined by various factors. In addition to the requirements regarding accuracy and speed of the signal transmission, the input data of the following devices, the properties of the signal being transmitted and the ambient conditions also need to be taken into account.

Current or voltage transmission
The initial criteria for selecting an isolator are the input signal to be processed and the output signal required. The output signal is generally determined by the following devices such as controllers, indicators, PLC, PCS etc., whereby many of these devices have either current or voltage inputs alternatively.

If both possibilities are available, current signals should be transmitted over longer transmission paths. Injected current signals are considerably less insensitive than voltage signals.

Input resistance
The input resistors of modern isolation amplifiers are generally dimensioned in a way that they are of sufficiently high resistance for voltage inputs and of sufficiently low resistance for current inputs so that the signal being processed is practically not loaded. Only in a few cases (very low voltage signals with a high source resistance or low-load capability current signals) would the input resistance be a selection criterion for isolation amplifiers.

The input resistance of the isolation amplifiers from the Varitrans® family series is specially developed for shunt applications is, at approx. 25 kOhms, relatively low compared with other isolation amplifiers. However, for shunt applications with resistances in the mOhm range, the resistance is always several times to the power of ten higher than required.

Input voltage drop
In various isolation amplifiers with a current input and loop-powered DC transformers, the load on the input signal is specified as a voltage drop and not as an input resistance. This voltage drop is constant during normal operation and is max. 500 mV in isolation amplifiers depending on the model.

In passive isolators, there is a voltage drop at the input resulting from the natural voltage requirement of the device plus the load voltage at the output. Before passive isolators are used, the load capability of the measuring signal and the load connected to the isolator output should be known.

Load capability of output
The load capability of voltage outputs is generally indicated by the max. current.

Almost all manufacturers specify a resistance value for the load capability at current outputs. This specification does not indicate the load capability of the output currents of Knick isolation amplifiers absolutely correctly. Therefore the output load capability is 'traditionally' given as a voltage value. A 20 mA current output with a load capability of 10 V can be loaded, for example, with 2 kOhms at 5 mA or 1 kOhm at 10 mA. The specification of the maximum permissible load voltage 10 V therefore applies for each current value, whereas 500Ohms would apply exclusively for 20 mA.

Transmission accuracy
Knick isolation amplifiers are distinguished by partly extraordinarily low transmission errors so that the accuracy requirements of practically all measuring tasks in industrial measuring technology are easily met. The long-term stability of Knick electrical isolators ensures maximum transmission accuracy past the 5-year warranty on Knick electrical isolators.

Measuring signal quality
As accurate a transmission of the input signal as possible is required not only for applications in test engineering. Signal distortions due to change in polarity, overshoots in the case of signal changes, extreme angles in square-wave transmission are the rule in many isolation amplifiers available on the market. These undesirable properties are not immediately visible to the user. They often do not become noticeable until inexplicable errors occur during operation. In the cyclical, digital scanning of measured values, signal distortions, for example, due to overshooting, can cause serious measuring errors. For this reason, Knick traditionally places great emphasis on the accurate transmission of signals in the development of its isolation amplifiers.

Residual ripple
The output signal of DC isolation amplifiers is principally superimposed by low interference voltages. These interference voltages are caused, for example, by the chopper frequency as well as by mains feedover. The amplitude of this interference voltage, referred to as residual ripple, should be as low as possible because otherwise measuring errors cannot be ruled out - especially with low modulation.

Temperature coefficient (gain droop)
The temperature coefficient or gain droop is a specification for changes in gain caused by temperature changes. Droop rates are specified as a relative variable in %/K or as an absolute value, for example, in nA/K or µA/K.

In absolute value specifications, you need to check whether the TC refers to the input or the output.

The temperature coefficient (at the output) of an isolation amplifier is, for example, max. 10nA/K. A change in temperature of 20 K causes a change in the output current of 20 x 10 nA = 200 nA.

The TC of an isolation amplifier is, for example, 0.0025 %/K. A change in temperature of 20 K causes a change in amplification of 0.05 %.

Offset voltage, offset current
In (real) amplifiers, the output variable is not exactly '0' even when the input signal is '0'. The (input) offset voltage of an amplifier is by definition (gain-independent) the voltage that needs to be applied to the input in order for the output variable to become '0'. It therefore acts as an input voltage or an additional voltage acting in series with the input signal.

The (input) offset current of an amplifier also acts as an additional input signal. In amplifiers with a voltage input, the offset current generates a voltage drop at the internal resistor of a voltage source that is added to the input signal.

The offset voltage and offset current are so low in Knick isolation amplifiers that they are negligible for normal applications. Offset influences should only be considered for very special applications, for example, the 1:1 transmission of very small measuring signals or the transmission or amplification of very high-resistance signals.

The polarity of offset variables depends on each model and therefore is given as an amount variable without a plus or minus sign.

Cut-off frequency
DC isolation amplifiers are basically designed for the transmission or amplification of DC signals. In order to be able to transmit fast changes in the measured value almost without delay, DC isolation amplifiers are only conditionally suitable for transmission of alternating variables. The upper cut-off frequency for Knick isolation amplifiers and DC transformers is up to approx. 12 kHz for sinusoidal signals depending on the model.

As an upper limit frequency, as is common in electronics and telecommunications, the frequency is defined at which the gain is attenuated by 3 dB (in relation to the DC gain) or which corresponds to the amount divided by √2 (corresponding to approx. 71 % of the DC gain).

Common-mode behavior
If the same voltage Vcm is applied to ground at both inputs of a (symmetrical) amplifier, the input voltage remains Vin = 0. This operating mode is called common-mode modulation. In an ideally symmetrical amplifier, the output voltage Vout would also remain at 0. This is not the case in real amplifiers, however, i. e. a voltage deviating from 0 will appear at the output. A common-mode modulation always exists when the signal voltage is not at ground potential, i. e. when there is a potential difference between the (two) input lines and the ground, for example, in voltage measurements on a shunt lying at a high potential against ground.

Common-mode voltages can also occur as common-mode interference voltages, for example, in switching processes, due to interference on the signal lines or due to compensating currents.
The ratio between an applied common-mode voltage and the resulting output voltage is known as common-mode gain. However, in practice, the deviation from the ideal common-mode behavior of an amplifier that is indicated as common-mode rejection is of greater interest. The common-mode rejection S is defined as a quotient between opposite-mode and common-mode gain or as the (logarithmic) ratio between an applied common-mode voltage Vcm and a signal voltage Vd that would produce the same output signal.

S = 20 * log (Vcm/Vd) [dB]

The common-mode modulation of an isolation amplifier with Vcm= 800 V causes a 'common-mode error' (at the input) of 800V/10 (120/20) = 0.8 mV with a common-mode rejection of 120 dB. In an isolation amplifier with an input sensitivity of 60mV, this results in a 'common-mode error' of approx. 1.3 % of the end of range value.

For common-mode voltages in the DC and low-frequency AC range (50 Hz), high common-mode rejection is usually easy to achieve. The common-mode error in this range is negligible in Knick isolation amplifiers.

The common-mode rejection of amplifiers is, however, frequency-dependent and becomes considerably lower as the frequency increases. This is essentially influenced by the coupling capacitance between the primary and secondary coils of the transformer used that cannot be reduced as desired.

Therefore the common-mode rejection is considerably lower with pulse-shaped common-mode voltages or fast common-mode voltage changes.

Transient common-mode voltages can be caused both by single and by periodic switching processes, for example, in thyristor-controlled convertors.

In the Knick Varitrans® Series Isolation Amplifiers, special constructional measures have been implemented to suppress this kind of common-mode pulse.

These isolation amplifiers are therefore particularly suited for measurements on shunts with which common-mode pulse voltages or rapidly changing common-mode voltages are to be expected.

The term T-CMR (Transient Common-Mode Rejection) has been chosen for the corresponding data specification. It describes the quotient between differential DC gain and common-mode gain of a transient (interference) signal with a rise speed of 1000 V/µs

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