Definition of Terms for A/C Applications
CURRENT FREQUENCY RATIO (I/f)
This specification is the frequency equivalent of the time-current product for pulse applications. If the ratio of peak current to frequency exceeds the rating, the core will saturate.
LOW-FREQUENCY 3 dB Point
This spec is the frequency equivalent to the droop. For accurate measurements, the lowest frequency to be viewed should be a decade higher than the lower 3 dB point. DC current will move 3dB point to a higher frequency.
HIGH-FREQUENCY 3 dB Point
This spec is the frequency equivalent of the risetime. As a first approximation a 10 ns risetime is equivalent to a 35 MHz high frequency 3 dB point. For accurate measurements, the highest frequency to be viewed should be a decade lower than the high-frequency 3 dB point. If higher frequencies have to be accurately known, a model with a superior high frequency response and/or a curve displaying sensitivity vs. frequency may be requested from the factory for the model in question.
MAXIMUM RMS CURRENT
Is determined by internal heating of the monitor. The listed values are for frequency of 1 kHz or less. Additional restrictions may apply for higher frequencies. For high sensitivity models, the 50Ω feed-through termination at the scope might be the principal limiting element. This occurs when the square of the RMS value of the output voltage [Vout= sensitivity x Primary Current] divided by 50 ohms exceeds the power rating of the feedthrough termination, which typically has a 2 watt rating. To alleviate this limitation, IPC offers for sale a 15W, 50Ω feed-through termination.
Placing an IPC current monitor around a current-carrying conductor is equivalent to inserting a small series resistance. This series resistance depends strongly on the sensitivity of the monitor. For sensitivities of 1, 0.1, and 0.01 V/A, the respective insertion resistances are 20, 0.6, and 0.06 milliohms.
This spec is the most common limitation for high current applications. If the product of peak current and pulse width exceeds the amp-second rating off the selected monitor, the core of the monitor saturates and the displayed signal is greatly diminished from that moment on. This limit is specified for a single, unidirectional pulse. For repetitive pulses, the limit is reduced even further as the repetition rate increases.
If none of the listed models have a satisfactory current-time product, it is often possible to meet the requirements by increasing the cross section of the core. A small DC bias will also extend the point of saturation (see section on Saturation and Bias). Physical reversing of the monitor will approximately double the ampere second limit, but only for one pulse. Saturation does not damage the monitor.
In pulse applications, the output voltage signal from the monitor will always decay more rapidly than the current pulse being measured. A model with lower sensitivity tends to have a lower droop. DC current will increase droop.
This spec lists the fastest risetime recordable while keeping the reproduced pulse to less than 10% under- or over-shoot.
MAXIMUM PEAK CURRENT
This value determines the maximum current the monitor can tolerate. This limitation is primarily determined by the voltage breakdown rating of the connector, which is typically 500V. Also important is the voltage and power handling capability of the 50Ω feed-through termination at the scope.
The choice of monitor sensitivity, expressed in output voltage per Amp of primary current is influenced by the type of signal to be measured. If the current to be measured is only a few mA and at high frequency, then a monitor with a high sensitivity, such as our CM-100-xx model, should be selected. On the other hand, if the current to be measured is very high and the frequency low, then a monitor with a low sensitivity such as our CM-01-xx or CM-1-xx model should be selected.