Aim TTi LCR400 Manual De Instrucciones página 11

frequencies requires a more complex equivalent circuit than the simple two component series or
parallel circuits discussed here. In practice the solution is to select component types to match
the frequency range of the application.
For the majority of resistors, where inductive and capacitive parasitics are minimal, both series
and parallel circuits will give identical results for resistance.
For resistors where inductance is the significant parasitic, the series equivalent circuit will give
the value which matches the manufacturer's data-sheet. For high value devices, capacitance can
start to be significant and the parallel equivalent circuit may be more appropriate.
Normally R+Q should be selected for resistors; the Q of a resistor will usually be very low –
especially at the low measurement frequencies used. However if the series and parallel
resistances at 10kHz differ significantly to those at 100Hz or 1kHz, the Q will be significant. Either
the inductance or capacitance of the resistor is producing an effect. Selecting either C+R or L+Q
will quantify the parasitic capacitance or inductance.
Low value resistors can be measured at any of the three LCR400 test frequencies but high value
resistors (>100kΩ) are best measured on the 100Hz range. The instrument warns if a
measurement is outside its maximum accuracy range by flashing the units annunciator; if
accuracy can be improved by changing the measurement frequency the frequency annunciator
will also flash, see Display section.
Capacitors
All capacitors have parasitic inductance and resistance in addition to their intended capacitance.
The leads of a capacitor can add significant inductance at high frequencies. Spiral wound metal
film capacitors can have significant parasitic inductance, which is why they are not used for
decoupling high frequencies. Some types of ceramic capacitors can provide excellent de-
coupling, i.e. have high capacitance with low series resistance and inductance, but can be very
lossy. Large value electrolytic capacitors can have significant inductance – this inductance can
even resonate with the capacitance at the measurement frequencies of the LCR400. This has the
effect of showing a known high value capacitor to have either negative capacitance or
inductance.
Capacitors have two main types of parasitic resistance. Firstly there is the physical resistance of
the dielectric and dielectric losses; this is normally specified in terms of the Dissipation Factor 'D'
or loss tangent and is frequency dependent. Secondly, there is the physical resistance of the
leads and the connections to the electrodes on the dielectric. The lead and connection resistance
are usually negligible, but on high value electrolytics, used to smooth power supplies, it can be
very important. The series resistance of such devices is often a manufacturers specified
parameter.
For most capacitors, other than high value electrolytics, the parallel equivalent circuit will give the
capacitance that matches the manufacturers data sheet. For low loss capacitors the series and
parallel equivalent capacitances will be the same.
Electrolytic capacitors are polarity sensitive and should be connected to the instrument correctly
and bias applied. For very high value electrolytics, for which the manufacturer specifies
Equivalent Series Resistance (ESR) the series equivalent circuit should be used.
The LCR 400 provides the means to investigate the losses of capacitors either in terms of
dissipation factor (C+D) or in terms of equivalent series or parallel resistance (C+R).
To get maximum resolution and accuracy, low values of capacitance, (<4nF) are best measured
on the LCR 400 at 10kHz after zeroing the capacitance with no component connected. Higher
values, (>10µF) should be measured at 100Hz. The instrument warns if a measurement is
outside its maximum accuracy range by flashing the units annunciator; if accuracy can be
improved by changing the measurement frequency the frequency annunciator will also flash, see
Display section.
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