Impedance Measurement

REW can make measurements of impedance by using both inputs of the soundcard. Impedance measurements of drive units can be used to calculate the Thiele-Small parameters. The general connection arrangement for impedance measurements is shown below:

Conections for Impedance Measurement

The sense resistor, which must be non-inductive, is used to measure the current flowing into the load, which will be (Vleft - Vright)/Rsense. The voltage across the load is Vright, so the impedance is voltage/current = Rsense*Vright/(Vleft - Vright). Note that the accuracy of the result is only as good as the accuracy of the value entered for the sense resistor.

Good results can be obtained using a headphone output to drive the load, with a 100 ohm sense resistor. If a line output is used the sense resistor typically needs to be much larger as line outputs have high output impedance and limited drive capability, try 1 kOhm but note that the results will have much higher noise levels.

An alternative is to drive the load via a power amplifier, which can deliver the lowest noise levels and most accurate results, but great care must be taken as the levels a power amplifier can generate can easily damage soundcard inputs. If using a power amplifier the sense resistor can be much lower, 33 ohms or less, but the soundcard inputs should be connected via a resistive divider providing around 20dB of attenuation and ideally the inputs should also be protected by back-to-back zener diodes to clamp the input to less than 5V.

The soundcard input connected to the load must be the same one which has been chosen as the input in the REW soundcard settings. In the diagram above that is the right input, but if the left is being used simply swap left and right in the diagram. If the left and right connections are the wrong way around your impedance measurements will show curves that are shifted up by approximately the value of the sense resistor. It does not matter which headphone output is used as REW puts the test signal on both outputs.

Making an Impedance Measurement

The result of the measurement is displayed in the graph area, information about the measurement appears in the Measurements Panel. Measurements are given a default name of the date and time at which they are made, a more appropriate name can be entered in the box at the top of the measurements panel.

When the mouse cursor is within the graph panel of the Impedance & Phase graph the equivalent series resistance + inductance or resistance + capacitance and parallel resistance||inductance or resistance||capacitance of the impedance at the cursor position is shown at the bottom left corner of the graph, this is useful when making measurements of inductors or capacitors to check their value. For capacitor measurements the values are most accurate at frequencies where the total impedance has dropped below a few hundred ohms.
Impedance Readout

For details of the various ways of viewing the measured data, including averaging multiple measurements, refer to the Graph Panel help.

Calibrating the Impedance Rig

Small gain differences between the soundcard input channels cause incorrect calculation of the load current and the impedance. These can be calibrated out by making a measurement with the load disconnected and the sense resistor shorted out. N.B. both soundcard inputs must be connected to the same output signal.
Impedance Calibration Setup

The calibration factor used for impedance measurements is shown in the measurement info panel next to the measurement thumbnail, along with the sense resistor value.
Impedance Measurement Info

Resistance of Test Leads

When measuring very low impedances the resistance of the test leads may become significant. To measure this, first calibrate the rig as above, then make a measurement with the leads shorted together at the point the load is attached (set the RLEADS value to zero before making the measurement). The measurement should be fairly uniform, perhaps showing variation at very low frequencies depending on the low frequency limitations of the headphone drive stage. If the result is more than one or two tenths of an ohm, check that the connections are tight and the leads are not too flimsy. Enter the resistance from the flat part of the measurement into the RLEADS box on the measurement panel.

Input Channels Swapped

If the input channels have been connected the wrong way around the impedance measurements will be too high by approximately the value of the sense resistor, make a test measurement of a resistor (of less than 100 ohms) to check that everything is wired correctly.

Impedance Measurement Quality

The main source of measurement noise is acoustic noise and vibration during the measurement. Loudspeakers act as microphones, generating small voltages in response to sounds and vibrations that are picked up as part of the load voltage. To minimise this effect use high drive levels, low sense resistor values, avoid noisy environments and isolate the loudspeaker from vibration. Using a power amplifier to drive the speaker provides high signal levels and allows a low series resistor to be used.

Another source of error is the input impedance of the soundcard, which is in parallel with the load. This limits the accuracy of the measurement of high load impedances, the method is most suitable for impedances below a few hundred ohms.

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