Instrument Impedance

This page applies to the instrument (your AC or RF voltmeter, multimeter, oscilloscope, or spectrum analyzer) that you use to display the output voltage of the sensors shown at Sensors for 5 Hz-1 MHz 

For best accuracy we recommend using a display instrument with high input impedance and low shunt capacitance, equivalent to or better than our calibration instrument, which has input impedance 10 Megaohms shunted by less than 30 pF capacitance. Below 30 kHz frequency we may use input impedance of 1 Megaohms shunted by 100 pF. 

If your display instrument has 50 ohm input impedance, we strongly recommend that you use a high impedance adapter or high impedance probe with low capacitance with your display instrument to obtain the calibrated result from the sensor.  Here is how to choose scope impedance

Also, for some resonant sensors with a sharp high resonant peak (i.e. models MC95R, MC95Rw, MC90R, and MC110R), the input impedance your instrument presents to the sensor can affect the frequency of resonance, and affect the output voltage near resonance and at higher frequencies above resonance.

Coax Length: At frequencies above 30 kHz, the length of coax cable you use to connect the sensor to your display instrument can also significantly affect results. If the sensor has no built-in coax cable, then for calibration tests we used a 37-inch (0.94m) long coaxial cable which has 66 pF total coax capacitance.

Sharp bending or yanking of a thin coax may break the wires inside the coax, which is usually seen as erratic readings.