[previous] [next] [contents]    Impedance measurement (pin to ground)

Impedance measurements require the use of the ATAP pins, together with the TBIC and ABMs, to apply and read analog test stimulae. The measurement of an impedance between a pin and ground may be done using the set up shown in figure 1.

Figure 1: Test set up for measuring an impedance between pin and ground.

The measurement procedure may be summarised as follows for this case:

  • A known current IT is applied through AT1, by way of switches S5 and SB1, to the unknown impedance (ZD)
  • The resulting voltage VT is measured at AT2, which is connected to ZD through switches SB2 and S6
  • The value of ZD may then be computed as ZD = VT / IT

Notice that the error margin in this procedure depends on the relative values of the unknown impedance (ZD), the voltmeter input impedance (ZV) and the impedance of each switch in the measurement path. The value obtained for ZD will only be acceptable if the following assumptions are valid:

  • ZV >> ZS6 + ZSB2 (to guarantee that the voltage drop in S6 and SB2 is negligible)

ZV + ZS6 + ZSB2 >> ZD (to guarantee that the percentage of IT which does not flow through ZD is negligible)