  In this paper the weighting field $E_W$ and the frequency dependence of the
admittance Y of n$^+$p pad sensors irradiated by 24 GeV/c protons to equivalent
fluences in the range $\Phi_{\rm eq} = 3$ to $13\times 10^{15}$ cm$^{-2}$ are
investigated. 1-D TCad simulations are used to calculate $E_W$. For $\Phi_{\rm
eq} < 10^{13}$ cm$^{-2}$ $E_W$ depends on position and time. However, for
higher $\Phi_{\rm eq}$ the time constant $\tau$ is much longer than the typical
electronics readout time and $E_W = 1/d$ ($d$ = sensor thickness). It is
demonstrated that the increase of the resistivity of the Si bulk with
irradiation is responsible for the increase of $\tau$. The admittance Y of
irradiated pad sensors has been measured for frequencies between f = 100 Hz and
1 MHz and voltages between 1 and 1000 V at -20$^\circ$C and -30$^\circ$C. For f
< 1 kHz the parallel capacitance C$_p$ shows a f dependence. A model with a
position-dependent resistivity is able to describe the data. It is concluded:
1. The weighting field of a highly irradiated sensor is the same as the
weighting field of a fully depleted sensor before irradiation. 2. Models with a
position-dependent resistivity describe the frequency dependence of C$_p$ for
irradiated sensors.
