  Electrical readout of spin qubits requires fast and sensitive measurements,
but these are hindered by poor impedance matching to the device. We demonstrate
perfect impedance matching in a radio-frequency readout circuit, realized by
incorporating voltage-tunable varactors to cancel out parasitic capacitances.
In the optimized setup, a capacitance sensitivity of
$1.6~\mathrm{aF}/\sqrt{\mathrm{Hz}}$ is achieved at a maximum source-drain bias
of $170~\mu$V root-mean-square and with bandwidth above $15~$MHz. Coulomb
blockade is measured via both conductance and capacitance in a quantum dot, and
the two contributions are found to be proportional, as expected from a
quasistatic tunneling model. We benchmark our results against the requirements
for single-shot qubit readout using quantum capacitance, a goal that has so far
been elusive.
