We have measured the intrinsic performance of a digital X-ray detector, the Medipix1, by examining the total detective quantum efficiency (DQE). We studied how the DQE depends on both the incident photon energy and spatial frequency. Reported here is the calculation of the detective quantum efficiency for the case of a 300 μm thick silicon diode detector attached to the Medipix1 readout chip. This was done by determining the modulation transfer function and the noise power spectrum; together these allow the frequency component of the DQE to be calculated. X-ray absorption efficiency in the detector gives the dependance on incident energy. This system was found to have a DQE that peaked at 0.118, using a dental X-ray source, and dropped to 0.049 at the Nyquist frequency of 2.94 line pairs per mm.