We propose a new digital-to-analog converter (DAC) for realizing a synapse circuit of mixed-signal spiking neural networks. We named this circuit "time-domain DAC (TDAC)". This produces weights for converting a digital input code into voltage using one current waveform. Therefore, a TDAC is more compact than a conventional DAC that comprises many current sources and resistors. Moreover, a TDAC with leak resistance reproduces biological plausible synaptic responses that are expressed as alpha functions or dual exponential equations. We will show numerical analysis results of a TDAC and circuit simulation results of a circuit designed by the TSMC 40 nm CMOS process.