A current-mode $\Delta\Sigma$ analog-to-digital data converter (ADC) for biomedical readout front-end systems is being presented. The core of this circuit is a current-controlled ring oscillator (CCO) whose oscillation frequency is proportional to the input current signal. To shape the quantization noise and improve the resolution, the CCO has been used as an integrator in $\Delta\Sigma$ configuration. Experimental results show that the proposed ADC can reach to 54 dB signal-to-noise ratio while the sampling frequency is 45.5 kHz and consuming 800 nW power (with an oversampling ratio of 64). The sampling frequency of this ADC can be adjusted from 700 Hz to 540 kHz with a scalable power dissipation, corresponding to 17.5 pW/Hz sampling frequency.