An analogue–digital circuit for thermal control of the resonance frequency of optical ring resonators is suggested. This approach is characterised by a low power dissipation of 209 µW and needs only 2800 µm 2 of chip area in 0.16 µm CMOS technology. Therefore, it is well appropriate to be used to control each of the cells of large optical switch matrices. Compared with a purely analogue heater control approach the power dissipation is reduced to 7%.