56-channel direct-coupled chopper-stabilized EEG monitoring ASIC with digitally-assisted offset correction at the folding nodes

We present a 56-channel neural recording interface with a chopper-stabilized DC-coupled front-end and a programmable mixed-signal DC cancelation feedback. Each recording channel has a fully-differential amplifier with 51-54dB of gain, an input-referred noise of 5μV_rms integrated from 10Hz to 5kHz and a CMRR of 65dB. Input DC-coupling allows for a simple chopping scheme without the area overhead of large capacitors and extra non-idealities compensation circuitry. Chopping is used to reduce the integrated input-referred noise from 7.5μV_rms to 4.2μV_rms over the bandwidth of 1Hz to 1KHz. Each channel contains a digital integrator and a 16-bit delta-sigma DAC in the feedback in order to cancel input DC offsets of up to α50mV and suppress the low frequency drift. Compensating the input DC offset at the folding node of the OTA provides an input-referred noise that is independent of the DC offset value. The recorded data by the array is digitized by 8 column-parallel SAR ADCs with 8-bit resolution and ENOB of 6.6 bits. Each channel in the neural recording array occupies 0.018mm². The 8.7mm² die, fabricated in a 0.13μm CMOS process, dissipates 1.07mW from a 1.2V supply. The integrated circuit has been validated in vivo in online intracranial EEG recording in freely moving rats.