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 5 mu V-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 7.5 mu V-rms to 4.2 mu V-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 +/- 50mV 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 0.018mm(2). The 8.7mm(2) die, fabricated in a 0.13 mu m 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.