A Low Mismatch and High Input Impedance Multi-Channel Time-Division Multiplexing Analog Front End for Bio-Sensors

A multi-channel time-division multiplexing (TDM) analog front end (AFE) with low mismatch and high input impedance for bio-sensors is proposed. To reduce mismatches among channels, a four-channel synchronous DC offset elimination system based on TDM technology is proposed, which can multiplex both the low-noise amplifier (LNA) and the variable gain amplifier (VGA). An input impedance boosting technology based on differential difference amplifier (DDA) is proposed to increase the equivalent input impedance of the AFE. The proposed improved digital-controlled DC-servo loop (DCDSL) is applied to four-channel synchronous calibration, which realizes the multiplexing of the first-stage amplifier, solves the problem of the comparator misjudgment, and breaks through the limitation of the channel numbers. The circuit is implemented in
$0.18~\mu \text{m}$
CMOS process. The core area and power of each channel are 0.048 mm
²
and
$2.48~\mu \text{W}$
from 1.2 V supply voltage, respectively. The measured results show that the gain mismatch among channels is 0.8%, and the equivalent input impedance is 1.29
$\text{G}\Omega $
. Within 130 ms, DCDSL can calibrate a DC offset voltage of ±300 mV. The input-referred noise over the range of 0.1~100 Hz is
$1.74~\mu $
Vrms.