A highly linear, Sigma-Delta based, sub-Hz high-pass filtered ExG readout system

High-Pass <inline-formula> <tex-math notation="LaTeX">$\Sigma \Delta$</tex-math> </inline-formula> Converter Design Using a State-Space Approach and Its Application to Cardiac Signal Acquisition

“…Due to process (P), voltage (V), and temperature (T) variations and poor circuit structures, pseudoresistors achieve a linearity of about -40 dB and compromise on the accuracy of the implementation of the high-pass cut-off frequency. Although the use of a g m stage in combination with current sources [9] leads to a power efficient solution, the g m of any transistor is inherently non-linear and is less robust to P, V and T variations. Off-chip digital solutions [10] can be used to obtain a highly accurate and linear high-pass cut-off frequency at the expense of power.…”

“…With the bandwidth of the ECG signal extending from sub-Hz to 200 Hz [5], a major challenge for an ECG readout system lies in implementing the sub-Hz high-pass cut-off frequency (f hpf ) as this translates into the realization of large time constants. To realize large time constants in the order of a few seconds, there exist techniques that employ pseudoresistors [6]- [8] or g m blocks [9] or off-chip solutions such as using an IIR-filter [10] or a resistor [11]. Recently, a new technique to implement large time-constants has been introduced, employing duty-cycled resistors [12].…”

Structured electronic design of high-pass ΣΔ converters and their application to cardiac signal acquisition

“…With the bandwidth of the ECG signal extending from sub-Hz to 200 Hz [1], a major challenge for an ECG readout system lies in implementing the sub-Hz HP cutoff frequency as this translates into the realization of large time constants on-chip. To realize large time constants in the order of a few seconds, there exist techniques that employ pseudoresistors [2] or g m blocks [3]. Although these techniques realize large time constants in an area and power efficient manner, they are heavily limited in both linearity and accuracy.…”