IEEE Circuits and Systems Magazine - Q3 2023 - 55

I. Introduction
E
merging applications in industrial instrumentation
like condition monitoring for predictive
maintenance, sonars, acoustic, and seismic
measurements use wide-bandwidth, low-noise sensing
technologies [1], [2]. Signal chains involving these
applications require ADCs with ultra-low noise spectral
density (NSD < −155 dBFS/Hz) and low distortion
(THD ≤ −100 dB). A CTΔΣM is an attractive architectural
choice to realize such ADCs due to the ease with which
it can be driven and its inherent anti-alias filtering.
These properties are better appreciated when system
level benefits are considered, as discussed below.
Fig. 1(a) shows the simplified schematic of a signal
chain employing a high-resolution discrete-time ΔΣ
ADC. It consists of a programmable-gain instrumentation
amplifier (PGIA) which excites a filtering driver.
This specialized input driver is needed to faithfully
drive the large switched-capacitor input impedance of
the ΔΣ modulator. A carefully chosen RC lowpass filter
is inserted between the driver and the ΔΣ ADC to better
isolate the glitches caused by the switched input capacitance
from the driver. The noise and distortion of the
driving filter must be much lower than that of the ADC.
In addition, the switched-capacitor load offered by the
ADC at its reference pins necessitates a buffer with sufficiently
wide bandwidth and low output impedance.
An alternative approach is to use a CTΔΣM-based
signal chain as shown in Fig. 1(b). Thanks to the resistive
input impedance and anti-alias filtering, the
Figure 1. Signal chain comparison (a) using a discrete-time ADC and (b) using a CT-ΣΔ ADC. (c) Board level practical signal
chain with a DT-ΣΔ ADC (left) versus a CT-ΣΔ ADC (right).
Raviteja Theertham is with Analog Devices Inc., Toronto, ON M5G 2C8, Canada.
Shanthi Pavan is with the Indian Institute of Technology Madras, Chennai 600 036, India.
THIRD QUARTER 2023
IEEE CIRCUITS AND SYSTEMS MAGAZINE
55

IEEE Circuits and Systems Magazine - Q3 2023

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