IEEE Circuits and Systems Magazine - Q1 2022 - 25

suited to implement reconfigurable ADCs [171]-[173],
which can be handled by ANNs in CR/SDR.
3) Time-Based Quantization
Another scaling-friendly approach-suitable for medium-low
resolution (< 10 bit) and GHz-range speed required
in RF digitizers-is that based on time coding
by using Voltage Controlled Oscillators (VCOs), rather
than using amplitude quantization. The idea consists of
translating the quantized information in the amplitude
domain to the time domain by means of a voltage-tofrequency
conversion. This can be performed by a VCO
circuit as conceptually depicted in Fig. 14, that shows
one of the first implementations of this concept [174].
The principle of operation behind this approach relies
on the use of a ring oscillator to count the number of
edges within a given time period. The result is directly
related to the input signal, thus obtaining a digital representation
of the amplitude. In addition to their higher
speed, they can operate at supply voltages below 0.5 V,
while reducing the analog content, which makes them
highly scalable. Moreover, VCO-based quantizers provide
an implicit first-order noise-shaping filter due to
their inherent differentiator operation required to implement
the frequency-to-voltage conversion in the digital
domain [100], [174]-[190].
The main limitation of VCO-based quantization is
the inherent nonlinearity of the voltage-to-frequency
conversion, requiring the use of either calibration or linearization
techniques [180]-[182], [191], [192]. Another
strategy consists of embedding VCO-based quantizers
in the back-end stage of a cascade2 RDM, such that their
nonlinearities can be attenuated by the noise shaping
of precedent stages in the cascade [193]. Indeed, this
idea has been used in other types of two-step hybrid
RD/Nyquist-rate ADCs [82], [194]. An alternative to VCObased
quantizers consists of using pulse-width modulation
(PWM) [195], [196] and a Time-to-Digital Converter
(TDC) that generates a time-quantized representation
of the signal. Indeed, embedded TDCs have been succesfully
implemented by a number of recent RDMs
[197]-[203]. Another approach is based on the so-called
Gated Ring Oscillators (GROs), which can attenuate the
nonlinearity of VCOs and be combined with active-RC
integrators, thus benefiting from both amplitude- and
time-based circuit techniques [204].
2 Cascade are also known as multistage noise shaping (MASH) RDMs.
op on
on
op
cm
φ2
φ1
φ1
φ2
Cs1
φ2
Passive SC
Filters
x (t)
Loop
Filter
Inverter-Based
OTAs
Frequency/
Time-Based
Quantizer
ADC
y (n)
Vtune
DAC
Quantizer
φ2
φ1
φ1
φ2
Vr+ Vr-
Figure 14. Scaling-friendly analog circuit techniques for mostly-digital RDM and hybrid RDM/Nyquist-Rate ADCs.
FIRST QUARTER 2022
IEEE CIRCUITS AND SYSTEMS MAGAZINE
25
φ2
φ1
φ2
yd
φ1
c0
z-1
c1
z-1
c2
FIR-Based
DACs
CLK
N-Bit Register
01 11 0
z-1
cN
C1
φ1
Cs2
φ1 Cs2
φ2
φ1
φ1
φ2
φ1
Cs2
φ1
φ2
C2
SC-DAC
Residue
H(z)
Logic
ADC
NS-SAR Quantizer
IEEE Circuits and Systems Magazine - Q1 2022

Table of Contents for the Digital Edition of IEEE Circuits and Systems Magazine - Q1 2022
