IEEE Solid-States Circuits Magazine - Summer 2020 - 24

variation of the ideal crossing point.
To address these amplitude modulation effects, several published
designs use a level shifter at every
VCO output phase; however, this can
take up a significant portion of the
ADC's power budget [10].

There is a very wide variety in published VCObased ADC designs, even when limited to highly
digital designs that do not require analog blocks
such as opamps.
drive requirements for the preceding circuits are lower than those
for the source-follower approach.
Conversely, using the current drive,
the transconductor approach allows
for easier setting of the oscillation
frequency and typically has a lower
voltage swing, leaving more voltage headroom to optimally size the
ring oscillator. For low-frequency
circuits, an additional consideration is the 1/f noise, which is typically lower for PMOS than NMOS
transistors, although chopping can
greatly relieve this aspect, as mentioned previously.
In their basic forms, both the
source follower and transconductor

drive circuits of Figure 10 are notoriously nonlinear. In this respect, an
improvement is the resistive drive
circuit of Figure 11(a) (the figure
shows the bottom drive version),
originally proposed in [48]. Various
implementations [10], [48]-[50] have
demonstrated up to 11 bits of linearity in a pseudodifferential VCObased ADC.
Whichever drive circuit is used
to tune the overall VCO, there will
always be a significant modulation of
the voltage Vring across the ring oscillator. As illustrated in Figure 10(c),
this leads to an amplitude modulation
of the VCO output waveform, which,
in turn, leads to a signal-dependent

Other Oscillator Circuits
A n imp or t a nt v a r ia nt of a r ing
oscillator with simple inverters is
one using differential delay elements [see Figure 12(a)] [51]. The
adv a nt ages to this a re that the
number of delay elements can now
be even and that the phase readout can be done with a differential
sense amplifier, combining excellent sensitivity with very low power
consumption [52]. This also makes
the readout much less sensitive to
modulation in the crossing point of
Figure 10(c), greatly relaxing metastabilit y problems and eliminating the need for power-consuming
level shifters at the phase readout. Finally, with differential delay
elements, the current in the supply lines has significantly smaller
spikes, which also reduces the power
supply noise.
The typical circuit for such a differential delay element is shown in
Figure 12(b). It consists of two main
inverters that form the main signal
path and two (small) auxiliary inverters that create a balancing mechanism, ensuring that the oscillator
cannot get locked in a latched state.
Improved variants with lower power
use a modified feedforward coupling

fVCO

Vdd

"Ordinary" Ring Oscillator

Ring-Oscillator
VCO ADC
Vin

Control

Resistively Tuned
Ring Oscillator

R
R
(a)

Vtune

(b)

FIGURE 11: (a) Resistively driving the ring-oscillator VCO-based ADC [48] improves (b) the
linearity of the VCO tuning curve.

N Stages

vin+

(a)

4×

4×
Main
Inverters

1×
vout+
Auxiliary
Inverters

SU M M E R 2 0 2 0

IEEE SOLID-STATE CIRCUITS MAGAZINE

W
Control

(b)

FIGURE 12: (a) A ring-oscillator VCO with differential delay elements and (b) their implementation based on inverters.

Vdd
2W

1×
vin-

Control

vout-

IEEE Solid-States Circuits Magazine - Summer 2020

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