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Description
Summary
Analog blocks are mandatory parts of today’s CMOS ASICs used in high energy physics
experiments. Such the blocks like supply voltage regulators, high quality ADCs and
DACs include a reference circuit. The common way to implement reference circuit with
a low temperature sensitivity and high power supply ripple rejection is the bandgap
reference circuit.
With steadily decreasing power supply voltages (Vdd) in present and future deep
sub-micron CMOS technologies a design of any bandgap voltage/current reference
on-chip becomes a non-trivial task.
The classical voltage summing bandgap reference circuit (BGR) featuring parasitic
diodes (p-diffusion in N-well) is not suited for a 0.13µm CMOS technology with a
maximum Vdd of 1.2V. It is so because the value of bandgap voltage (Vgap) in silicon
(1.12V) turns out to be very close to the nominal supply voltage of the process.
This causes the circuit to fail.
We use a new structure called a dynamic-threshold MOS transistor (DTMOST) in place
of conventional diodes in the circuit. Such a combination constitutes a high-quality
reference circuit able to fit into the reduced supply voltage range of a 0.13µm CMOS
technology.
In order to construct a model of the DTMOST device suitable to run simulations,
pre-design characterizations have been carried out.
The circuit has been submitted in CuTe2 MPW submit in May 2004.
Testing has shown that the measured performance of the circuit is in good agreement
with the performance predicted by simulations.
The circuit is able to operate at supply voltage in range from 1.4V down to 0.85V.
Sensitivity of the reference voltage to the power supple voltage variation is
3.3mV/V.
The circuit can be externally trimmed in order to reach the operating point where
the reference voltage has the lowest temperature coefficient (less than 1mV in the
range from 0ºC up to 80ºC). However, due to statistical variation the trimming is
dedicated to the chip. With no trimming the temperature coefficient is typical
0.05mV/ºC.
The circuit will operate in the radiation environment of high energy physics
experiments and therefore radiation tolerance is an issue for the design. We used
the CERN’s in-house X-ray facility for the irradiation of the chips. The effect
caused by irradiation consists in the shift of the reference voltage while the
circuit remains fully operational.
The change of the reference voltage has been monitored in the time of the
irradiation. We tested 5 chips and have seen that the reference voltage does not
demonstrate an identical behaviour. The only thing to conclude is that the reference
voltage deviates in the range of ± 12mV. It indicates that the irradiation is the
dominant factor of instability of the reference voltage.
In some applications not only stability of the reference voltage is important but
its absolute value as well. The absolute value differs from chip to chip and is
caused by fabrication process variation. With few samples available we can roughly
estimates chip-to-chip spread of the value of the reference voltage. It turned out
to be confined in the range of ± 15mV.
A commercial 0.13um CMOS technology is suitable to design a high quality bandgap
voltage reference circuit in spite of a low power supply voltage.
Redesign of the circuit is needed in order to improve chip-to-chip spread of
absolute value of the reference voltage as well as its sensitivity to irradiation.
