Download On the design of a very high resolution DAC at 1 kHz for

On the design of a very high
resolution DAC at 1 kHz for
space applications
D. Mitrovgenis, K. Makris, D. Fragopoulos,
G. Tsiligiannis, C. Papadas
Integrated Systems Development (ISD S.A)
Athens, Greece
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Overview
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Context of the project
Architecture
Radiation and low frequency noise
Preliminary results
Conclusion
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Context of the project
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
The 24-bit Digital-to-Analog data converter is
intended to be suitable for space and is designed
using the 0.13um CMOS technology of STM.
The converter will be used by ESA in the Laser
Interferometer Space Antenna (LISA) experiment
for detecting and observing gravitational waves in
the frequency range 10-4 to 10-1 Hz from massive
black holes and galactic binaries.

This work is supported by an ESA contract.
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Key Features
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The core of the converter is a 3rd order ΣΔ modulator with an internal
multibit quantizer for achieving excellent dynamic performance and
monotonic behavior in the range of 0.1 mHz up to 1 kHz.
It supports both serial and 24-bit parallel input format.
Target SNR performance ~120 dB at full bandwidth.
Programmable Oversampling Ratio for sampling rates of 6 or 12 kHz.
Slave I2C for register configuration.
Differential current output (0 to 5.84mA).
Reference current of internal DAC may be fixed through an external
resistor.
1.2V/3.3V for the digital and analog part respectively.
Separate Digital and Analog power down features.
Power consumption <70 mW.
Cascade of multiple DAC devices by using common biasing signals.
This converter can be used as a stand alone device or embedded as an
IP core.
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Specifications for radiation tolerance
LET for SEL immunity:
≥ 70 MeV/mg.cm-2
TID tolerance:
≥100 krad
Temperature range (functional) :
Temperature range (full performance) :
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-55 °C < T < 125 °C
0 °C < T < 50 °C
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Architecture
24-bit
digital
input
Input
interface
I2C
slave
IRP1
x2
IRP2
x2
Dynamic Element
Matching
Algorithm (DWA)
32
Generation of
reference
current
IRP3
x2
3rd order ΣΔ
modulator
Thermometer encoded data
Matrix Array of 32 current sources
Biasing
scheme
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SINC
x32/x16
Differential
current output
24-bit D/A
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Architecture – Digital Part (1/4)
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Efficient implementation of the digital interpolation implies using a
multi-stage structure (reduced computational complexity) for the
filtering, which is inherent in the interpolation procedure.
FIR filters have linear phase and symmetric coefficients (symmetric
direct-form topology).
Interpolation filters are designed using a a MAC architecture based on
a polyphase implementation, which is optimal for x2 upsampling.
Linear interpolation is achieved by a SINC Filter (x16/x32):


Fs
Simple HW implementation.
Medium attenuation of image replicas.
FIR Equiripple
Linear Phase
(IRP1) x2
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Half Band
(IRP2) x2
Half Band
(IRP3) x2
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SINC
x32 or x16
OSR·Fs
7/28
Architecture – Digital Part (2/4)
3rd order ΣΔ architecture employing
oversampling and quantization
noise shaping.

Multi-bit quantization for stability
and relaxation of the requirements
of the analog post filter.
2
 3  2L  1  N

DR  10 log10   2 L   2  1 OSR 2 L 1 

2  

L: Order of the ΣΔ modulator
N: Number of quantization bits
OSR: Oversampling Ratio

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Architecture – Digital Part (3/4)
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The modulator under DC
excitation or with very low
frequency signals may
produce periodic patterns,
which give rise to idle tones.

The in-band tones will not be
suppressed by the postreconstruction filter.
• A 19-bit pseudo-random signal generated by a 35-bit Linear Feedback Shift
Register (LFSR) is used to break the periodicity of the tones.
• This additional random noise will be shaped to higher frequencies by the ΣΔ
modulator with negligible SNR degradation.
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Architecture – Digital Part (4/4)
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The inferior linearity of the internal DAC
in multibit architectures degrades the
overall performance.
The selection of elements is based on
the DWA algorithm, which is used so as
to suppress in-band harmonic distortion
that stems from element mismatch. The
DWA circuitry selects the DAC elements
in a rotational way, resulting in a firstorder shaping of the error due to element
mismatch.
The algorithm gives good results for σE
≤ 0.1%.




 2 N OSR3 
ENOB  log 2 

  1- 1  
E
N

 2  

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Architecture - Analog Part (1/6)
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Matrix of 32 PMOS differential current sources
(correspond to 5-bit DAC).
Level shifter for adjustment of digital signals
from 1.2V to 3.3V.
Reference current is generated internally and
can be tuned by an external resistor.
Appropriate biasing scheme.
Low frequency design (because of flicker
noise) is a challenge.
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Architecture - Analog Part (2/6)
10uA
Current
Bias
Bandgap
1.2V
+
LPF
LOW NOISE
OPAMP
-
Array of current
sources
IRef
M1
Iout+ IoutRRef
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Rext
The Bandgap block is used for generating a stable voltage tolerant to
temperature variations.
The Low Pass Filter before the amplifier is used to decrease the noise bandwidth
of the Bandgap reference block.
The Operational amplifier is biased by a 10uA current in order to generate a fixed
reference current IRef through transistor M1.
The reference current can be fixed inside or outside by using an external resistor.
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Architecture - Analog Part (3/6)
1:32
M2
M3
Current
Bias
1.2V
Bandgap
LPF
IRef
I2n,in
+
LOW NOISE
OPAMP
-
M1
Iout
R2
I2n,out
I/V
converter
Vout
RRef
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The Bandgap and the operational amplifier are low noise designs.
The noise from the various blocks is additive and its total amount at the
output of the converter is related to the number of current sources that steer
current at a given time.
An I/V converter which converts the current (and each noisy component) to
voltage before the later is fed to the reconstruction filter is used in the noise
calculations.
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Architecture - Analog Part (4/6)
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The Bandgap reference
block provides 1.2V at
low temperature
coefficient.
Resistors with different
temperature coefficient
are used for generating
the reference voltage.
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Architecture - Analog Part (5/6)
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Low noise and distortion
implementation based on a two
stage implementation: a
differential first stage followed by
a push-pull output gain stage.
Large devices are used in the
input stage to achieve low noise
levels.
The amplifier senses the voltage
drop across RRef and fixes the
current IRef by biasing
appropriately transistor M1.
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10 uA Bias
current
+
LOW NOISE
OPAMP
-
M1
IRef
RRef
15/28
Architecture - Analog Part (6/6)
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Differential folded cascode PMOS
current sources for increased output
M4
impedance controlled by a
complementary 32-bit signal.
In this Regulated Cascode topology,
the drain source voltage of M2 is kept
stable for obtaining high overall output Amplifier
impedance against channel length
modulation.
This is accomplished by a feedback
loop consisting of an amplifier (M4 and
M3) and M1 acting as a follower,
Increased transistor size for high
linearity and decrease of flicker noise.
The layout follows the commoncentroid technique to avoid systematic
errors and achieving better matching.
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M2
Follower
M1
M3
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For addressing the radiation (1/3)
The design of a radiation-hardened chip follows the general design
flow with some manufacturing and design variations that reduce the
susceptibility
to
high-energy
subatomic
particles
and
electromagnetic radiation, such as would be encountered in outer
space, high-altitude flights or nuclear reactors.
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The 0.13 um CMOS technology of STM is qualified for space
applications, disposes a Radiation Hardened digital library, whereas
it assures small threshold voltage drifts.
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Measurements with heavy ions and protons no latchup nor
hard fail occurred up to 120 MeV/mg.cm-2
High intrinsic radiation hardness measured with gamma
rays. The irradiated chip functionality was 100% at 100
krad (Si) with no over-consumption.
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For addressing the radiation (2/3)
fsm_state
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For the digital part the
robust cells from the
library are used during
synthesis.
Moreover:
 Fault
masking by
TMR at FSM level.
 Synchronous reset.
fsm_state_next_1
FSM1
FSM2
fsm_state_next_2
FSM3
FSM1
control_signal
control_signal_1
VOTING
SCHEME
fsm_state_next_3
D-flip flop
control_signal_3
VOTING
SCHEME
control_signal
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For addressing the radiation (3/3)
 For the analog part:
 Surrounding each n-channel device with a p+ guard
ring, which cuts any possible radiation induced
parasitic path.
 Increase the W/L ratio of the transistors, which
increases the capacitance and the driving power.
 Adding additional capacitances to the most sensitive
nodes.
 Increase the distance between the p+ diffusion in the
well and the n+ in the substrate.
 Increase the number of the substrate and well
contacts.
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For reducing the flicker noise (1/2)
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Flicker noise is the main limiting factor and affects the
low frequency performance of the device.
The flicker noise is mainly generated by the trapping and
releasing of the charge carriers whose time constant is
finite and the phenomenon is maybe not accurately
modeled by the circuit simulator during current source
switching.
For an acceptor like trap:
τe=emission time
τc =capture time
Electrical noise and RTS fluctuations in advanced CMOS devices
G. Ghibaudo, T. Boutchacha, Microelectronics Reliability 42 (2002) 573–582
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For reducing the flicker noise (2/2)
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Our strategy is based on:
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Increasing the device size.
Using very low noise biasing schemes.
Reducing the time slot for which the current sources
drive current in order to avoid quasi-static phenomena.
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Preliminary results(1/4)
Flicker noise is dominant in
low frequencies.
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Preliminary results(2/4)
SNR improvement
when large devices are
used (flicker noise
reduces by increasing
the WL product).
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Preliminary results(3/4)
By further improving the
bandgap.
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Preliminary results(4/4)
SNR results vs
temperature at 10-4 Hz.
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Output current variation vs temperature
The output current
variations are in the order
of uA.
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Conclusion
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A high resolution Digital to Analog Converter for
space application has been presented.
The DAC exhibits more than 120 dB SNR above 0.1
Hz and 80 dB in the 0.1 mHz range.
The DAC exhibits low power characteristics and has
a very small footprint about 1 mm2 with pads.
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Suitable for stand-alone and embedded applications.
Multiples instances of the same DAC.
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Thank you for your attention !
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External filtering considerations
OUTP
Differential
current
output
Analog
Part
VOUTP
Low Pass
Differential
Differential
I/V converter
voltage Reconstruction voltage
Filter
output
output
OUTM
VOUTM
DAC



VOUTP
VOUTM
The reconstruction filtering (stop-band attenuation, ripple) is related to the
content of the application.
An I/V converter should be placed before the continuous time reconstruction
filter.
It is desirable for the driven load to exhibit a low pass characteristic.
Passband
(kHz)
Stopband
(kHz)
Stopband
Attenuation
Passband
Ripple
Filter type
Order
1.01
45
100 dB
0.001 dB
Chebyshev-II
4
1.01
45
80 dB
0.01 dB
Chebyshev-II
3
1.01
45
70 dB
0.01 dB
Butterworth
3
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