Written by:
Gabriel Heifets
Alexander Zaprudsky
Instructor:
Evgeniy Kuksin
Project Goal
Build a complete multipurpose DAQ:
Find the best price/performance in class
components.
Create schematics design.
Design PCB layout and produce it.
Block Diagram
Power Supply (USB)
4 Analog
Output
Channels
ADC
MSP430
Drivers
8 GPIOs
F5529
2 PWM
Generators
USB 2.0
eZ430
Analog inputs
Analog Input requirements:
• 8 independent channels.
• +/- 10v Input Range.
• At least 1MOhm Input impedance.
• BW>100khz.
Analog inputs
Voltage input range problem
The issue is that we want to sample +/- 10 volt
with our DAQ system, but the internal MSP430
ADC is capable to handle 0~3 voltage. To achieve
our goal, three possible solutions were tested.
Analog inputs
Voltage input range problem
Method 1:
The first solution was to interface each ADC
channel to a PGA AD8250, which is capable to
handle +/- 13.5 volt, and reduce it to an ADC
valid voltage.
+/- 10V
Analog
Inputs
MUX
PGA
AD8250
+/- 10v power source
+ 3V
Reduced
Analog
Inputs
ADC
Analog inputs
Voltage input range problem
Method 1:
Advantages:
1: No voltage translation is needed, PGA is
capable to handle up to +/- 13.5 volt
2. Rin > 1Gohm
3. Cin < 0.5pF
Analog inputs
Voltage input range problem
Method 1:
Disadvantages:
1: One input PGA, we will have to use 8 of it’s kind,
or a MUX.
2: A high BOM and volume price (AD8250 = 5$)
3: An additional power solution for the PGA’s
power demands: +/-15 volts
Analog inputs
Voltage input range problem
Method 2:
Each ADC input is interfaced to a simple OpAmp
with a constant gain with a low offset voltage.
Three OpAmp were compared: OPA4188,
AD8624, LTC1151
Analog inputs
Voltage input range problem
V Supply
Vin
[+/- Volt] [+/- Volt] V offset [μV]
Manuf.
Part #
Ti
Opa4188
18
18.5
Typ=6,
Max=25
Analog
Devices
AD8624
15
13.8
Typ=10,
Max=125
18.3
Typ=0.5,
Max=5
Linear
Technology LTC1151
+/- 10V
18
Offset drift [μV]
Price $ (1k)
4
8.8
100M/6pF
2.94
Typ=0.5 Max=1.2
4
11
1G/3pF
3.56
Typ=0.01 Max=0.05
2
5.8-6.81
OPA4188
+ 3V Reduced
Analog Inputs
Analog
Inputs
OpAmp Noise
package [nV/√Hz] Input Imp.
OPA4188
+/- 10v power source
ADC
Analog inputs
Voltage input range problem
Method 2:
Advantages:
1: BOM price is lower then in Method 1
2. Rin > 100MOhm
3. Cin < 6pF
Analog inputs
Voltage input range problem
Method 2:
Disadvantages:
1: One 4 inputs OpAmps, we will have to use 2 of
it’s kind.
2: OpAms with a low offset voltage are expensive.
(5.88 $, 7.12$, 23.2$ accordingly)
3: An additional power solution for the OpAmps
power demands: +/-10 volts
4: OpAmps have a constant gain.
Analog inputs
Voltage input range problem
Method 3:
An analog inputs are interfaced to an 8 inputs
PGA116. The PGA’s valid voltage levels are 0~4, so a
voltage dividers (implemented with low tolerance
resistors and reference voltage) will be added to the
PGA’s inputs.
Vref
3v – 0v
Reduced
Analog
Inputs
+/- 10v
Analog
Inputs
Voltage
Dividers
PGA 116
ADC
Analog inputs
Voltage input range problem
Method 3:
Advantages
1: Low BOM price: only one PGA is needed for
an 8 inputs. The PGA’s price is relatively low,
PGA116 = 2.25$.
2. Supply voltage: 4V, no comprehensive power
solution is needed.
3. Programmable output gain.
Analog inputs
Voltage input range problem
Method 3:
Disadvantages:
1: Low PGA’s inputs voltage range tolerance –
additional voltage translator required.
2: Low input impedance of the complete circuit.
Analog inputs - Schematics
Analog Outputs
Analog Output requirements:
• 4 independent channels.
• +/- 10v capable outputs.
• Iout Current > 5mA per output.
• 12bit Resolution.
• BW>1kHz
• Internal flash or E2PROM for values retention.
(Optional)
Analog Outputs
Analog Voltage Generation
Analog Outputs
Analog Voltage Generation
Analog voltage generation is implemented with
an MCP4728 DAC. This DAC choice among the
other DACs will be featured in upcoming table.
To extend the output voltage range to +/-10V, an
optional amplifier circuit (based on an LM2903),
was added.
Analog Outputs
Choosing DAC
Part
INL
DNL
Noise
Digital
PS
Offset
Error
AD5686
2
1
6uVpp
1.8-5.5 1.5mV 0.10%
LTC2635
2.5
1
MCP4728
2
DAC7714
1
750uVpp 5.5 - 2.7 5mV
0.2 290uVpp 5.5 - 2.7 20mV
2
65nV
0-3.3
Gain
Error Protocol
Supply
Voltages
Vref
Static Power
Dissipation
SPI
5.5 - 2.7
External
1.8mW
80-
-
7-11$
0.80%
I2C
5.5 - 2.7
Internal
1.8mW
80-
-
3-8$
3%
I2C
5.5 - 2.7
Internal
4.4mW
57-
+
1-2$
*+15V-15V External
45mW
90-
-
13-15$
SPI
PSRR EEPROM Price
Depending on the parameters benefit, and it’s low cost, MCP4728 DAC was chosen.
Analog Outputs
Analog Voltage Generation
The MCP4728 DAC voltage outputs are
controlled through I2C interface. To reduce the
over analog voltage outputs BOM price, the
MCP4728 IC was chosen although it’s lower
accuracy. To compensate this disadvantage, a
feedback calibration circuit implemented.
Analog Outputs
Analog Voltage Calibration
Two calibration method were considered and
BOM optimized:
1: The four voltage outputs are introduced to a
four channel MUX, and the MUX output divided
by a high precision resistors to accommodate
MSP430 valid voltage level.
Total BOM price for this method:
0.172$+2.8$=2.872$
Analog Outputs
Voltage Calibration
2: Each one of the four channels will be divided
by a high precision resistors to an MSP430 valid
input voltage.
Total BOM price for this method: 0.688$
During to lower BOM price, the second method
was implemented in our solution.
PWM
PWM requirements:
• opticaly isolated OD/OC.
• Isink > 2A.
• Vmax > 48v.
PWM
PWM
PWM outputs are generated by MSP430, and
optically isolated by an optic solid state relay
VO14642AT. To drive the VO14642AT inner LED
MOSFET driver was added.
GPIO
GPIO requirements:
• 8 GPIO channels.
• 5V TTL levels.
• Drive capability of 5mA.
GPIO
GPIO
A two methods were exanimated to implement
the GPIOs level translation.
1: A dedicated solution by a level translators
that are available on the market
2: Single MOSFET based voltage translation for
an each GPIO.
During the BOM consideration, METHOD 2 was
chosen.
Power Management
Power requirements:
• High accuracy Vref to achieve +/- 2 LSB in ADC
sampling accuracy.
• LDOs for power supplies.
• Step up converter to +10v for the OpAmp
supply to achieve analog outputs +10v
requirement (Optional).
Power Management
Power Management
External Vref
We need accuracy of +/- 2 LSB with 12bit ADC
i.e.
4
100%  0.1%
4096
The accuracy of the ADC sampling depends of
+/- Vref:
N ADC
Vin  VR 

VR   VR 
Power Managment
External Vref
The MSP430F5529 has an internal Vref with
accuracy of:
VREF _ max  VREF _ typ
VREF _ typ
2.5375v  2.5v
100% 
100%  1.5%
2.5v
But in order to achieve accuracy of +/- 2 LBS the
accuracy of the Vref should be at least 0.1%.
So the LM4132BMF an external Vref with
accuracy of 0.1% was chosen.
Power Management
StepUp Converter
In order to get +10v from +5v USB power for the
optional OpAmp power supply in the Analog
Outputs circuit we decided to use charge pumps
instead of Boost converter to prevent possible
noises and to reduce BOM price.
Because we can’t trust that the USB power is always
+5v and not less, we couldn’t use only one charge
pump to double it, so we needed two charge
pumps with more than +5v operating voltage range.
We found only one charge pump with such
operating voltage range - ICL7660S
Power Management
LDO
In order to supply stable power of 3.3v and 4.5v
to the components we used TLV70233DBVR and
TLV70245DBV accordingly because of they
relatively low price – 0.16$ for each.
Making PCB layout
PCB design software:
• Orcad Alegro PCB Editor
Components library:
•A standard HSDSL footprints library
Making PCB layout
Layers:
• The PCB contains four layers. The top layer is
dedicated to a components’ placement and a
signal routing.
• The second layer dedicated to the powers’
planes
• The third layer dedicated to the ground plane
• The bottom layer dedicated to the signals
routing
Making PCB layout
Signal types:
• One differential pair. This pair used for an USB
communication, and demands 90ohm
impedance. The PCB manufacturer will have to
make the desired calculations according to the
applied material.
• Power signals: all power signals, except the
planes, where routed with a traces at least 20 mil
width.
Making PCB layout
High noising components:
Switching components that are implements our
charge pump, were placed in an isolated part of the
PCB, and surrounded with a grounded shield.
Making PCB layout
High noising components:
Analog and Digital signals and Power supplies are
separated one from another to prevent parasitic
leakage and crosstalk to achieve a better accuracy.
Making PCB layout
USB differential 90ohm pair:
To calculate the width of the USB data traces the following
equation was used:
87
 5.98H 
z
ln 

0.8
W

T
 r  1.41 

Where:
Z=45ohm (desired impedance)
Er=4 (PCB dielectric –layer C)
H=5 (dielectric thickness between the pair layer (A) and a power
plane (D) )
T= 1.4 (thickness of copper traces)
W= 9.5 The calculated traces width
Making PCB layout
High accurate components:
• The vref generator that is dedicated for the
calibration purposes is placed as close as possible to
the vref sense point to prevent additional noises.
• Vref routing was implemented a star design and not
as a daisy chain.
Making PCB layout
Free PCB areas:
All the PCB areas that are left free from the routing traces
were filled with a solid ground planes to make better EMI
protection. A dozen of VIAs were added to provide better
connection between the ground planes, and to make the
current return path as short as possible.
Making PCB layout
PCB housekeeping:
All island that were created during solid planes
adding where detected and cleared. One way planes
and traces that are formed so called “antennas”
were eliminated or grounded.
Optional features
1. Firmware upgrade over USB channel (Using TI
BSL feature).
2. Making traces width to match the component’s
pin’s footprint width or wider – to reduce ESR.
3. Adding maximal quantity of solid ground traces
to the decoupling capacitors (trace to each side of
the capacitor pin’s footprint) – to reduce
impedance by paralleling traces.
4. Using an arc instead of sharp angles on a trace
turns to reduce signal reflections
5. Adding “tear drops” on every connector pins