Low Cost DAQ Implementation

Low Cost DAQ Implementation

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

MSP430

F5529

ADC Drivers

Power Supply (USB)

8 GPIOs

4 Analog Output

Channels

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.


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.

-/+10VAnalogInputs

MUX ADC

+3VReducedAnalogInputs

PGA AD8250

-/+10v power source


Method 1:Advantages:1: No voltage translation is needed, PGA is capable to handle up to +/- 13.5 volt2. Rin > 1Gohm3. Cin < 0.5pF


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


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


Price $ (1k) Input Imp.Noise

[nV/√Hz]OpAmppackage Offset drift [μV] V offset [μV]

Vin [+/- Volt]

V Supply [+/- Volt] Part # Manuf.

2.94 100M/6pF 8.8 4 Typ=6,

Max=25 18.5 18 Opa4188 Ti

3.56 1G/3pF 11 4 Typ=0.5 Max=1.2Typ=10,

Max=125 13.8 15 AD8624Analog Devices

5.8-6.81 2 Typ=0.01 Max=0.05Typ=0.5, Max=5 18.3 18 LTC1151

Linear Technology

-/+10V

Analog

Inputs

OPA4188

ADC+ 3V Reduced Analog Inputs

OPA4188

-/+10v power source


Method 2:Advantages:1: BOM price is lower then in Method 12. Rin > 100MOhm3. Cin < 6pF


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 volts4: OpAmps have a constant gain.


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.

ADC

3v – 0vReducedAnalogInputs

PGA 116Voltage Dividers

Vref

-/+10v

Analog

Inputs


Method 3:Advantages1: 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.


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 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 NoiseDigital

PSOffset Error

Gain Error Protocol

Supply Voltages Vref

Static Power Dissipation PSRR EEPROM Price

AD5686 2 1 6uVpp 1.8-5.5 1.5mV 0.10% SPI 2.7 - 5.5 External 1.8mW -80 - 7-11$

LTC2635 2.5 1 750uVpp 2.7 - 5.5 5mV 0.80% I2C 2.7 - 5.5 Internal 1.8mW -80 - 3-8$

MCP4728 2 0.2 290uVpp 2.7 - 5.5 20mV 3% I2C 2.7 - 5.5 Internal 4.4mW -57 + 1-2$

DAC7714 1 2 65nV 0-3.3 SPI +*15V-15V External 45mW -90 - 13-15$

Depending on the parameters benefit, and it’s low cost, MCP4728 DAC was chosen.


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 market2: 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 ManagementExternal Vref

We need accuracy of +/- 2 LSB with 12bit ADC i.e.

The accuracy of the ADC sampling depends of +/- Vref:

in RADC

R R

V VNV V

4 100% 0.1%4096

Power ManagmentExternal Vref

The MSP430F5529 has an internal Vref with accuracy of:

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.

_max _

_

2.5375 2.5100% 100% 1.5%2.5

REF REF typ

REF typ

V V v vV v

Power ManagementStepUp 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 ManagementLDO

In order to supply stable power of 3.3v and 4.5v to the components we used TLV70233DBVR andTLV70245DBV accordingly because of they relatively low price – 0.16$ for each.

Making PCB layout

PCB design software:• Orcad Alegro PCB EditorComponents 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 layoutSignal 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 layoutHigh 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 layoutHigh 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 layoutUSB differential 90ohm pair:

To calculate the width of the USB data traces the following equation was used:

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

87 5.98ln0.81.41r

HzW T

Making PCB layoutHigh 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 layoutFree 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 layoutPCB 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 reflections5. Adding “tear drops” on every connector pins

Low Cost DAQ Implementation

Documents

supply voltage

voltage dividersvref

reference voltage

voltage translation

low offset voltage

analog inputs opa4188adc

inputs opamps

adc input