Implementation Strategy

INTRO

What is it? If the Use Case is the WHY and the Design Requirements is the WHAT, then the Implementation Strategy is the HOW

The HOW evaluates relevant options and chooses a path to design and build an instrument.

You transform the Requirements (functions, features) into an Implementation (physical structure, techniques, algorithms).

The arrows above tell a realistic story - several different implementations may achieve the target. Similarly, there may be multiple options that fail to hit the requirements.The strategy requires

• Research
• Experience, intuition, creativity
• Preliminary design and calculations

Strategy also requires courage to select an implementation based on what you know now, while having confidence to adjust later as you learn and discover more during the actual design.

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BLOCK DIAGRAM

The Block Diagram defines a physical structure for the implementation as shown in a typical instrument below.

• Attenuator - scales a 20V input to < 4V for the ADC.
• Filter - reduces 60Hz, clock or RF pickup.
• ADC - converts analog signal to Digital Word.
• Microcontroller - executes key hardware / software functions such as conversion, scaling, range selection and data transfer to PC.

INPUT ATTENUATOR

A search on the web returned two potential circuits for an input attenuator . Which one better fits our requirements?

• CIRCUIT REQUIREMENTS
  • 4V Range: K=1
  • 20V Range: K = 4V/20V = 1/5
  • Rin = 1M both ranges

OPTION 1: R-Divider with Single Switch.

Single switch connects R2 into circuit for divider action.

Gain	Conditions	Analysis / Results
1 (4V Range)	SW1=Off	Rin = Open K = 1
1/5 (20V Range)	SW1=On	Rin = R1 + R2 K = (R2+Rsw)/(R1+R2+Rsw)

• EVALUATION
  • Gain equation contains resistance of switch, an added error source of inaccuracy. (CON) .
  • Rin is not fixed for both ranges. Rin = Open on 4V Range. (CON)
  • Only requires 1 switch (PRO)

OPTION 2: R-Divider with Dual Switch.

Two switches select either direct connect or resistor tap for divider action.

Gain	Conditions	Analysis / Results
1 (4V Range)	SW1=On SW2=Off	Rin = R1 + R2 K = 1
1/5 (20V Range)	SW1=Off SW1=On	Rin = R1 + R2 K = R2/(R1+R2)

• EVALUATION
  • Gain equation does not contains resistance of switch, better accuracy. (PRO) .
  • Rin is fixed on both ranges. (PRO)
  • Requires 2 switches (CON)

Strategy Decision

For better accuracy and a fixed Rin on both ranges, Option 2 will be implemented at the cost of a second switch.

LOW-PASS FILTER

A filter reduces external noise sources (60 Hz mains, system clocks, radio frequencies, etc.) from corrupting the signal being digitized by the ADC.

• CIRCUIT REQUIREMENTS
  • Simple RC Low-pass filter
  • For a 10x attenuation of a 60Hz mains pickup, choose a filter bandwidth of fc = 60Hz / 10 = 6Hz.

• EVALUATION
  • For fc = 6Hz, RC = Tau = 1/(2*pi*fc) = 0.026
  • Settling time to 1%. Ts = -ln(0.01)/ln(e)*Tau = 0.12s. < 1sec
  • Approved.

ADC CONVERTER

The development board (Arduino UNO R3) is equipped with an ADC on the Atmel Microcontroller (ATmega328P). Does this ADC fulfil the accuracy requirements?

 

• ADC Specifications at 25C
  • Resolution: 10 bits (1024 counts)
  • Vref: 5V +/-50mV (Arduino Bd Vcc)
  • Offset Error:  2 LSBs
  • Gain Error:   2 LSBs at Full Scale.

• CIRCUIT REQUIREMENTS
  • Offset Error: 15mV
  • Gain Error: 1.5% of Reading

• EVALUATION
  • ADC Resolution: 
      V_LSB = 1LSB/2^10*Vref = 1LSB/1024*5V = 4.88mV
      (<15mV PASS)
  • ADC Offset Error: 
      Voff = 2LSB/2^10*Vref = 2LSB/1024*5V = 10mV
      (<15mV PASS)
  • ADC Gain Error: 
      Kerr = 2LSB / 2^10 * 100% = 0.2%
      (<1.5% PASS)
  • Vref Gain Error: 
      50mV/5V*100% = 1%
     (<1.5% PASS)
  • Approved (based on preliminary analysis).

MICROCONTROLLER

The Microcontroller Board is a small computer that provides key functions: execute software, convert analog signals to digital form, control hardware via digital lines and communicate with the PC.

For this project, the selected microcontroller is the Arduino UNO. Why?

• my past experience with the UNO found it powerful, easy-to-learn and fun.
• the growing familiarity of Arduino with students provides an easy on ramp.
• an explosion of online tutorials for Arduino basics, projects and code make for a broad resource of support / ideas.

What are the relevant Arduino UNO specs for this project?

• Microcontroller IC: ATmega328P
• 14 digital input/output pins
• 6 analog inputs
• a USB connection
• Arduino Software (IDE) - write and upload code to the board.

• EVALUATION
  • 1 analog input needed, 6 available
  • 2 digital outputs needed (range switch control). 1 digital input needed (range select pushbutton), 14 available
• Approved.

Other viable Microcontroller Board options.

• MPS430 (TI)
• STM320 (STMicroelectronics)
• A growing universe of powerful options depending on scope of project, capability and preference.

PROJECT PROTOYPE

As a Proof of Concept we'll build a prototype using a Solderless Prototying Board (Analog Circuit), an Arduino UNO Board and a PC.

ARDUINO INTERFACE

What signals connect our instrument front-end and Arduino?

Signal	Description
+5V	Power for Opti-Isolated solid-state relay and pushbutton.
GND	Ground reference for circuit.
A0	Analog Input for voltage measurement.
D11	Digital Output control for SW1.
D12	Digital Output control for SW2.
D13	Digital Input for Pushbutton (V Range Select).

HANDS-ON

Play in the Excel file - modify values, see what happens!

• Excel File:  DVM1_Implementation_Strategy1.xlsx
• Right Click on the filename, select "Save link as...".

ADC Resolution

• Suppose a measurement accuracy of < 2mV is required. How many ADC bits N with a 4V reference are needed to resolve it?

Bandwidth (Settling Time)

• Your design application requires a Low Pass filter to settle to 1% in 5ms. What bandwidth fc is needed?

 

ADJUST THE IMPLEMENTATION?

Are these implementations set in stone? During development, the design approach can be refined and adjusted for a number of reasons.

• Assumptions or initial theoretical understanding were incorrect.
• Further analysis finds short coming or deficiency of approach.
• The requirement specifications have changed.

 

NEXT UP

Given an Implementation Strategy, we can jump into the Circuit Design.

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