Design Requirements Spec
INTRO
If the Use Case provides the WHY, then the Requirements define the WHAT
for the instrument to be created.
A Design Requirements Specification describes the product in
terms of its Features. Functions and Performance.
Where are the requirements derived from? These specs can
- fall naturally out of the Use-Case
- reflect industry standards
- require creating a strategy of making these decisions
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FUNCTIONS
- Measure voltages listed in Use Case: 1.5V to 20V
- Provide 2 selectable Voltage Ranges: +4V and +20V Range.
FEATURES
- Pushbutton selects voltage range
- Laptop window displays voltage.
- No Calibration
PERFORMANCE
- Accuracy
- 4V Range: Verror = ±15mV ±1.5% of Reading
- 20V Range: Verror = ±75mV ±1.5% of Reading
- Measurement overange: 20%
- 4V Range: 4V x 1.2 = 4.8V
- 20V Range: 20V x 1.2 = 24V
- General
- Input R: 1M both ranges
- Input polarity: positive only (unipolar)
- Input V max: +/--50V (must safely handle overvoltages)
- Display update: 0.25 to 0.5s (industry standard)
- Settle to within accuracy < 1 sec
VOLTAGE RANGE STRATEGY
Why two voltage ranges, why not a single 20V range?
- Many ADCs have an
input voltage range of 0V to 4V (or 0V to 5V).
- A 4V input range is an easy choice to directly digitize the voltage
below 4V.
- However a 20V range requires an 5:1 attenuator to scale down
the 20V input to a 4V input level of the ADC. Unfortunately, scaling
down comes with a price. Any error of the ADC, 10mV for example, will
scale up by 1:5 for a 50mV equivalent error at the battery, degrading the
accuracy at lower voltages!
- So, we'll keep a separate lower range for better accuracy for inputs
below 4V.
ACCURACY STRATEGY
Where did the accuracy requirement come from? The answer came from
digging into the Use Case and making some reasonable assumptions .
- The Use Case states that customers typically replace a
1.5V AA at 1.1V and a 9V at 7.0V.
- An Industry Rule-of-Thumb: the instrument should be 10x more
accuracte than characteristic to be measured.
1.5V AA Accuracy Required
- What is the expected voltage change from nominal to depleted
battery?
dV = 1.5V - 1.1V = 0.4V
- For a measurement with 10x more accuracy:
dVerr =
0.4V/10 = 40mV
9V Accuracy Required
- What is the expected voltage change from nominal to depleted
battery?
dV = 9.0V - 7.0V = 2.0V
- For a measurement with 10x more accuracy:
dVerr =
2.0V/10 = 200mV
GAIN & OFFSET ERROR STRATEGY
Most instruments exhibit two significant non-ideal behaviors: an
Offset Error and a Gain Error. The goal is to select values for
Offset and Gain Error that will meet the overall accuracy of 40mV (4V
Range) and 200mV (20V Range).
With some trial and error, as well as past experience with
voltmeters, we settle on some reasonable Offset and Gain Errors. Let's check
if they pass spec at 20% charge
remaining.
- 4V Range: 15mV + 1.5% of Reading
- Check Verror = 15mV + 1.5%/100 x 1.13V = 38mV (< 40mV PASS)
- 20V Range: 75mV + 1.5% of Reading
- Check Verror = 75mV + 1.5%/100 x 7.1 = 158mV (< 200mV PASS)
BANDWIDTH STRATEGY
Ideally, the voltmeter should respond quickly to a voltage change - implying a
high bandwidth.
However, the long battery test leads may pickup unwanted signals
like 60Hz mains or a multitude of radio frequencies.
Unfortunately, these signals can cause fluctuating readings or harrnful
transients. What's the fix? Add a Low Pass filter before the ADC to reduce
these interfering signals. The filter bandwidth should be
- LOW enough to attenuate 60Hz mains pickup.
- HIGH enough to settle to specified accuracy without a significant
perceptable delay to the user.
HANDS-ON
Play in the Excel file - modify values, see what happens!
Offset and Gain Error
- Suppose the system had a larger offset error. Increase Voffset from
15mV to 25mV. What is the Verr at 1.5V?
- Can you reduce the Gain Error (Kerr) sufficiently to still
meet the spec of Verr = 40mV at 1.5V?
ADJUST THE SPECS?
Are these specifications set in stone? During development, specs can be
refined and adjusted for a number of reasons.
- A competitor has an improved performace.
- The product achieved better than expected performace.
- The accuracy cannot be met given the budget or technology.
- The assumed needs were over or understated.
NEXT UP
The next step is to draft the Implementation Strategy to answer HOW we can achieve the Design Requirements.
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