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Analog to Digital Converters
Representing a continuously varying
physical quantity by a sequence of
discrete numerical values.
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By: Engr. Muhammad Muizz Bin Mohd Nawawi
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Successive Approximation
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Flash Comparison
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If N is the number of bits in the
output word….
Then 2N comparators will be
required.
With modern microelectronics
this is quite possible, but will be
expensive.
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Pro and Cons
Slope Integration & Ladder Approximation
Cheap but Slow
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Pro and Cons
Flash Comparison
Fast but Expensive
Slope Integration & Ladder Approximation
Cheap but Slow
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Pro and Cons
Successive Approximation
The Happy Medium ??
Slope Integration & Ladder Approximation
Cheap but Slow
Flash Comparison
Fast but Expensive
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Resolution
Suppose a binary number with N bits is to represent an analog value ranging from 0 to A There are 2N possible numbers Resolution = A / 2N
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Resolution Example
Temperature range of 0 K to 300 K to be linearly converted to
a voltage signal of 0 to 2.5 V, then digitized with an 8-bit
A/D converter
2.5 / 28 = 0.0098 V, or about 10 mV per step
300 K / 28 = 1.2 K per step 8
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Resolution Example
Temperature range of 0 K to 300 K to be linearly converted to
a voltage signal of 0 to 2.5 V, then digitized with a 10-bit
A/D converter
2.5 / 210 = 0.00244V, or about 2.4 mV per step
300 K / 210 = 0.29 K per step
Is the noise present in the system well
below 2.4 mV ?
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Quantization Noise Each conversion has an average uncertainty of one-half of the step size ½(A / 2N) This quantization error places an upper limit on the signal to noise ratio that can be realized. Maximum (ideal) SNR ≈ 6 N + 1.8 decibels (N
= # bits) e.g. 8 bit → 49.8 db, 10 bit → 61.8 db
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Signal to Noise Ratio Recovering a signal masked by noise
Some audio examples
In each successive example the noise power is reduced by a factor
of two (3 db reduction), thus increasing the signal to noise ratio by
3 db each time.
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Example 1 Example 2 Example 3 Example 4
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Conversion Time
Time required to acquire a sample of the analog signal and
determine the numerical representation.
Sets the upper limit on the sampling frequency.
For the A/D on the BalloonSat board, TC ≈ 32 μs,
So the sampling rate cannot exceed about 30,000 samples
per second (neglecting program overhead) 12
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Data Collection – Sampling Rate
The Nyquist Rate
A signal must be sampled at a rate at least twice that of the
highest frequency component that must be reproduced.
Example – Hi-Fi sound (20-20,000 Hz) is generally sampled at about
44 kHz.
External temperature during flight need only be sampled every few
seconds at most.
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