ADC (Analog to Digital Converter)
Block Diagram of
Partial Discharge In Data Storage
Data from the sensor will flow to analogue-digital
converter (ADC). In this part, the data will convert from analogue to digital.
It will separate regarding the level of the voltage or the waveform itself. If
sampling rate at the waveform are low the bit after ADC is also low and
sampling rate are high the bit on ADC will be high. The quality of the waveform
will be determined by the variable. The ADC has to work depending on the
frequency of the sensor and the voltage to make it more accurate or same as
original waveform and simple to construct the circuit in the ADC with 8 bit
resolution.
The data from the ADC will go to storage, but in
storage there will be another circuit and program that have been done to ensure
that the data will store in the storage. The circuit is to control the data to
go to specified program to save in some format file and store it in the
storage. The specified program is to manage the data not be separate in any
ways but it have to be structured according right sequential. This will ensure
that the data can be saved in one format and not to be interrupted by the other
files in the storage.
Internal
Error in Analogue to Digital Converter
In
process converting analogue to digital output, a quantization error is inherent
in all analogue to digital converter while sampling in amplitude axis. For example
a four bit ADC which is has resolution of (1 cont/100mV) shows results for the
different range for the output, for 1 count its between minus 0.5 to plus
0.5
in the range of the input. To more understanding, the figure below is illustrated;
The quantization error at amplitude
The figure
show an example that all analogue voltage values from 50mV to
150mV is produce same digital output, 0001; and between 150mV to 250 is
0010. Thus it will count as one value in digital. However, in analogue input is
between from 250mV to 350mV and the value in digital is 0011, converting the
value again into analogue the value is shown as 300mV but the actual value is
not 300mV cause the original value is between 250mV to 350mV. Thus in this case
the quantization error for this digital value is ±50mV. So the internal problem
in making the analogue to digital converter circuit its can only be minimize.
Analogue to digital converter in PIC
PIC is a family of Harvard architecture microcontrollers made by
Microchip Technology, derived from the PIC1640 originally developed by General
Instrument's Microelectronics Division. The name PIC initially referred to
"Peripheral Interface Controller”. PICs are popular with both industrial
developers and hobbyists alike due to their low cost, wide availability, large
user base, extensive collection of application notes, availability of low cost or
free development tools, and serial programming (and re-programming with flash
memory) capability. PIC16F877A is a small piece of semiconductor integrated
circuits. The package type of these integrated circuits is DIP (Dual Inline
Package) package. This package is very easy to be soldered onto the strip board.
However using a DIP socket is much easier so that this chip can be plugged and
removed from the development board. The purpose of using PIC 16F877A is writes
a program into it. It is very easy to assemble and it can be programmed and
erased up to 10,000 times.
The waveform sampling in discrete time
The figure above show the analogue sample converting to discrete
time.
This time sampling is the important part because its will show the
quality of converting to digital. If the time sampling is not high so the
quality wave form is worse.
Then
the discrete-time signal is quantized to generate the digital signal
Sampling in digital signal
After converting to digital, the
above figure show the line red is in digital unit of waveform. Digital waveform
did not same as the original value because had quantization error. To reduce
the quantization error, must put a resolution to make sure that digital output
most same as the analogue input.
In PIC16F877A has built in ADC
ports, which is port a is a input and port e is output. In PIC, it can be configured
according to resolution will need to enhance the quality waveform in 8 bit or
10 bit resolution. The digital signal after conversion represents the digital
version of the analogue signals and because it’s have 2 type of resolution, for
10 bit, the accuracy is 1 part in 1024, better than
0.1% at full scale or also 8 bit,
the accuracy is 1 part in 256, about 0.5%. There are a few function of ADC
ports.
• For 10 bit resolution, the
conversion of an analog input signal results in a corresponding 10-bit digital
number.
• The A/D module has high and
low-voltage reference input that is software selectable to some combination of
VDD, VSS, RA2 or RA3.
• The A/D converter has a unique
feature of being able to operate while the device is in Sleep mode.
• To operate in Sleep, the A/D
clock must be derived from the A/D’s internal RC
oscillator.
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