Analog to digital conversion notes


An analogue signal is similar to a sine wave and is
generally less than 5v (5,000mV) in amplitude. Low-level signals
are generally expressed in mV, to make them instantly
recognisable and easy to talk about.

Analog to digital conversion notes

Fig: 1shows an analogue signal. It may be as low as a few
millivolts or hundreds of millivolts. In general this type of
signal will be too small to be detected by a microcontroller. A
micro needs a signal greater than about 3,500mV so that the
waveform appears on the input line as a HIGH, during the peak of
its excursion. It should be as high as 5,000mV as possible
for reliable detection of the signal. The instruction to “read”
the HIGH is:

BTFSS 06,3 ;       Test input line GP3

This instruction tests line GP3 and if the bit is HIGH (the
line is SET – the line sees a HIGH) the micro will skip the next
line in the program and execute the line that follows in the
program.

The shape of the waveform does not matter. It can consist of
large excursions and small excursions as shown in fig: 2.

Analog to digital conversion notes

Only the large excursion(s) will be detected by the micro as
the others may be too small to be detected. To increase the
analogue signal to as high as 5,000mV as possible, an amplifier
is needed, as shown in fig: 3.

Analog to digital conversion notes

The amplifier maybe one or tw1o stages, depending on the amplitude of the original signal.

Each stage of an amplifier will increase the size of the
signal about 70 times. If you are very lucky, you may get an
amplification of 100x (100 times). Thus a 5mV signal with one
stage of amplification will produce a 350mV signal. This is not
sufficient to be detected by the micro. Another stage will easily
produce a full 5,000mV signal and the micro will detect its
presence.

The second stage only needs to amplify the signal about 10 to
12 times to be sufficient for the micro, and the added gain
provided by the stage simply drives the waveform into “bottoming”
and “cut-off” as shown in fig: 4.

Analog to digital conversion notes

This means the waveform will be “clipped” at the top and
bottom and converted to a fairly “square-ish” shape. Normally
this would be called distortion but in digital terms we don`t
worry about it being distorted and simply use it for its
“triggering” properties.

Suppose you have a waveform that is higher than 5mV (say 30 –
50mV) and want to know if it will trigger the micro after a
single stage of amplification.

Connect the circuit in Fig: 5 to the micro and connect a LED with 470R resistor in series, to pin GP0.

Burn the following routine into a PSEUDO`508 and the LED will illuminate when the waveform is detected as a HIGH.

Program for a Pseudo`508:

BSF 03,5 ;Select Page1

MOVLW 08 ;GP3 input

TRIS 06 ;Load TRIS

BCF 03,5 ;Select Pag0

Test1 BTFSS 06,3 ;Test input line

GOTO Test2

BSF 06,0

GOTO Test1

Test2 BCF 06.0

GOTO Test1

There is only one problem with the circuit in Fig: 5.

Analog to digital conversion notes

At the end of a whistle or speech, the LED may be illuminated
or extinguished. It all depends on the last cycle of the
waveform. The circuit sits with the output approx mid-rail and
the micro does not know if this is a high or low, and takes the
reading by the direction of the last cycle.

Some of the inputs of the micro are Schmitt Triggered. This
means a HIGH has to be 85% to 100% of rail voltage for it to be
seen as a HIGH and betw1een 20% and 0% to be seen as a LOW.

The non-Schmitt Trigger inputs see a LOW as 20% to 0% and a HIGH as above 2v for 5.5v operation.

If the last cycle went from zero to mid-rail the micro will
see the waveform as a low on Schmitt Trigger inputs and a HIGH on
the other inputs. If the cycle came from a high to about
mid-rail, the waveform
will be seen as a high. This
problem can be overcome via a few instructions in the program, by
looking at the input and if it is not changing, it is ignored.
Suppose the circuit only just activates the micro with a loud
whistle. To improve the sensitivity of the input another
stage will be needed.

Analog to digital conversion notes

This can be a DIGITAL STAGE as shown in fig: 6 or an analogue stage as shown in fig: 7.

Analog to digital conversion notes

A digital stage simply means the biasing resistor is left off
the base of the second transistor so the transistor turns on
fully when a signal greater than 650mV is detected and is fully
turned off at other times. This stage is ideal for the micro as
only tw1o voltage levels are delivered. Either 0v or rail voltage
(5v).The other advantage is the stage does not take any
quiescent (idle) current.

This stage is only suitable if you are sure you have plenty of
“over-voltage” to drive the transistor into saturation. By this
we mean you must have at least 1v (1,000mV) drive signal so you
can be sure the transistor will turn on (saturate).

The fast rise and fall times means you have a “clean” period
of time during a HIGH or LOW for counting operations. A micro
determines the frequency of an incoming signal by counting the
length of time for the HIGH or LOW (betw1een a rise and fall) and
coming to a decision as determined by the program. If the
lengths of the HIGHs and LOWs are very regular the micro can
come to an accurate decision very quickly. This is very
important if you are detecting a particular frequency out of a
possibly range of frequencies as the program will compare the
incoming frequency with a table value and it may keep sampling
until an exact match is produced.

Fig 7. Shows tw1o analogue stages with “over-drive.” The
circuit produces a signal that is larger than necessary for the
micro, and when the input waveform is very low, the output will
still be sufficient to
trigger the micro.

This circuit is more sensitive than fig: 6 and should be used
if you want to pick up faint whispers and turn on equipment (such
as a tape recorder). It should also be used for magnetic
pick-ups (such as a
metal detector, nail finder or
pipe locator) as this type of pick up has a very low output (can
be as low as 1 to 5mv).

THE PIEZO DIAPHRAGM

Piezo transducers such as piezo diaphragms can also be used as
microphones. They produce a very “tinny” sound (like talking
into a tin) but this is not important in our case. The output is
about the same as an electret microphone (about 10 to 30mV for
low level speech and up to 50mV for loud speech).

Some diaphragms are very sensitive while others are very
insensitive. You can check the sensitivity with a CRO, by simply
whistling and viewing the waveform.

Diaphragms that operate on low voltages, such as those in a music card seem to very sensitive.

The only other way to determine a sensitive piezo diaphragm is
to connect it to a music chip. The output it produces reflects
its sensitivity as a microphone.

The piezo diaphragm has an advantage over an electret microphone. It does not need a power source.

Analog to digital conversion notes

The diagram of fig: 8 shows how to connect it to a transistor
amplifier. Using the program above, whistle into the diaphragm
and determine if a single stage is sufficient. If not, a second
stage, exactly like the microphone stage can be added.

Analog to digital conversion notes

Fig 9 shows a simpler circuit to interface a piezo diaphragm
to the input of a micro. The first transistor is held in
“bottoming” and the second in “cut-off.” This offers the lowest
current consumption and produces the greatest output. The second
transistor is “turned-off” because the first transistor is “ON”
and the voltage across its collector-emitter terminals is
about 0.35v and this is below the turn-on voltage of 0.65v for
the base of the second transistor. The first transistor is fully
turned on but the turn-on current is very low and this allows the
maximum waveform to be produced by the piezo for a given sound
intensity. In other words the circuit puts the lightest load on
the piezo.

When no-signal is present, the output of the circuit is HIGH.

A signal from the piezo produces a full-rail waveform to exactly match the waveform from the piezo.

By monitoring the output of the circuit you will be able to
detect the instant the waveform produces a LOW. Instructions that
loops until a low is detected are shown below:

Loop1 NOP

BTFSC 06,0 ;Test GP0 for a LOW

GOTO Loop1

The only problem with loop1 is it holds up the micro from doing any other operations.

The solution is to “poll” the input line by producing a
program that “looks” at the input on a regular basis and if a low
is detected, a “bit” is placed in a flag register. The Main
routine constantly looks at the flag register and when the bit is
SET, it carries out an operation and cancels the flag.

MAGNETIC PICKUP

A 10mH choke (with both wires coming out the same end so the
magnet can pass as close as possible to the core) can be used as a
magnetic pick-up to detect rotating shafts etc. The circuit in
fig: 10 will connect it to a micro.

Attach a strong magnet to a rotating shaft and have the
magnetic sensor close to the magnet as possible without touching.
The amplitude of the waveform produced by the coil (choke) will
depend on the strength of the magnet, the distance betw1een the
magnet and coil, the speed of the rotating shaft and the number
of turns on the coil.

Analog to digital conversion notes

Fig: 10 “AC” couples the coil to the amplifying circuit so
that the biasing of the first transistor can be determined by the
value of the base-bias resistor. The coil cannot be connected
directly to the transistor as the low impedance (resistance)
of the coil will upset the bias on the base.

If the coil is directly connected to the base, the voltage
across the coil will be only a few millivolts and the coil will
have to produce 600mV or greater to turn on the transistor.

With our arrangement, 100mV or less will turn off the
transistor (a 100mV waveform will turn the transistor ON more
during the “positive half” of the waveform and turn it off during
the other half of the waveform).

Any coil of wire of any size will be suitable and to make it
an effective collector of magnetic flux it should have a magnetic
core such as a ferrite.

Simply rotate the shaft at half the required RPM and if the
circuit detects the magnet, the coil is suitable. No other
impedance-matching or resistance measurements are necessary.

The output of the circuit is exactly the same as for the piezo
diaphragm except the frequency will be considerably less.

THE LDR

The cheapest light sensor is the LDR. It has a resistance of
about 100 ohms in bright light and more than 1M in darkness.

It can be connected to a transistor amplifier as shown in fig: 11.

Analog to digital conversion notes

The pot alters the sensitivity of the LDR so that the output
changes at the desired light intensity. This circuit is only
suitable for large changes in intensity. If you need to detect a
very slight change in illumination you need a very sensitive
detector.

PHOTO DARLINGTON TRANSISTOR

A more sensitive device is a photo-darlington transistor, such
as the MEL-12. A further stage of amplification and a pot will
adjust the sensitivity level. This is shown in fig: 12.

Analog to digital conversion notes

VOLTAGE LEVELS

Some waveforms change very slowly. They may take a day or so
to rise and fall. The are classified as VOLTAGE LEVELS and can be
produced by thermocouples (a thermocouple PRODUCES a voltage due
to the heating of tw1o different metals in contact with each
other). Other devices DELIVER a voltage by being supplied
with a voltage and the resistance of the device causes a certain
voltage to appear across it. You can call it a VOLTAGE-DIVIDER
principle. Devices that deliver a voltage include the thermistor,
diode (and transistor) and strain gauge. This is very important
as it determines how the device is connected to either an
amplifier or the input of the micro.

THE FREQUENCY OF A SIGNAL

Once you have a waveform that triggers the micro (use the test
program given at the beginning of this article to make sure it
is being detected by the micro) you can get the program to simply
detect its presence or determine the frequency.

The frequency can be determined in tw1o ways:

1. Counting the number of HIGHs and LOWs in one second.

2. Counting the time taken for a HIGH-to-LOW transition – one half cycle.

Method 1 is a very slow way to obtain a frequency in cycles per second.

A micro works so fast that it can determine things in a fraction of a second.

Method 2 is much faster but it can be very inaccurate as the
half-cycle is multiplied-up to get the value in Hertz. To reduce
the inaccuracy you can count 10 cycles or 100 cycles and perform a
multiplication. A higher cycle-count reduces the
inaccuracy.

The following steps are required to produce the value of a
frequency (in Hertz). Refer to fig: 13 for the definition of a
HIGH and LOW.

The input is looped until a HIGH is detected.

The program then goes to a short delay.

The micro comes out of the delay and increments a “count” file.

The program looks at the input for a LOW. If a LOW is not detected the program goes to step B.

When a LOW is detected the program has a value in the “count” file – this is half a cycle.

By adding-up the number of instructions carried out during the
“count” increment, the program can work out the frequency of the
waveform.

Sometimes a frequency in cycles per second is not required.
You may want to compare a frequency with a value in a table. This
is called TONE DETECTION.

TONE DETECTION

– also called Tone decoding

The frequency of an incoming tone can be detected and
“flagged” when it is exactly the correct tone. This is handy for
detecting DTMF tones on the phone line. In this way DTMF tones
can be used for turning on equipment (such as lighting, heating,
alarms etc). The accuracy of the tone can be set by the
program. It is best to allow a margin of error so you can
guarantee the tone will be recognised.
For best detection,
the incoming tone should be “good quality” i.e full amplitude and
free from noise, so the determination can be speedy. Fig: 13
shows a typical waveform.

Analog to digital conversion notes


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