Laboratorio -1

The error between theoretical conditions and simulation is

= 2,72 − 2,5 = 0,22.

practically null.

1.3 Analog to Digital Converter

On this part, we integrated a PWM-modulated analog-to-digital converter, using a comparator

and a sawtooth signal generated by a waveform generator.

A PWM analog-to-digital converter is a type of ADC that uses the principle of pulse width

modulation to convert an analog signal to a digital signal via the duty cycle of a rectangular

digital signal.

The requirement is to have a PWM signal at a frequency with a duty cycle that varies

= 100

linearly with respect to the input signal from = 0% ℎ = 0° =

100% ℎ = 100°.

We have already implemented the current bridge circuit, which converts a linear change in

temperature into a linear change in voltage. At this point, we simply need to make the

connection between the of the PWM signal and the voltage In other

_.

words, the must be proportional to the value of the ensuring that the

_,

PWM behavior reflects the linear temperature change provided by the current bridge. The

with a was chosen.

082 = ±15 Figure 16 – Comparator

Figure 17 – Active current bridge + Voltage Reference + Comparator on LTspice

The sawtooth waveform parameters were chosen to satisfy the above request:

= 100 → = 0.01, = = 0.005

= 100% = 100° → = 9,8 = 5

Equal rise and fall times are used (no so that there can be correct modulation of the PWM

)

signal at the comparator output.

The selection of and is based on the maximum duty cycle () of A margin

100%.

of is included to ensure that, when , the signal always remains

0.1 = 100° _

higher than the signal.

ℎ

Same procedure for = 0.

After configuring the generator to produce the desired signal, the circuit was simulated on

LTspice.

Figure 18 – Vout_bridge (in blue), Vsawtooth (in red), and Vout_PWM with T = 10ms (in green), the value circled in red is the

Ton of the signal.

From the simulation we obtained a PWM signal with perfectly in line with

25%,

the request since the voltage _ 2,5.

Next, the circuit was mounted on the breadboard. We obtained the following results:

Figure 19 - Vout_bridge (in pink), Vsawtooth (in yellow), and Vout_PWM with T = 10ms (in blue), the value circled in red are the

Vout of the bridge and Ton of the PWM signal respectively.

The value is , higher than the ideal value. However, the behavior of the PWM

_ 2,72

signal confirmed the effectiveness of the signal conditioning: the measured

was of, in line with the expected value, ensuring a perfect linearity

27,2%

between _ ℎ .

ꮄ

The overall error is We can notice that the signal

= = 27,2% − 25% = 2,2%.

conditioning didn’t add more error.

1.4 Error Correction

The last phase of the laboratory was dedicated to correcting errors, which emerged clearly

during the measurements. To address these issues, a potentiometer compensation circuit was

implemented to balance the current bridge and improve the accuracy of the system.

Figure 20 – Circled in red are the potentiometers used to compensate the errors

The first potentiometer was used to adjust the gain of the circuit, with the goal of obtaining a

bridge reference voltage exactly equal to In this case, the value of the potentiometer

10 .

is not critical, as it is used to make a simple partition of the voltage on the output.

The second potentiometer was used to balance any mismatch between the two resistances

1

in the current bridge. The value of this potentiometer has been chosen to be in an order of

magnitude lower than . This choice was made considering the errors introduced by the

1

circuit, in particular those due to the tolerances of the resistances (about 10%), and at the same

time trying to keep the initial model of the circuit as intact as possible.

Adding a resistor in the bridge results in a slight reduction in the current flowing through it,

causing a decrease, even if minimal, in the output voltage. To compensate for this reduction

and ensure proper operation of the circuit, the bridge reference voltage has been increased to

a value slightly higher than

10 .

For simplicity, the same values as 1 2.

1 = 2 = 10Ω