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A square wave generation from 600mv to 1.2v

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SAILLESH.SABARISH

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I would like to generate a square wave with a frequency of 300-500HZ and the amplitude can be in the range of .6v - 1.3v not more than that.

currently i am using a function generator forto have a square wave for my project . I wonder if this will be available in the form of an IC .

can any one suggest some good ideas to generate a square wave or mention the IC if it is available at above mentioned specifications.


thanks in advance.
sash
 

alexan_e

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You can use a 555 timer in astable connection 555 and 556 Timer Circuits
you have to use a voltage divider (two resistors or trimmer) to the output to lower the voltage to the level you want

Alex
 

SAILLESH.SABARISH

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but i would like to have an rise time and fall time that is very small ...may be 10 -20 nano sesonds...is it easy to achieve with the method you are telling??
 

KerimF

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but i would like to have an rise time and fall time that is very small ...may be 10 -20 nano sesonds...is it easy to achieve with the method you are telling??

Let us assume you get a square wave using gates of a fast logic series. The output would be likely 0-5V. To lower the square wave signal, we may add at the output a resistor (say 1K) followed by a rather fast diode (1N4148 low-cost and may work); its anode connected to the resistor and its cathode to ground.

The output will be at the anode of the diode (approximately 0.65). (You may like to use two diodes in series to double the output amplitude).
 

FvM

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To lower the square wave signal, we may add at the output a resistor (say 1K) followed by a rather fast diode (1N4148 low-cost and may work); its anode connected to the resistor and its cathode to ground.
Feeding the pulse to the RD network will also affect the risetime. A low impedance resistive divider, e.g. with 50 ohm output resistance, will achieve a better pulse repsonse.
 

TuAtAu

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use a crystal ?

or use a timer555+addition opamp to add the step~
 

SAILLESH.SABARISH

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I have a 50% duty cycle PWM signal from the microcontoller with 3.3v and i am using a PMOS as a switch with the resistor between collector to ground to have the desired voltage drop of 1.1v at 40mA current . but i face an offset in the output.
will this type of circuit can make a desired voltage drop at specific current without any offset?
 

KerimF

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So you have a p-channel MOSFET. A resistor is connected between its drain (right?) and ground. The source pin is connected to Vdd (3.3V of the MCU, right?).

When the MCU pin is high, the PMOS is off ==> no current ==> output at its drain is 0V.
When the MCU pin is low, the PMOS is on (I suppose Vgs=3.3V is good for it), a drain current will develop a positive voltage on the load (resistor).

This is how I see your circuit so far.
Now, what do you mean by 40mA, is it related to PMOS current when on?
And is 1.1V the voltage drop on the load (Vd) or PMOS (Vds)?

Sorry if my questions look rather silly but you know, you are familiar with your problem much more than I :wink:

Kerim
 

alexan_e

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I have a 50% duty cycle PWM signal from the microcontoller with 3.3v and i am using a PMOS as a switch with the resistor between collector to ground to have the desired voltage drop of 1.1v at 40mA current . but i face an offset in the output.
will this type of circuit can make a desired voltage drop at specific current without any offset?

You said that you want 10-20 nano second rising/falling edge, how will you accomplish that speed with a mosfet driven by the mcu PWM with just a few mA?
Even with a very small device having charge of 2nC you will need at least 100mA.

Alex
 

SAILLESH.SABARISH

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yes the connections is as you said .
but the source is connected to 5v supply and the PWM signal is applied to the gate of the PMOS.
I have put a resistor in between the drain to the ground . such that when i measure across the resistance
when the PWM of 3.3v is across the gate i need an 1.1vPWM signal as the output with 40ma current .

---------- Post added at 19:53 ---------- Previous post was at 19:51 ----------

You said that you want 10-20 nano second rising/falling edge, how will you accomplish that speed with a mosfet driven by the mcu PWM with just a few mA?
Even with a very small device having charge of 2nC you will need at least 100mA.

Alex
But the maximum current from my MCU is 350mA will this be enough to drive a mosfet.
 

KerimF

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i need an 1.1v PWM signal as the output with 40ma current .

This is what has confused me the most. If the output should be 40mA then the load will determine the voltage on it. And the reverse is the same, if the output voltage should be 1.1 V, also the load will determine the current it needs at 1.1V

Edited:
so only R as a load which is equal to 1.1/0.04 Ω can satisfy both.
 

SAILLESH.SABARISH

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so when i use this resistance between the drain to ground can i get expected output without any offset?

---------- Post added at 20:15 ---------- Previous post was at 20:14 ----------

You are using an mcu that has an output of 350mA from a single pin?
What is the model you are using?

Alex

I am using EVK1100 board with AVR32uc3a0512 controller.

---------- Post added at 20:16 ---------- Previous post was at 20:15 ----------

This is what has confused me the most. If the output should be 40mA then the load will determine the voltage on it. And the reverse is the same, if the output voltage should be 1.1 V, also the load will determine the current it needs at 1.1V

Edited:
so only R as a load which is equal to 1.1/0.04 Ω can satisfy both.



so when i use this resistance between the drain to ground can i get expected output without any offset?
 

KerimF

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so when i use this resistance between the drain to ground can i get expected output without any offset?

This was just the first step to the solution.
Do you mean, we can assume from now on that the resistor value is 27.5 Ω ?
 

SAILLESH.SABARISH

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This was just the first step to the solution.
Do you mean, we can assume from now on that the resistor value is 27.5 Ω ?


yes i could be assumed to be 27.5 ohm..

---------- Post added at 20:31 ---------- Previous post was at 20:30 ----------

This was just the first step to the solution.
Do you mean, we can assume from now on that the resistor value is 27.5 Ω ?

yes it could be assumed as 27.5 ohm.
 

alexan_e

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KerimF

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Just to be sure, do you mean by getting an 'offset' voltage that you get a small voltage when MCU is high though in this state the output should be zero?
 

SAILLESH.SABARISH

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Your specification is not correct, the datasheet http://www2.atmel.com/System/Redirect.aspx?dest=/dyn/resources/prod_documents/32058S.pdf
says (page 40) 370mA for all pins, in page 41 you will see that the output high level and output low level current is 4-8mA (max) depending on the pin

Alex

yes i am wrong ....but will this be enough to drive a PMOS??

---------- Post added at 02:49 ---------- Previous post was at 02:43 ----------

Just to be sure, do you mean by getting an 'offset' voltage that you get a small voltage when MCU is high though in this state the output should be zero?

no i get an offset voltage all the time....even the output goes high or low....
 

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