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AD7819YNZ and DAC0808

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itzme

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hey all,
I would like your opinion on something. As from the data sheet of the adc chip, ad7819 it states that the Vin is between the range of 0V to Vref. After doing throught the adc ann then dac, a full sine wave is observed. But if the Vin is has a range from 0V to Vref i thought after the DAC i would only be able to see the positive half cycle.:confused:
 

ericgibbs

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hey all,
I would like your opinion on something. As from the data sheet of the adc chip, ad7819 it states that the Vin is between the range of 0V to Vref. After doing throught the adc ann then dac, a full sine wave is observed. But if the Vin is has a range from 0V to Vref i thought after the DAC i would only be able to see the positive half cycle.:confused:
hi,
If you use a opa before the A2D input and add an 'offset' of say Vref/2, then the 8 bit output will be a full sine wave.

A 8bit value on the output of 127 will represent 0V.

Ok.?:)
 

itzme

New Member
hi,
If you use a opa before the A2D input and add an 'offset' of say Vref/2, then the 8 bit output will be a full sine wave.

A 8bit value on the output of 127 will represent 0V.

Ok.?:)

im sorry. but can i know what is opa? do u mean opamp? so even though the Vin is from a range of 0 to Vref i can still observe the full sine wave as long as the offset is set to Vref/2? but i dont have a offset pin on my dac :confused:
 

ericgibbs

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im sorry. but can i know what is opa? do u mean opamp? so even though the Vin is from a range of 0 to Vref i can still observe the full sine wave as long as the offset is set to Vref/2? but i dont have a offset pin on my dac :confused:
OPA, is short for operational amplifier.

If you used a Vref of +5.0V say and an offset of Vref/2. [ +2.5V centre]

The maximum swing of the input sinewave to the opa would have to be 5Vppk [ thats 5V from peak tp peak].

So the output of the opa would swing from 0V, representing the -Vpk, and +5V represnting the +Vpk... the centre voltage of 2.5V is 0V of the sine.

This means the A2D output is [00Fh] decimal 00 for the -Vpk and [FFh] decimal 255 for the +Vpk... the centre line output is [7Fh] 127 decimal.

Do you follow.?
 
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itzme

New Member
OPA, is short for operational amplifier.

If you used a Vref of +5.0V say and an offset of Vref/2. [ +2.5V centre]

The maximum swing of the input sinewave to the opa would have to be 5Vppk [ thats 5V from peak tp peak].

So the output of the opa would swing from 0V, representing the -Vpk, and +5V represnting the +Vpk... the centre voltage of 2.5V is 0V of the sine.

This means the A2D output is [00Fh] decimal 00 for the -Vpk and [FFh] decimal 255 for the +Vpk... the centre line output is [7Fh] 127 decimal.

Do you follow.?


ok got it! :) but the thing now is i did not use an op-amp to the input of my ADC. The input into my ADC is a sinewave directly from the function generator and the 8-bit binary output is directly into the DAC. Hence, im curious while im seeing a full sine wave
 

ericgibbs

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ok got it! :) but the thing now is i did not use an op-amp to the input of my ADC. The input into my ADC is a sinewave directly from the function generator and the 8-bit binary output is directly into the DAC. Hence, im curious while im seeing a full sine wave
Its possible that the output of the FG [func gen] has a 'dc' offset, some FG'a do have an adjustable offset.

The best way to check the FG output is to use a scope set for reading 'dc', see what you get.

What will the 'real' sinewave [ ac] input come from, for the A2D.?
 

itzme

New Member
Its possible that the output of the FG [func gen] has a 'dc' offset, some FG'a do have an adjustable offset.

The best way to check the FG output is to use a scope set for reading 'dc', see what you get.

What will the 'real' sinewave [ ac] input come from, for the A2D.?


The output from the FG has been observed using the oscilloscope and i've adjust the amplitude of the sinewave to say 2.5Vpp. After going through the A2D and going throught the D2A again, the reconstructed sinewave is observed to have almost the same amplitude (2.5Vpp) as the input signal. A full sine wave can be observed. hmm
 

ericgibbs

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The output from the FG has been observed using the oscilloscope and i've adjust the amplitude of the sinewave to say 2.5Vpp. After going through the A2D and going throught the D2A again, the reconstructed sinewave is observed to have almost the same amplitude (2.5Vpp) as the input signal. A full sine wave can be observed. hmm
Whats the D2A ic type, is it powered from +/-V supplies.????

EDIT:
Summarising.
You have a FG driving a A2D the output of this input to a D2A, where you are seeing the original sinewave on its output.???
 
Last edited:

ericgibbs

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That sounds a little strange.

This is the transfer function for the AD7819, its in line with what I would expect.

Are the input/outputs showing the same frquency.?
 

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itzme

New Member
That sounds a little strange.

This is the transfer function for the AD7819, its in line with what I would expect.

Are the input/outputs showing the same frquency.?
Yes. This graph is obtained when i observed the output using LEDs. The input however is a dc voltage. When i use a AC voltage instead, the LED is observed to be blinking and increasing from 00h to FFh. However when i use a DAC to check the reconstructed signal, i assume i shouldnt see a full sine wave as a level shifter is not use yet. yes. The frequency and the Amplitude of the input signal are almost similar to that of the signal from the DAC. Strange huh?:confused:
 

ericgibbs

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Looking at the AD7819 datasheet, shows two clamp diodes on the Vsig input, the lower lower clamp diode [~0.7] should clip any below zero input voltages.!

EDIT:
Just because the LED's are showing a full sweep when using an 'ac' input dosnt mean its a sinewave output.
Its most likely a half cycle swing, due to the clamp diode' you are seeing on the LED's

I thought you were using a scope, showing a true sinewave.!!
 
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itzme

New Member
Looking at the AD7819 datasheet, shows two clamp diodes on the Vsig input, the lower lower clamp diode [~0.7] should clip any below zero input voltages.!
That means by right i should only observe the positive half cycle, am i right? could it be the problem with the dac0808?
 

ericgibbs

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That means by right i should only observe the positive half cycle, am i right? could it be the problem with the dac0808?
I would use a scope to check the shape/form of the output sinewave that you see on the LED's
 

itzme

New Member
I would use a scope to check the shape/form of the output sinewave that you see on the LED's
it is possible not possible to observe the output sinewave directly from the ADC as it is an 8-bit binary. Thats why im using a DAC to check the reconstructed signal to confirm the input signal
 

ericgibbs

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it is possible not possible to observe the output sinewave directly from the ADC as it is an 8-bit binary. Thats why im using a DAC to check the reconstructed signal to confirm the input signal
I mean the output of the D2A.:)

Im convinced that the output of the A2D is not a true sinewave, when you are driving the input from sinewave thats swinging about zero, ie +/-V over the full input range.

The LED display is misleading you, IMO.:)
 

itzme

New Member
I mean the output of the D2A.:)

Im convinced that the output of the A2D is not a true sinewave, when you are driving the input from sinewave thats swinging about zero, ie +/-V over the full input range.

The LED display is misleading you, IMO.:)
hmm so the problem is not with the DAC but with the ADC u mean?:confused:
 

ericgibbs

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hmm so the problem is not with the DAC but with the ADC u mean?:confused:
IMO it sounds like there isnt a problem.. its the way you are reading the LED's thats causing an 'apparent' problem.

Do you have a scope handy.??
 

itzme

New Member
IMO it sounds like there isnt a problem.. its the way you are reading the LED's thats causing an 'apparent' problem.

Do you have a scope handy.??
oh gotcha! [= yes i have! [=

i've observed the output from the DAC using the scope. and it's a full sine wave that im seeing
 
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