AD7840 DAC question

[okbro]

hi,

I had already asked question regarding AD7840 here https://www.electro-tech-online.com/threads/using-ad7840-14-bit-dac-as-8-bit-dac.161167

now i have question again on using it,

1) how or where to connect REF IN pin? should it connected to +3.3V?
2) as i understand Vdd can be connected to +5V, but it gets very hot soon, even with 3.3V
3) can i permanently ground the CS pin?
4) why is that audio is produced whichever way I connect the 8 bits output to MSB(D13) to D6 or D0(LSB) to D7?

datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/AD7840.pdf

thanks

 

[Ian Rogers]

Think about it.... Its only an ADC.. Shouldn't get hot at all.

So.. the majority of pins are input... The only pins you'll need to check are outputs... ie Vout and Ref out etc... Show me your circuit.

 

[danadak]

1) Ref in should be connected to Ref out.
2) Vdd should be +5V, Vss - 5 V.
3) Yes, but these notes apply -

4) DAC does not know what bits it gets are MSB or LSB so yes you will get audio but if you
shift the 8 bit byte to lower pins the DAC pk-pk output will drop. Ie. all you would be doing is
scaling the DAC output.

If chip got hot sounds like you connected a pin to a V > + 5 or V < - 5. May have ruined
it.

Regards, Dana.

 

[Pommie]

Its only an ADC.

Dyslexic?

Mike.

 

[Ian Rogers]

Dyslexic?

Mike.

yep.... have always been a typo man

 

[okbro]

i have connected Vss to ground, ground means -5V here right? or am I missing something?

 

[JimB]

i have connected Vss to ground,

Bad mistake.

ground means -5V here right?

Wrong.

am I missing something?

Yes.

Look at figure 25 in the datasheet.

+5v is connected to Vdd

-5v is connected to Vss

There are connections marked AGND (analogue ground) and DGND (digital ground).
I would prefer that these were also identified as 0v (zero volts). This is where the other wire from the two power supplies connects.


/RANT MODE = ON
When you have a 5v PSU, one wire is +5v, the other wire is 0v, NOT -5v
the +5v connects to Vdd and the 0v connects to GND, or as I would prefer to call it 0v
/RANT MODE = =OFF

This can be very confusing for a newcomer.

JimB