dac8800FPZ glitch

Your voltages are low across the board, so I suspect something else is going on.  Have you verified this on more than one channel?  What is the load on the output?  The DAC has ~12k ohms output resistance, so it cannot really drive a load very well.  Remove all loading on the output and test again, and also check the other channels to see if the same thing is happening.  Also check for any changes on the power supply.  Since you are using the supply as the reference, any change there will affect the output voltage.

Regards,

Tim

The output of channel 1 is connected to the +input of an LM358 opamp. I have also tried other channels (unloaded) apart from connecting my fluke meter (high impedance load) to each output. I get the same result. The 12v dc supply is fed from a 7812 linear regulator and the output has been checked with a CRO and is stable.  I have 25pcs and all exhibit the same problem, am I overlooking something obvious as the circuit is not that complicated!

A closer look at the datasheet has (most likely) revealed the issue.  The REF inputs (high side) have to be at least 4v from the positive rail.  Thus, with a 12 volt supply, you're limited to 8v input on the reference.  You've most likely got the output stage saturated.  Can you provide a lower reference voltage and run the test again?

Tim

Hi Tim,

I changed the supply regulator to 15v and adjusted the reference to 10v the problem actually appears to have deteriorated (see below)

VDD = 15v

Vrefh = 0v (both)

Vrefl = 0v (both)

VSS = 0v

Data

255 = 11.24v

254 = 11.14v

253 = 10.25v

252 = 11.02v

251 = 9.76v

250 = 10.08v

249 = 9.68v

248 = 10.81v

247 = 9.40v

246 = 9.51v

245 = 8.99v

244 = 9.79v

243 = 8.91v

242 = 9.37v

241 = 9.17v

240 = 10.45v

and so on...

Attached is a schematic, I have chaged the 7812 to a 7815 in this latest test.

Hi-

I moved this thread to the Precision DACs community.  Please continue the discussion here.

Thanks,

AndyR

EngineerZone Community Manager

Hi Daliseau,

I had hoped Tims headroom suggestion would have solved it.

So you have a single supply system:

Vdd=15V

Vrefh=10V

Vrefl=0V

Vss=0V

Has this issue been resolved ?

Hi Liam,

Yes it is a single supply system with the parameters you outlined

Tim raised a good point regarding the Ref voltage but unfortunatly it appears to be unrelated to this issue.

ie. The problem is still evident, any clues?

thanks

Darren

Hi Darren

Have you scoped all SPI writes to the part to makes sure that the data you are writing is actually seen by the part ?

I assume you have checked all grounds / supplies etc,

You could try the specified single supply setup on page 2 of datasheet:

Vdd=12V

Vss=0V

VrefH=5V

Vrefl=0V

See if this gives any further improvement.

-Liam

Hi Liam,

I am currently on leave until the end of the month, however prior to heading off on holidays I pulled the prototype unit (that was in storage after moving) and it worked perfectly, this has an older IC from a different batch!

Without swapping out the DAC8800 from the prototype, I suspect I have a bad batch of IC's.

Will post an update on my return.

Darren

Hi Darren,

All these parts are production tested to the highest standard before leaving the factory - however it's possible the chip is damaged through some other method.

-Liam