AD7845 single supply voltage range

Hi se.adam,

It cannot , this is a very old part, you may want to take a look at AD5445 which is a 12-bit MDAC, parallel interface and no negative supply needed for the part.

Cheers,

Ivan

Hello,

The things is, the AD7845 is used in our designs for years with a single supply +15V and it doesn't seem there are problem with it.

But when I looked at the datasheet, I saw the troubles. I'm currently working about a new design with a correct DAC and supply.

With the confirmation that the AD7845 isn't guaranteed with a single supply we either need a solution to replace to replace the AD7845 on older design.

Regards,

Sebastien

Hi se.adam,

As the Vss basically supplies the negative supply of the output buffer to create negative output voltages, but looks like your design has a 4-quadrant multiplication configuration and does not intend to output negative values. Again, this might work but there's no guarantee on the performance of the part.

Have you taken a look at the AD5445? Let me know if you have questions please. 

Cheers,

Ivan

Hi Isantos,

We finally choose the AD5725 which works with single voltage +15V, isn'it ?

We test one with :

- AVdd = +15V

- AVss = 0V

- DGND = 0V

- Vrefp =+10V 

- Vrefn = 5V

According to the datasheet we're in the specifications.

We want the conversions at a specific time with the first DAC, so we cable the circuit like that : 

The datas come from logic circuit 4516, W/R is normally high (+15V) and is active low (0V) to have the conversions.

During the test, we observe that pins already have voltage when not connected.

Cause of that, we have high current with pins we want at +15V and need a resistor, even for DBXX input.

Is that normal ? Cause of the Double-buffered inputs ? iCMOS process technology ?

Hi se.adam,.

Apologies, was not able to give you a response ASAP. Do you still need help with your query? 

Which pins have voltage readings even when not connected? What are the power sequence used (including the availability of logic line levels)?

Cheers,

Ivan

Hi, isantos,

We finally manage to make it work.

The first problem was the voltage level we applied on the digital pins.

Before we use our +15V and output from logical circuit supply in +15V, so many inputs didn't like it.

After reading more carefully the datasheet, we saw the maximum +7V on digital pins, thus we use know +5V.

The other problem was our way to use it, as the AD7845 by putting down the /WR pins to have the output taking the conversion voltage.

Thus in our design we force /LDAC to "0" and /WR to "1" unless we want the conversion.

But finally we force /WR to "0" and /LDAC to "1" and switch /LDAC to "0" when we want the DAC output to take the right conversion voltage.

Is that the right way ?

Hi,

Glad it works now!

Sorry, before we proceed, just want to make sure we're talking about the same device, the AD7845 does not have an /LDAC pin, are we talking of another device?

I look at the AD5725 which has an /LDAC pin, but does not have /WR pin (only R//W).

Cheers,

Ivan

Hi,

Indeed, it's a confusing.

The first topic was about the AD7845, but since we change to the AD5725

Sebastien