DAC80508: Query regarding the output voltage of DAC80508

Hello,

1. The DAC80508 VDD max is 5.5V, so the output will not be able to go beyond that. You could add an external op amp with a gain of 2 to achieve the 0-10V output. I would suggest the DAC81408 to get a 0-10V output. 

2. The settling time is typically given in DAC datasheets for ¼ to ¾ scale and ¾ to ¼ scale settling. You can expect smaller settling for smaller steps. The settling time is also given to ±2 LSB in the DAC80508 datasheet. Your application may not require the output to be settled to ±2 LSB, so you might consider the device to be settled faster than the datasheet considers the device to be settled. 

3. The datasheet for this device does not mention 1MSPS. So I'd like to confirm what you are referring to as the 1MSPS sample rate. If you use the 50MHz clock speed then each DAC update will take 0.48us. If each DAC needs to be updated individually then it would take 3.84us to update all 8 DACs. The settling time would start for each channel after each channel is written to unless you are using the LDAC trigger to update all of the DACs simultaneously. If all 8 channels require the same data, then you can write to all of them at the same time using the broadcast DAC register. 

4. I would suggest the 8-channel DAC81408 or 4-channel DAC81404 to get a 0-10V output.

Best,

Katlynne Jones

Hello ,

Thank you for the Response. Please find below requirement.

Simultaneous DAC channel : 8 

All 8 input data rate is 1 MSPS , but different. (Not same data) 

Required sampling rate for each DAC is 1 MSPS 

Required output voltage for each DAC: 0 to 10 V

DAC resolution: 14/16 .

So, could you please suggest a suitable 8 or 4 channel DAC based on above requirement.

Hello,

I have gone through the DAC81404 datasheet and calculate update rate from below equation.

Tupdate = Tsclk x 23 + Tcss + Tcsh + Tcshigh = 0.505 uS , F update = 1.98 MSPS 

that means , update rate is good. Based on this , what would be the final DAC sample rate for simultaneous usage of all 8 DAC channels? 

As , my requirement is to use simultaneous 8 DAC channels with 1 MSPS sample rate. Please clarify and provide guidance.

Also, I want to connect the DAC single ended output to 50 ohm load through coaxial cable, as DAC voltage output is buffered, it's impedance is low. so, can you please suggest a way(application note) on how to convert single ended DAC output impedance to 50 ohm? thank you in advance for your support.

Hello,

You would need to divide the update rate by 8 to calculate the total time it would take to update all 8 DAC channels with unique data. You would also add the settling time for the last DAC channel to the update rate to get the total sample rate. The DACs will update immediately after you write to each register (unless you use the LDAC trigger), so the last DAC will finish its slew last. 

We do have some parallel DACs that would be able to update faster like the DAC8228. I'm not sure if you had a requirement for the communication protocol. It looks like the multichannel parallel DACs have a settling time of a few microseconds. There are single channel, parallel, MDACs that will have a much faster settling time. They require an output buffer, but you can select a buffer that will meet your settling requirements. You'll likely have to add a buffer to the output with an amplifier that can support a 50Ω load at 10V anyways. I don't think any of our DACs can drive that load.

Another question, what type of waveform are you trying to output? The DAC81408 also does have a toggle feature that can switch between two voltage values with a toggle input if you are trying to output a square wave. This will be much faster than updating the DAC for each new value.  

Best,

Katlynne Jones 

Hello,

We need to interface octal channel DAC with FPGA platform board , And there are also other interfacing components would be there, so we are thinking to use lesser interfacing I/O pins as possible. that's why we are going for multi channel simultaneous DACs instead of 8 different single channel DACs. 

As this DAC5228 has parallel input interface , it will have total 20 pins require for parallel interface to FPGA. other than that 7 more interfacing for control logic. so in this way, it will take so many pins for interfacing.

So, we are preferring SPI protocol. 

Our output is voltage output, variable DC and all outputs from each channel needs to be output simultaneously.

So, can you suggest any multi channel DAC that can fulfill our requirement of 1 MSPS sample rate with output voltage range of 0-10 V?

Hello,

If you cannot use a parallel DAC then you would need to find a DAC that supports ~8MSPS in order to update all 8 channels at 1MSPS. We do not have a DAC like this that supports SPI. 

You mentioned that the output is variable DC. Is this a square wave? If so, please check if the toggle feature of the DAC81408 will work for you. It looks like the max toggle frequency is 100kHz which might still be too slow for you. 

Best,

Katlynne Jones

Hello,

Ok, I will check about the output and will let you know. So, you are suggesting if we use parallel interfacing DAC like DAC81408 , Then it would be suitable for 1 MSPS sample rate requirement, right?