DAC60508: DC output impedance affects Vout accuracy?

Hello,

You can expect a response by tomorrow morning US time. Thank you for your patience.

Kevin,

Thank you for taking care of this thread.

Yes, I can wait till tomorrow morning US time.

Best regards,

K.Hirano

Hi Hirano,

Maximum output impedance will be at full code or zero code and minimum will be at center code as per the datasheet.

So max value is 15 ohms. As such we can guarantee load regulation only when DAC is sourcing or sinking 10mA current, so connecting 500 ohms load to DAC out will have 10mA current flowing through the same and regulation will be 85uV/mA.

Also please note that short circuit current is 30mA to GND

Hope this information helps

Regards,

AK

AK,

Thank you for your response.

However, I am sorry I do not know what the “load regulation” spec is.

Could you elaborate?

Best regards,

K.Hirano

Hi Hirano,

This term defines the incremental change in output voltage for a change in load current. This change is generally small, but should be considered in high-accuracy applications. Note that this is more important with some DAC topologies such as R-2R ladders, while resistive string topologies are less susceptible.

In our case its 85uV per 1mA of current, meaning there can be a change of 85uV in output voltage when you are drawing 1mA load current from DAC.

Hope this clarifies.

Regards,

AK

AK,

Thank you for your response.

Let me ask a couple of questions.

Q1:

DC output impedance is minimum at DAC code = midscale, and maximum at DAC code = zero/full scale.

How about between zero scale and mid scale, or mid scale and full scale? Do you have plots of DC output impedance vs DAC codes?

Q2:

My customer would like to estimate the worst case.

How much is maximum DC output impedance at DAC code = midscale and DAC output at GND or Vdd?

Best regards,

K.Hirano

Hi,

We don't have plot of DAC output impedance vs Code. But I can get you sim values from my design team, if there exists.

Having said that, worst case will be 15 ohms at full scale or GND. You can expect a 10% variation on these values.

Regards,

AK

AK,

Thank you for your response.

If you could get the sim values from your design team, please let me know that.

Best regards,

K.Hirano

Hi,

I will try to get this data very soon.

Regards,

AK

AK,

Thank you for taking care of this request.

I look forward to getting the data.

Best regards,

K.Hirano

AK,

I am sorry for reminding you again.

But, I received a reminder from my customer again.

Did you have a chance to get the data?

Best regards,

K.Hirano

AK,

How is it going?

Did you have chances to get the data?

Best regards,

K.Hirano

Hi Hirano,

I am following up with my design team. Please hold on. They need to do some simulation to get you the data.

Regards,

AK

AK,

Thank you for telling me the current status.

I look forward to getting the data once the simulation is completed.

Best regards,

K.Hirano

Hi Hirano,

1. You dont need to worry about dc ouput impedance, its a closed loop ouput impedance, so your calculation Vout = Vout target * load /( DC output impedance + load) is not valid.

2. you should look into output voltage headroom specifications for your load current specification to arrive at worst case error.

so if you drawing 10mA current with 500 ohms load at 5V output, have a look at headroom requirements below.

its written as 300mV, so minimum supply if you can provide as 5.3V, there wont be any drop in voltage.

For other load currents, you can calculate similar way by looking into voltage headroom specifications.

Regards,

AK

AK

Thank you for your response.

But just in case, let me reconfirm.

Do you mean that since the output impedance is closed loop impedance, there is no voltage drop due to the output impedance if Vout target <= Vcc – 0.3V when Iout = 10mA?

Best regards,

K.Hirano