STM32F446RET6 DAC output

[Junzhe]

Hi,

For STM32F446RET6, I want to use the DAC function of it. But I have some question.

1. If I don't use the output buffer, what is the output range? From the datasheet I think it should be 0.5mV to 3.3V±1LSB？

2. If I use the output buffer, what is the output range? From the datasheet I think it should be 0.2V to 3.1V？

(My VDDA=VREF is 3.3V)

3. For this 12-bit DAC, its minimum output cannot be 0V (regardless of whether a buffer is used). So how should I adjust its output? When calculating the formula X/4095 (where X is my setting), if I set X to 1 and do not use an output buffer, should its output be 0.5mV + (1/4095) * 3.3V?  

When calculating the formula X/4095 (where X is my setting), if I set X to 1 and use an output buffer, should its output be 0.2V + (1/4095) * 3.3V?

Thanks a lot for your help.

Best,

Junzhe

[mƎALLEm]

Hello,

This is what STM32 Sidekick answered your questions :) :

1. DAC Output Voltage Range (Buffer ON/OFF)

• With Output Buffer ON:
  The DAC output voltage range is from a minimum of 0.2 V up to (VDDA – 0.2 V). For your setup (VDDA = VREF+ = 3.3 V), this means the output can swing from 0.2 V to 3.1 V. The minimum and maximum output voltages are not exactly at the digital code extremes (0x000 and 0xFFF), but are limited by the buffer characteristics.

• With Output Buffer OFF:
  The output voltage range is from a minimum of -0.5 mV up to (VREF+ – 1 LSB). For your setup, this means the output can go from approximately -0.5 mV to just below 3.3 V. Again, the minimum output voltage is not exactly 0 V, but slightly negative due to the DAC architecture.

2. Calculating Output Voltage for a Given Digital Value

To calculate the output voltage for a given digital value (X), you must consider the minimum and maximum output voltages as described above.

• With Buffer ON:

  VOUT = 0.2 V + [X / 4095] × (3.1 V – 0.2 V)
  

  Where X is your 12-bit digital value (0 to 4095).

• With Buffer OFF:

  VOUT = -0.5 mV + [X / 4095] × (3.3 V – (-0.5 mV))
  

  This formula accounts for the slightly negative minimum output.

3. Minimum Output Voltage and Practical Limitations

• True 0V Output:
  The DAC cannot achieve a true 0V output, regardless of buffer configuration. The minimum output is either 0.2 V (buffer ON) or -0.5 mV (buffer OFF). This is a practical limitation of the DAC design.

• Formula Adjustment:
  When calculating the output for X = 1 (buffer OFF), the output is:

  VOUT = -0.5 mV + (1/4095) × 3.3 V
  

  This means the output starts slightly below 0V and increases linearly with the digital value.

---

Summary

• With buffer ON: Output range is 0.2 V to 3.1 V.
• With buffer OFF: Output range is -0.5 mV to just below 3.3 V.
• The DAC cannot output a true 0V; minimum output is limited by hardware.
• Use the adjusted formula for output voltage calculation, considering the minimum output voltage for your configuration.

[Junzhe]

Hi,

When I use an output buffer and set X to 1, my DAC output is 0.063V. Why is that? And why is my DAC outputting 0.6V when I don't use the output buffer and set X to 1?

[waclawek.jan]

With Buffer ON:

VOUT = 0.2 V + [X / 4095] × (3.1 V – 0.2 V)

Of course not. Thou shalt not trust any AI.

VOUT is always (VREF+-VREF-) * X/4095; when buffer is ON, it's saturated, i.e. for DAC_OR values which would result in voltages close to the extremes, it won't output those voltages but the limiting voltages (the datasheet value is worst-case).

JW