DAC buffer on/off

[flute2k3]

I noticed that with analogWrite() function, even I put 0, the output is not zero, something around xx - xxx mV, I happened to find that it is due to DAC buffer on - maybe by default? refer to below link ... I have no negative power available so this non-zero output is a little bit disturbing.

Is that possible we can turn the buffer off to get the zero voltage output using Arduino way? I anyway have to connect a buffer using low drift opamp so it does not matter.

https://stcommunity.st.com/t5/stm32-mcu-products/minimum-dac-output-voltage-on-stm32f30x/m-p/410862

[maxgerhardt]

The DAC buffer is indeed turned on by default.

ArduinoCore-GD32/cores/arduino/analog.cpp

Lines 73 to 125 in dfa7ccb

	void set_dac_value(PinName pinname, uint16_t value)
	{
	#if DAC_NUMS != 0
	uint32_t dac_periph = pinmap_peripheral(pinname, PinMap_DAC);
	uint8_t index = get_dac_index(dac_periph);
	if (!DAC_[index].isactive) {
	pinmap_pinout(pinname, PinMap_DAC);
	rcu_periph_clock_enable(RCU_DAC);
	// only do reset of DAC clock domain once when *every* DAC is being inactive.
	bool do_reset = true;
	for(uint8_t i = 0; i < DAC_NUMS; i++) {
	if(DAC_[i].isactive) {
	do_reset = false;
	break;
	}
	}
	if(do_reset) {
	dac_deinit();
	}
	#if (defined(GD32F1x0) && defined(GD32F170_190)) || defined(GD32F30x) || defined(GD32E50X)
	dac_trigger_disable(dac_periph);
	#if defined(GD32F30x) || defined(GD32E50X)
	dac_wave_mode_config(dac_periph, DAC_WAVE_DISABLE);
	#endif
	dac_output_buffer_enable(dac_periph);
	dac_enable(dac_periph);
	dac_data_set(dac_periph, DAC_ALIGN_12B_R, value);
	#elif defined(GD32F1x0) && !defined(GD32F170_190)
	dac0_trigger_disable();
	dac0_output_buffer_enable();
	dac0_enable();
	dac0_data_set(DAC_ALIGN_12B_R, value);
	#elif defined(GD32F3x0)
	/* only has 1 DAC at maximum, no need for a parameter... */
	dac_trigger_disable();
	dac_wave_mode_config(DAC_WAVE_DISABLE);
	dac_output_buffer_enable();
	dac_enable();
	dac_data_set(DAC_ALIGN_12B_R, value);
	#endif
	DAC_[index].isactive = true;
	} else {
	//set dac value
	#if defined(GD32F30x) || (defined(GD32F1x0) && defined(GD32F170_190)) || defined(GD32E50X)
	dac_data_set(dac_periph, DAC_ALIGN_12B_R, value);
	#elif defined(GD32F10x) && !defined(GD32F170_190)
	dac0_data_set(DAC_ALIGN_12B_R, value);
	#elif defined(GD32F3x0)
	dac_data_set(DAC_ALIGN_12B_R, value);
	#endif
	}
	#endif
	}

Sounds like we should disable the DAC altogether when analogWrite(0); is called and output LOW as a GPIO pin? How bad is the offset at analogWrite(1);? is it a general offset that is always there as long as the buffer is enabled?

[flute2k3]

the output changes proportionally with 1, 2 ... etc, it is a constant offset as far as I tested.

[maxgerhardt]

So when you put

dac_output_buffer_disable(DAC0);
dac_output_buffer_disable(DAC1);

after every analogWrite() call, the offset problem is mostly gone and the DAC output curve is more ideal?

[flute2k3]

I tested writing 0 on a standalone bluepill plus board,
without dac_output_buffer_disable(DAC0);
PA4 = 6.303 mV
with dac_output_buffer_disable(DAC0);
PA4 = 0.228 mV
it makes notable difference at 0 output now.

[maxgerhardt]

We can definitely add an API for this but I'm not sure if this should be the default behavior. The datasheet says that the purpose of the buffer is to provide a low-impedence output that can directly drive stuff without needing an external opamp. I can't repredict the user's application for this, whether the drive capability or voltage accuracy is more important :( .

A static +6mV offset in a total voltage swing of 3300mV doesn't seem neck-breaking to me, but I'm also willing to be convinced of the opposite.

[flute2k3]

keep the default maxgerhardt, no action needed, I will use dac_output_buffer_disable(DAC0) for my application :-) cheers ~