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Strange output of DAC when Buffer_Off and use external OpAmp

Question asked by su.xing on Mar 23, 2018
Latest reply on Mar 26, 2018 by su.xing



Question: Does DAC buffer enable or disable when use external OpAmp?
Reason: I want generate high frequency sine wave use DAC,range from 100~100Khz,0.1Vrms~1Vrms.

when generate 100Khz, I want a smoth sine wave,so i set Timer2_trigger speed is 4.5MHz, sample point is 45.

That is the maximum sampling rate of DAC when use STM32F3 which is specified in AN4566(Page8).

And also as ST's "AN4566-Extending the DAC performance of STM32 microcontrollers" write,
it's better use external OpAmp to buffer the DAC output due to internal buffer limitation(Page5).

So I buy some LMH6645 accordding the AN4566(Page9),and build an test pcb just as it(Page14).

Here is the output when not use external buffer:(Yellow is DAC output)

But When I turn off DAC buffer,the external buffer output is not sine,but it's square wave.

(Yellow is DAC output,Blue is external opamp output).


While if I turn on DAC buffer, the external buffer output is well follow input,

(Yellow is DAC output,Blue is external opamp output).


but as we use external opamp for high frequency,
so it's should not turn on DAC buffer,otherwise why we use external buffer?

And I test other even better opamp,such as LMH6646,LMH6642,OPA2354,it's the same problem.


So what shoul i do if i want use external opamp to generate high frequency sine wave?

and what's the  reason about strange output of external OpAmp when disable DAC buffer?
Hardware: STM32F3-Discovery,which use STM32F303VC,it's DAC1 has internal buffer,which can be turn off.


Software: Example(DAC_SignalsGenerations ) in "STM32F3-Discovery_FW_V1.1.0",


This example provides a short description of how to use the DAC peripheral to
generate several signals using DMA controller.
DMA2 channel3 is configured to transfer continuously, a unit16_t Array[450] buffer to the DAC register DAC_DHR12R1.
DAC channels conversions are configured to be triggered by TIM2 TRGO triggers and
without noise/triangle wave generation.



Attatch is the sine wave(0.6Vrms,1Khz) test.



Thanks,and looking forward anybody's reply and discussion.