AnsweredAssumed Answered

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

Hi,

 

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).

DAC_BufferOff_With_External_OpAmp

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

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

DAC_BufferOn_With_External_OpAmp.bmp

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.

Outcomes