5 Replies Latest reply on Sep 12, 2019 5:37 AM by LePo_1062026

    wavedac8 rising time


      hi all,


      I am using Wavedac8 for generating waves from psoc 5lp. after starting  DAC , wave take time to rise maximum value even in square wave .so how can i remove that or what is time to take arbitrary values by DAC after starting DAC ?

        • 1. Re: wavedac8 rising time

          What risetime do you see? Do you use an Opamp to buffer DAC?


          • 2. Re: wavedac8 rising time


            No i am not using any opamp  .I am simply using Wavedac arbitrary file option and generate square wave for some amplitude (as simple as that 5 samples of 0 and 5 samples of amplitude value) , when ever my values of amplitude goes up (0 to some amplitude)its give me slop or rise/fall ramp for some microsecond.is DAC take some time for one sample to another? how can i remove these?

            • 3. Re: wavedac8 rising time

              The output impedance of VDAC in 0-4V is 16k. So the scope probe/cable capacitance will affect the risetime. Switch probe o 10x to see the difference. Normally, if the VDAC output goes outside the chip it should be buffered by Opamp. Enable Opamp buffer in WaveDAC8 settings to see the difference. Note that PSoC Opamps have low slew rate of <1V/us, which limits risetime of 0V to 4V step to approx. 4us. You can switch VDAC range to 0-1V to reduce output impedance to 4k, and get faster risetime, but if 0-4V range is necessary, you have to use some external Opamp with GBP>10MHz.


              • 4. Re: wavedac8 rising time

                Thank you for your response,

                These is exactly what happening with me,i am already use buffered 0-4 v and fast slew rate and its take 4 us .Are there any special purpose pin to reduce it ? moreover I have one another thing that i tested is even i am use one  normal pin and toggle it by some millisecond delay that it's also take 2us for 0 to 1 or 1 to 0 and Documents says its take only nanoseconds. So what is possible way to reduce these delay , is there any specific settings for pin?

                • 5. Re: wavedac8 rising time



                  As /odissey1 indicated, using the WaveDAC output direct has a fairly medium impedance.  If you connect the WaveDAC to an Opamp before using the signal externally you should significantly reduce the slew.


                  Here is a copy of the 5LP AC characteristics for the opamp:

                  /odissey1 indicated a SR of <1V/us however this datasheet indicates a SR of > 3V/us.  Note:  This is at 200pF capacitive load.   If your real load is much less, the slew rate should be even faster.  Remember that the slew rate is dependent on the series resistance and the load capacitance.  This is an RC equivalent circuit.  Lower the R and/or the C and you should exhibit a faster slew rate.


                  I should also note another issue if you are going to Opamp buffer your WaveDAC signal:  Select either pins P0.0, P0.1, P3.6 or P3.7 for your Opamp outputs.  This is because these pins have the lowest resistance connection to the Opamp output (~0.1 ohm).  Choosing other pins can result in 250 to 450 additional ohms of series resistance.