VDAC Output current rating

[Mrinal]

Hi

I have searched through the TRM and datasheet and still not able to find the output current rating of the VDAC. All I can see is 2us settling time when connected to 25pF load, that too through internal buffering.

I am using the DAC along with a 4 channel analogue de-mux in a scanning configuration. Sample and hold capacitors are connected to physical pins of the MCU.

How much current can the VDAC supply when unbuffered?

How much current can the buffers supply in different power modes?

The datasheet of all the components are lacking electrical specification. Even the output voltage range is not mentioned anywhere for the buffers. The initialization window of the opamp only draws a yellow shaded area at the bottom range of the allowed output voltage! Shall we guess the value of the yellow line or can we find it somewhere?

Please help 

[Vasanth · Accepted solution]

Hi Mrinal,

The internal buffer mode uses the Opamps(CTBm) blocks in the device.  The output current ratings of the opamp is the current that can be drawn from CTDAC in those cases.

Best Regards,
Vasanth

[Mrinal]

Thank you for the reply.

I was also able to find the output resistance of VDAC in the PSOC-6 family datasheet. It is 15KOhm.

[Len_CONSULTRON]

Mrinal,

With a 15K ohm output resistance you can see why they recommend a buffered output.  In general, 15K ohms is considered a medium impedance drive.

Therefore with VDAC12 in unbuffered mode with VDDA = 3.3V then the output drive is:

Iout = VDACout/(15K + Routload)  ; where Routload is the resistance of the load connected to the output.

However with internal buffered (Opamp drive) Mode 1 high or medium, Iout can be as high as 10mA with a loss of output voltage of 0.5V.

Therefore the adjusted equation should be:

Iout =(VDACout(buffered)-0.5V)/Routload.

With the Opamp buffer, the output source impedance (Rs) can be extrapolated to: 

Rs = VDACout(buffered-0.5V)/Iout

where Iout is 10mA and VDACout(buffered-0.5V) = VDDA-0.5V

Then

Rs = (3.3V-0.5V)/10mA = 2.8V/10mA = 280 ohms.

If you're driving relative high capacitance, a medium impedance (15K) will take longer to rise or fall to. 

Using a lower impedance (280) with the cap will get there faster.

Additionally using a lower impedance with a lower impedance load (Routload) will lose less VDAC voltage to the drive impedance.