DAC904: The choose of I/V opa of DAC904

Hi Hanlin,

I'm sorry to hear you're unsure of your design.  Unfortunately we do not have the bandwidth to support University projects at this time.

Regards,

Geoff

Hello Geoff,

Thank you for your reply.

But I don't understand the meaning of "the bandwidth to support University projects".

I just want to know if there is suitable OPA for the I/V trans of DAC904 output with 50kHz~100kHz square wave.

Hello Hanlin,

I do not see an immediate issue using the OPA891.  Could you share a block diagram or circuit here?

As your waveforms are fairly slow, the slew rate of the OPA891 should not be an issue.  Could you share your input signal to the OPA891?

Best,

Alec

Hello Alec,

Thank you for your reply.

Here is my schematic:

The impedance match is according to: DAC904 Impedance Match.pdf

And the problems now are:

When ±5V is on, all DAC904 digital input pins are 0, the final output is: 

When ±5V is on, all DAC904 digital input is programmed suqare wave, the final output is: 

I'm trying to locate the noise source.

Hello Hanlin,

What frequency is the noise?

The OPA891 does prefer to be loaded with a higher resistance load than 50 Ohms.  You could try putting the load of the first OPA891, R73, to 150 Ohms.  You may also use 1kOhms for improved THD.

Best,

Alec

Thank you for your reply!

The noise frequency is about 65kHz.

Here are the signals at Iout+, Iout-, R73(50 Ohms):

It seems the 65kHz noise is from DAC904 current output?