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I encountered a problem while designing my own waveform generator. In my design, I am using a DAC (DAC5662) to generate different types of signals, and then I amplify the signal using an operational amplifier (THS391). Schematic looks like that: enter image description here

As seen on the schematic, IOUTA_1 and IOUTA_2 form a differential pair which my DAC generates. R18 and R19 are resistors that determine the voltage points. I then amplify the signal using an operational amplifier in the circuit as a differentiator. After that, I only have some elements for the ESD protection and overvoltage protection.

Firstly I programed my FPGA to just generate a Sawtooth wave. After that I started measuring my circuit and on my output I got:

enter image description here

which doesn't look like a sawtooth wave. So I measured what I get on my inputs of my OP-AMP(or outputs of my DAC) and got that:

enter image description here

Again not the signal I wanted so now I have decided to unsolder my two 0ohm resistors R14 and R16 and measured it again:

enter image description here

Now it looks like what should it look like. The voltage of the sawtooth which I except from my DAC is exactly as calculated. So now my question follows: What did I do wrong with my OP-AMP. I tried changing resistors from smaller to bigger values but nothing helps.

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  • \$\begingroup\$ Improved visual aid of schematic: i.stack.imgur.com/frokm.png \$\endgroup\$
    – Andy aka
    Jan 8, 2022 at 19:55
  • \$\begingroup\$ Is it meant to be a 40 kHz sawtooth? If it is then how could it be with those resistors removed? \$\endgroup\$
    – Andy aka
    Jan 8, 2022 at 19:58

1 Answer 1

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I think than if R14 and R16 are 0 Ohm, it is working as a comparator, with feedback at V-. You have to fix these resistors, once you decide the gain of the diff-amplifier.

https://i.stack.imgur.com/dxFKDl.jpg

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