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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.sstatic.net/frokm.png \$\endgroup\$
    – Andy aka
    Commented 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
    Commented Jan 8, 2022 at 19:58

2 Answers 2

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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.sstatic.net/dxFKDl.jpg

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Your main problem is PD pin (pin 16) which is grounded. It mean that the amplifier is disabled, so the amplifier don't work at all.

R16 and R14 should be 750ohm to work as a differential voltage follower. It will be good start point for futher tests. When you get correct opamp operation you can try to increse gain.

Tj is output pin connected to ground. The pin is internally connected to current source so you don't burned the pin.

This amplifier design has relatively low input impedance and can distorted signal from DAC. Just be aware of that.

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