Hello,
I just migrate to kicad 6.0 fun !
I create a new schema. The output of one operational amplifier is the reference for a dac.
Unfortunatly. the DAC MCP7425 as only one pin for the reference and the power supply.
How can I tell kicad for this behaviour ? in the library the pin name is VDD, as “power_input”.
that’s correct, but the output of an operationnal amplifier can not be used as “power_input” for an other integrat circuit ?
Thanks for your help.
Thierry
Look for the PWR_FLAG symbol under the power ports section. Then add one to the net that is the OPAMP output and DAC rail. This will inform Kicad that “power” is provided on this net
This all sounds like an interesting example of difficulties with ERC. BTW if it is OK I would be interested in seeing how you designed this portion of the circuit. A screen shot of the schematic showing op amp and surrounding components would be adequate. I have used an op amp in this way. Depending upon what you have, I may have some cautionary recommendations. The potential issue is driving a capacitive load with an op amp. This can be done well but (perhaps depending upon the load capacitance and the op amp) it usually requires a few added passive components.
There are also specialist opamps that are actually specified to drive capacitive loads
edit eg LM7321 from TI
Yes but depending upon needs and preferences, those may not be necessary. I usually like to use more generic ICs when I can do so. @Vorms may or may not feel that way…
I solved the issue with the modification of the DAC symbole.
The property of pin 3 was "power supply. This is not compatible with the outpout of an operationnal amplifier. I change this property with “input”.
You can see a portion of the schema. There is a 10V reference. This reference is divided by two and the operational amplifier is used to have a “low” impedance for powering the DAV
I was not sure, so I checked datasheets for the MCP4275 and TL074.
The MCP4275 DS recommends 100 nF power supply bypass. You may encounter noise or instability without that.
The TL074 appears to tolerate capacitive loads of up to a few hundred pF.
So while your design might work as shown, I do not recommend building it like that.
Basically, the DAC wants a supply bypass, but the op amp does not want a large capacitive load. These modifications I am recommending avoids that contradiction and should work well. The component values need not be so precise, but keep them within 1X to 2X of what I am recommending.
Hello BobZ
I thank you very much for your suggestions !
I will add a RC between the output of the op amp and the DAC.
I know this op amp is “stable” because it’s not a fast op.
So, I think it’s not necessary to place a capacitor between input pin.
My origginal question was about kicad, the VDD pin is a power input and it’s not compatible with an output of op. I solve this issue by modifying the pin category of the symbol of the DAC. Is there another mean to solve this issue ?
Regards
Be careful!
I think this is a 5 MHz op amp and even slower ones (such as LM324) can oscillate. If you connect only some of the parts, you risk introducing instability or a DC error. I will drop this discussion if you prefer. But if you want to follow through, please reply with a screen shot of the op amp portion of your modified schematic diagram.
See https://www.analog.com/en/analog-dialogue/articles/techniques-to-avoid-instability-capacitive-loading.html. I am recommending a design like this but with Rin left open for unity gain.
from https://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-25.html
Any guesses on the problem with it? 
This particular KiCad question is one on which I am not an expert and I prefer to “pass”. I largely skip schematic error checking.
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