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schematic

simulate this circuit – Schematic created using CircuitLab

The output is always outputting 5V, and I have no idea why. I have checked the inputs to the comp, I even switch them around its never hits 0 volts. Yes I am double checking that input from the OPA350 is greater than the Vrf.

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5
  • \$\begingroup\$ is the ground pin connected? \$\endgroup\$ Commented Oct 24, 2018 at 5:08
  • \$\begingroup\$ Yeah, weirdest thing, Pin 4 grounded VSS \$\endgroup\$
    – Leoc
    Commented Oct 24, 2018 at 5:11
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    \$\begingroup\$ What did you do with the CSn pin of the MCP6548? \$\endgroup\$
    – The Photon
    Commented Oct 24, 2018 at 5:21
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    \$\begingroup\$ Needing to know details like this is why you should post a complete schematic of your circuit, not just the parts you think might be important. \$\endgroup\$
    – The Photon
    Commented Oct 24, 2018 at 5:28
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    \$\begingroup\$ Oh, sorry I didnt know that was important I left it floating (Unconnected) \$\endgroup\$
    – Leoc
    Commented Oct 24, 2018 at 5:29

1 Answer 1

5
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It seems most likely that you have either tied the \$\rm\overline{CS}\$ input high, or you left it unconnected and it randomly drifted high due to static charge or capacitive coupling to nearby objects.

With \$\rm\overline{CS}\$ high, the output goes into a high-impedance state as described on p. 16 of the datasheet, which allows R3 to pull the output high.

Edit

In comments you now confirmed you left it unconnected.

As a general rule, you should never leave CMOS inputs floating, as this can produce unpredictable circuit behavior (even oscillation and circuit self-destruction in certain worst case scenarios). If a chip has an internal pull-up or pull-down allowing a logic input to be left unconnected, it will nearly always be mentioned in the datasheet.

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  • \$\begingroup\$ Wow, amazing. It works now. Thank you so much. (I grounded the CS pin) \$\endgroup\$
    – Leoc
    Commented Oct 24, 2018 at 5:39

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