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Intention is to design a comparator with following specification:

Input: 0V to 1.65V, Output: Low

Input: 2.1V to 5V, Output: High

This is the TI design doc I have refereed for the design.

Please see the circuit diagram for your reference: (Highlighted in green rectangle) enter image description here

For my design requirement, I have recalculated the value of Rx, Ry and Rh as follows,

Rx = 10K Ry = 5.689K Rh = 36.67K ( VL = 1.65V, VH= 2.1V and VCC = 5V)

Formulas used are also from the TI doc enter image description here

But the simulation result is as follows:

enter image description here

enter image description here

Obs: As per the LTSpice simulation VL is 2.02V and VH is 1.65V It is not matching with the design I have done.

Please do advice why there is a mismatch between the design and simulation. Kindly let me know if further any information required.

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    \$\begingroup\$ Can you label the nodes in your circuit so we can tell which trace in the chart is which? \$\endgroup\$
    – The Photon
    Commented Oct 16, 2017 at 16:13

1 Answer 1

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You are assuming the op-amp will output 5V when high and 0V when low, they do not.

Look at the green output in your image, notice high is under 4.5V and low is above ground.

You would need to use those voltages in your math.

enter image description here

Also be aware, Voh and Vol of the op-amp will vary from device to device, with temperature, and depending on what load is attached to the output. Using op-amps as comparators is generally not a great idea.

ADDITION:

The above is partly the reason why most actual comparator devices are open collector outputs.

schematic

simulate this circuit – Schematic created using CircuitLab

\$Th_{lo} \approx V_{cc} * R2||R3/(R1 + R2||R3)\$

\$Th_{hi} = V_{cc} * R2/(R2 + R1||(R3+R4))\$

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  • \$\begingroup\$ Ideally it should be 5V high and 0V low right? Why this change? How can I make 5V high and 0V low at output? \$\endgroup\$
    – vt673
    Commented Oct 16, 2017 at 16:14
  • \$\begingroup\$ @vt673 you can pick a better op-amp that gets closer to the rail, but you will never get exactly 5V and 0V. You need to figure out how much tolerance you can accept on the thresholds and work it back from there. \$\endgroup\$
    – Trevor_G
    Commented Oct 16, 2017 at 16:16
  • \$\begingroup\$ I was referring the op-omp datasheet,link. I am bit confused as I failed to find the tolerance of LT1006. What parameter can be referred against the the change in high and low of the op-amp output? \$\endgroup\$
    – vt673
    Commented Oct 16, 2017 at 16:33
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    \$\begingroup\$ @vt673 Maximum Output Voltage Swing.. at 5V ~ 4.4V typical. Can be as low as 4V could be closer to 5V.. \$\endgroup\$
    – Trevor_G
    Commented Oct 16, 2017 at 16:41

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