This is a simple circuit to sense whether a voltage is present/ not present. If present, it will give 0, if not present, it will be high. My common reference is 4.5V, Using different opamps in the IC, i will connect each one inverting input to 5V, 12V,15V, 24V. My supply is 24V, my output pullup is connected to 3v3.
Need assistance.
1. Can you review the schematic. I'm using LM239DR2G, because of no stock for TLV1704.
2. How do i add hysteresis
3. what things i should take care of in design ?
Hi Mohammed,
We have reviewed your schematic and we would like to make a couple of adjustments to resistor values to implement hysteresis.
As the simulation above depicts, we set your hysteresis thresholds to toggle low at ~4.8V and toggle high at ~4.4V.
If you would like to adjust the simulation please download the Tina Schematic below.
We can adjust the threshold values depending on your needs.
Please let me know if you have any questions or need further clarification.
If you need more information on how to implement hysteresis for an inverting configuration, please access this cookbook: https://www.ti.com/lit/SNOA997
Best Regards,
Joe
Applications Engineer
Linear Amplifiers Business Unit | Comparators Product Line (CMPS)
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Hi Mohammed,
I usually choose the resistances in my circuits in such a way, that the error caused by input bias current is less than the error caused by the input offset voltage. Here, this is the case when the resistance seen by each input is less than
R = 9mV / 300nA = 30k
Adding a resistor in the same range or something below at the -input will furtherly decrease the error then.
But it always depends on your application. If you can tolerate a bigger error and small current consumption is what counts in your application, you can increase the resistor to some degree. Then, you can furtherly decrease the error a bit by keeping both resistances seen from the inputs of LM139 identical.
The input bias current cancellation resistor works the better the smaller the input offset current is compared to the input bias current. So it's always necessary to have a look into the datasheet of comparator to see how to proceed.
The input bias currents of CMOS comparators are mainly ultra low leakage currents where such an input bias current cancellation resistor would make no sense.
Of course, there's more to be considered when choosing the resistors arround a comparator. The performance especially at higher frequencies suffers from too high resistances, because parasitic impedances like stray capacitances and input capacitances of comparator begin to show a negative impact. High frequency circuits can even become instable with too big resistors. Also, choosing low resistances makes the circuit less immune against capacitive stray coupling from 60Hz electrical fields and other sources.
Having said this, installing a resistor in the 100R or 1k range in series to the -input (or +input, if this is the input of your circuit) is recommended for many reasons. It provides a current limiting when the input sits on a wrong and dangerous potential. It limits the current into the comparator (input capacitance) when the input signal has steep edges. It can isolate the source from the input capacitance of comparator. It can dampen parasitic LC resonances at the input. It can provide some low pass filtering in combination with the input capacitance of comparator.
Kai
