Adjust resistor/capacitor values to supply change (12V to 5V)

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DonutChan

Joined Mar 3, 2019
33
I have a comparator circuit that has a 12V power supply. I wanted to change the comparator to one that is powered by 5V for my project. I don't really know how to go about choosing new values for resistors. (IN_ANALOG comes from a circuit that is powered by 5V).
I'd appreciate any kind of insight on how i should go about this.
 

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Alec_t

Joined Sep 17, 2013
12,890
Bear in mind that the common mode input range doesn't go as high as the supply voltage Vs. It's only about 0V to 3.5V with a 5V supply. So, depending on your signal and comparison threshold, you might need input attenuator resistors.
What is the threshold value?
 

Irving

Joined Jan 30, 2016
2,769
As Eric says, in general, no changes needed, but a further explanation might help you understand why, and alert you to a reason not to reduce the supply voltage.

You appreciate the comparator compares the voltages at in+ and in- and the output is low if in+ < (in- + Voffset) and high otherwise. Voffset is the comparator's offset voltage - the uncertainty area - and is typically 2mV for this device (max 7mV). The voltage at in+ is directly driven from IN_ANALOG. The voltage at in- is directly driven from X1; we don't know what drives X1 but we'll assume its a DC level for now. In any case none of the input resistor networks is dependent on Vcc so no changes needed. - for now...

We don't know the range of voltages for IN_ANALOG and X1. If IN_ANALOG is driven by a circuit powered by 5v, you would expect that the range of voltages would be, say, between 0 - 5v, and taking it as a simple sine wave, it would be centred on 2.5v. This is known as the DC offset or the common-mode voltage. There is another parameter for this comparator, the input common-mode voltage Vicr which is defined as -0.3v to Vcc-2v. As long as one of the two inputs is within that range the comparator will work as expected; outside those ranges it won't. So with a Vcc of 12v and an input of 0 - 5v there is no problem. But if you reduce Vcc to 5v then Vicr is 3.v and your comparator output wont be what you expect.....

Here's a simulation - first with Vcc = 12, X1 = 2.5 IN_ANALOG at 1.5v peak sinewave on 2.5v offset

1655892374220.png

Now on 5v with the same inputs...

1655892443556.png

Now with inputs reduced... this is just on the limit... you'd actually want to go slightly lower

As @Alec_t says, you'll need input attenuation or other adjustments...

1655892606521.png

[EDIT} A simple fix could be to add a couple of resistors to ground at the comparator inputs, to divide the inputs by 2... but this will depend on the nature of X1.

1655894355436.png
 
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LowQCab

Joined Nov 6, 2012
2,170
Depending on the Frequency that this Circuit needs to operate ...........
You could just use a Rail to Rail Op-Amp,
this could simplify the Circuit quite a bit,
but You didn't specify the Input or Output Voltage-Ranges or Impedances.
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