Resistor connected to comparator non-inverting input - Page 1
Forum section: electronics / projects

author: electronx
author_rank: Regular Contributor
timestamp: November 10, 2024, 11:28:48 am
content: I came across such a circuit.When a voltage above 0 is applied to the noniverting input, the output remains at 3.3v. So far, normal.When I pull the input directly to gnd in the pspice simulation, the input continues to remain at 3.3v.A trick is made here, a 330kohm R37 resistor is attached. Then, when it is pulled to gnd via 330k, the noniverting input appears negative. For this reason, the output is pulled to the low output state and I see a value like 20mV.I have not come across such a structure before. Is the purpose of r37 here to pull the output low when 0V input comes?

author: Manul
author_rank: Super Contributor
timestamp: November 10, 2024, 12:46:02 pm
content: I get suspicious just by seeing that the signal flow is from right to left.Anyway, valid reasons for the R37 might be:1. Enables driving the input with a signal way outside supply range without damaging the input;2. Crude way of low-pass filter (in this case roughly 100kHz);On the other hand, it effectively makes input offset specs worse, because it allows input bias current to develop a voltage over R37. So normally, for best precision and lowest noise (in case of Op Amp amplifier perhaps) the input impedance should be kept low if possible. In other words, such resistor generally degrades performance. Low pass filtering might also be unwanted if not intentional.Small negative voltage is some kind of simulation artifact, something not right with the math in the model. It should not be there.In order to switch reliably, this circuit must be driven with negative signal, with a margin of more than maximum offset voltage for this comparator.

author: PGPG
author_rank: Frequent Contributor
timestamp: November 10, 2024, 02:05:43 pm
content: Quote from: electronx on November 10, 2024, 11:28:48 amIt could be the design idea but if so than in my opinion it is not good project.According to LMV331 datasheet input bias current is typically 15nA so voltage drop at 330k is typically 5mV while max input offset is 7mV.I would expect this working as intended only with 99,9% of ICs (so not 100%). Designing to work with 100% of ICs allows you to not select them before assembling.As your simulation shows 8.2mV than probably your model not follows typical bias current info from datasheet. It is also possible that input current changes when inputs are driven with some offset, but I would rather expect current of input driven more negative (by r37) be smaller than typical and not higher. But as the max is specified as 250nA the model author could think it would be better to have input current higher than datasheet typical value.May be the author of this design used someway the same model as you and selected r37 to reach higher then 7mV offset believing the Spice more that anyone should. I never use Spice to simulate anything else than simple transistor circuits and supply filters (taking into account parasitic elements in capacitors and chokes). For ICs I always just follow their datasheets.Interesting is what will happen if input current will be 250nA (or 400nA). For 250nA we get 82mV. Don't knowing what is the input signal tested by this circuit you can't be sure if it is not too big for the circuit to still be working as expected.Quote from: Manul on November 10, 2024, 12:46:02 pmYou don't see the whole picture. I suppose main signal is going from left to right and this is only feedback. Imagine how OpAmp circuits would look like if you force that signal should always go from left to right (including feedback circuits).

author: r6502
author_rank: Regular Contributor
timestamp: November 12, 2024, 12:45:06 pm
content: Hi all,the output signal of the comparator is named "RFID Carrier". Could it be that the input is connected to the the RF circuit of a RFID reading system, there you often have higher voltages than 3.3V.Guido