bananasiong
New Member
Hi,
What is DC balanced? What can make the PNP transistor DC balanced?
What is DC balanced? What can make the PNP transistor DC balanced?
Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
bananasiong said:Hi,
What is DC balanced? What can make the PNP transistor DC balanced?
bananasiong said:so..
the input bias current of the LM393 is 25nA, 250nA max. with the 100k resistor, it gives only 2.5mV. it should be more than 5mV according to the input offset voltage. right or wrong??
bananasiong said:don't understand..
the voltage provided by the external resistor (by multiplying the resistance with the input bias current) should be more than the input offset voltage listed in the datasheet in order to make it balance???
Nono... nothing to do with the magnetic field..Nigel Goodwin said:Perhaps you might like to explain why you're asking this?, is it still to do with your magnetic field sensor?.
is it caused by the resistor from the non-inverting to GND? How if the magnitude is higher?To correct for offset voltage, you add a voltage source of equal magnitude and opposite polarity to the offset voltage in series with the non-inverting input of the op-amp.
dknguyen said:Offset voltage is ALWAYS there, and is not produced by external components (although I assume it could be affected by them). It comes from the halves of the differential inputs inside the op-amp not being perfectly matched.
However, a SECOND offset voltage can be made because the bias current of the two op-amp inputs is travelling through different resistances. You can correct for this by adding resistors/capacitors to the non-inverting input to make the bias currnts travel through the same resistance (and therefore get common-mode rejected by the op-amp differential inputs). That is probably why you connected the non-inverting input to ground through a resistor rather than just connecting it directly to ground. The resistor is there to make both opamp inputs see the same impedence. This offset voltage, however, is not in the datasheets because it depends on the components around the op-amp.
audioguru said:You connected a 100k resistor to ground at one input to force that input to typically +2.5mV due to the typical 25nA of bias current through the resistor.
You are correct in thinking that it should be more than the max input offset voltage of 5mV, because at only 2.5mV the output could be the reverse voltage from what you want if the input bias voltage is max and the opposite polarity to that created by the 100k resistor.
bananasiong said:You mean the resistances for both the inputs of the comparator should be the same? But in my circuit, non inverting is conected to the 100k and the 0.1uF to the GND. While the inverting input is conected to a tank circuit. How can their resistance to be the same? The circuit works well if i replace the 100k with a 270k.
this is the circuit.dknguyen said:Can I see the circuit? If you did not design the circuit (I am assuming you didn't since you don't seem to be sure about certain things), then the designer might have already compensated the impedences at the inputs for you. That is probably why the non-inverting input is connected to ground through a capacitor and resistor. From what it sounds like so far (as far as I can say without a schematic), it seems the circuit would work just fine if you connected the non-inverting input straight to ground. Then the reason the capacitor and resistor is there is to compensate for the impedences at the inverting input. It may have already be done for you.