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| Input impedance of a (generic) IC? LM 339? |
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| conducteur:
Hello, This isn't mentioned in the datasheet i found of the LM339 quad comparator, but what is a typical input impedance of an input pin on such a device? To what extent will this input impedance affect for example a simple RC first order low pass filter with it's output connected to the + or - input? If the resistor of the RC lowpass filter is to high, I assume it will become more or less a voltage divider, rather than a lowpass filter? |
| Alti:
The inputs are current sources. The input bias current is specified to be around 20nA typical, offset current +-3nA. So the input cannot be replaced by a voltage divider, in general. |
| T3sl4co1l:
Right, the impedance is high. It's not infinite, there is a slope to the current. Like maybe, Idunno, 1GΩ or so? It goes down substantially for differential voltages near zero (where the current shifts from one input to the other; ~MΩ?), and near the supply rails where either the current drops towards zero (or still further out, reverses), or the substrate diode turns on. Tim |
| magic:
(Dynamic) input resistance is sometimes specified in opamp datasheets. LM358 (similar to LM393) is missing it, but typical numbers for bipolar inputs are hundreds of kΩ to single digit MΩ. edit T3sl4co1l is right, it's not going to be that simple for a comparator. But we have the schematic: inputs are PNP emitter follower with active loads, driving a hidden differential pair inside. Any variation in input current will be due to Early effect in the follower and its load. Off the top of my head, I'm not sure how much to expect. It probably depends on the (unknown) standing current of this stage too? Good news is that it should be largely independent of what the other input pin is doing. |
| duak:
Page 8 of the LM358 data sheet from TI shows Zin: http://www.ti.com/lit/ds/symlink/lm358.pdf Both the '339 & '358 have Darlington PNP input circuits with similar bias currents so I wouldn't be surprised if they had similar Zin, at least in the linear range. I had remembered values of Zin for emitter followers in the range of 100K to 1M from h-parameter analysis in college (before SPICE). The values in the data sheet make sense give the Darlington inputs. Back to conducteur's question: The comparator's Zin will be in parallel with the RC network's capacitor. This will cause the signal at that point to be attenuated to approximately Zin/(R +Zin) in addition to the frequency selective attenuation of the RC network. The corner frequency (fc) of the RC network will be increased in frequency because the Thevenin equivalent resistance at that point will be equal to R in parallel with Zin. for example lets say R = 100K and Zin = 1M0. The signal applied to the comparator input will be approximately 90% of what it is without the comparator loading. The corner frequency of the filter will be increased by about 10%. |
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