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| Over-voltage protection for inverting amplifier |
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| iMo:
And the calculator: http://earmark.net/gesr/opamp/ |
| rstofer:
--- Quote from: imo on November 29, 2018, 05:56:06 pm ---And the calculator: http://earmark.net/gesr/opamp/ --- End quote --- Thanks for the link. I went looking for a calculator, TI even mentions one, but I couldn't find it. I have bookmarked the page, it is that helpful! |
| Zero999:
--- Quote from: rstofer on November 29, 2018, 03:42:18 pm ---There's a very methodical procedure for fitting an input range into an output range including offset using op amps. It does not, however, protect for voltages outside the input range specified. http://www.ti.com/lit/an/sloa097/sloa097.pdf A couple of days ago I was working on this for another thread so I'll just repost the MATLAB script. This is specific to the case where both 'm' and 'b' are positive as discussed in the paper. You could specify the output range to avoid getting close to the rails. Maybe leave 0.1V either side when using a rail-to-rail op amp. Unfortunately, I can't post a filetype of .m so I had to post it as .txt The only reason I wrote this was to document the process. The calculations are easy on a calculator. --- End quote --- That doesn't work when m < 1 and the TI app note also fails when m > 0 & <1. It generates negative resistance values. The minimum gain of a non-inverting op-amp is 1, so it can't work. When m < 1 & >0, either use an inverting amplifier, then invert again or a potential divider, like the one I posted and add a unity gain buffer to it, if necessary. --- Quote from: imo on November 29, 2018, 05:56:06 pm ---And the calculator: http://earmark.net/gesr/opamp/ --- End quote --- I've seen that before and was looking for it yesterday, but my Google skills were letting me down. It's good, but doesn't show the exact values, only the nearest standard resistor values, which aren't even optimised as you have to manually select R1, which might not always be what you want. |
| iMo:
--- Quote from: Hero999 on November 29, 2018, 09:26:28 pm ---It's good, but doesn't show the exact values, only the nearest standard resistor values, which aren't even optimised as you have to manually select R1, which might not always be what you want. --- End quote --- At the end of the day people simply take resistors from their junkboxes and calibrate the ADC output in software.. :) |
| David Hess:
Yes, it will work fine and transistors operating as transistors are often used in place of diode clamps for lower output impedance. Even more commonly, a transistor is used to provide a low impedance point to *drive* a low leakage diode clamp because the Vce transistor connection has additional leakage. Internally, precision bipolar operational amplifiers usually have sets of anti-parallel diodes across their inputs which do the same thing and especially prevent damaging base-emitter breakdown of the input transistors. Watch out for the added capacitance from the transistors or diode clamps at the inverting input which lowers the phase margin of the operational amplifier. A small amount of capacitance across the feedback resistor will cancel the shunt capacitance and increase phase margin if this is a problem. |
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