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| Protecting op amp input from negative voltages |
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| David Hess:
--- Quote from: guymo on July 19, 2018, 10:00:42 am ---The LM339 input is also not supposed to go below -0.3V -- are the protection concerns the same as for the op amp as discussed in this thread? The schematic of the input stage looks roughly similar so I suppose they are but worth asking! --- End quote --- Although it is not shown in the simplified schematics, the 324 and 339 do not use identical input stages but the difference is irrelevant; the 324 includes transconductance reduction which is required for stable linear operation (there is an extra set of collectors drawing current out of the input transistors) and the 339 does not. The input protection concerns all the same; avoid drawing excessive current out of the input by using a series resistor and if necessary, a clamping diode. If you do draw excessive current, abnormal behavior like phase inversion or weirder is possible. |
| Zero999:
You could also use a comparator with a push-pull output, such as the TSX3702 or LMC6762 (dual) or TSX3704 or TLC3704 (quad). The output stage is a couple of MOSFETs which looks like a resistor and will have less voltage loss, than a bipolar transistor output stage, at very low currents. http://www.ti.com/lit/ds/symlink/lmc6762.pdf https://www.st.com/resource/en/datasheet/tsx3702.pdf https://www.st.com/resource/en/datasheet/tsx3704.pdf http://www.ti.com/lit/ds/symlink/tlc3704.pdf |
| guymo:
Thanks again. I had looked at those kinds of devices before, but didn't read the data sheets beyond the headline output voltage level specs which are for way more current than I need. These things might actually be perfect, powered from 5V and 0V and feeding a summing op amp with reasonably high valued resistors to keep the current draw low. I really appreciate all the suggestions in this thread and the chance to draw on the experience, knowledge and kindness of people in this forum. Thanks! |
| David Hess:
The usual solution in the past if there was no other way was to drive an analog switch with a comparator. CMOS output comparators are almost as good. |
| Zero999:
--- Quote from: guymo on July 19, 2018, 02:36:41 pm ---Thanks again. I had looked at those kinds of devices before, but didn't read the data sheets beyond the headline output voltage level specs which are for way more current than I need. These things might actually be perfect, powered from 5V and 0V and feeding a summing op amp with reasonably high valued resistors to keep the current draw low. I really appreciate all the suggestions in this thread and the chance to draw on the experience, knowledge and kindness of people in this forum. Thanks! --- End quote --- If you only need it to be powered from 5V maximum, the list of CMOS comparators expands considerably, as they're optimised of interfacing with digital logic. MCP6541/1R/1U/2/3/4 http://ww1.microchip.com/downloads/en/DeviceDoc/21696e.pdf TS3022 https://www.st.com/resource/en/datasheet/ts3022.pdf MCP6561/1R/1U/2/4 http://ww1.microchip.com/downloads/en/DeviceDoc/22139C.pdf TLC3702/4 http://www.tij.co.jp/jp/lit/ds/symlink/tlc3704m.pdf http://www.tij.co.jp/jp/lit/ds/symlink/tlc3702m.pdf MAX976/MAX978/MAX998 https://docs-emea.rs-online.com/webdocs/12b1/0900766b812b1435.pdf |
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