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| variable gain current amp |
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| Simon:
I am looking to use the https://www.ti.com/lit/ds/symlink/lmp8645.pdf so that i can switch the gain resistor to change ranges. But just to have a gain of 50 I have calculated that i need 250k, to do my more sensitive range i need 25M! have i got it wrong or is this chip pretty hopeless? |
| coppercone2:
well its common with modern parts to use high Z there to save power. It's something you would use guard traces with if you follow the datasheets for other parts with similar nuances (i.e. LT) With the voltage regulators, I think it made sense to use a through hole part with a guard trace under neath IIRC But those parts were ground referenced. This is floating. It's pretty high (usually the VGA IA's have like 1 meg max) In the data sheet, maximum gain is 100. Don't know if this corresponds to a noise figure, or just the GBW setting. You are saying you want a gain of 5000, from a part that has a datasheet gain of 100 max, with 25meg, assuming the formula rgain (25Meg)/ 5k (internal) Single stage 5000 gain is going to be ugly. It looks like they want maximum to have a value there of 5Meg. |
| Simon:
Yea I think basically I need too much gain I'm at 50 with 250k but the other range is 100x higher. Maybe i need to follow with an opamp to do tho 100x |
| coppercone2:
--- Quote from: Simon on May 14, 2020, 03:35:57 pm ---Yea I think basically I need too much gain I'm at 50 with 250k but the other range is 100x higher. Maybe i need to follow with an opamp to do tho 100x --- End quote --- you might also get results if you filter prior to heavy gain because its a semiconductor facing something with RF noise that might do weird things, so you go from DE to SE with light gain, filter, then go to high gain. You need to analyze and also look at CM chokes and DE filtering prior to gain possibly. Greater then 1000 gain with a single stage is a little crazy IMO |
| Simon:
Looking at http://www.farnell.com/datasheets/2299421.pdf I don't see a gain figure just a maximum of 500µA output. |
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