Electronics

Walter · Joined: 03 May 2006 Posts: 1

I have an input voltage of 2.73 to about 3.2 Volts that I am measuring via a 8 bit analog to digital converter but as the a2d is to setup for 0 to 5 volts it gives me a resolution 20mv which is too large.

I need to subtract 2.73 volts then multiply this by 10 which looks like a good job for an opamp.

I have found several docs on it and did a simulation in spice and the design looked good but in real life it did not work (nearly zero volts).

I am using a LM358 running from a 5V supply but I think 9 is needed to get the gain.
I used the design on page 74 of http://focus.ti.com/lit/an/sboa092a/sboa092a.pdf

The resitors where 680K & 68K to get the gain but do the actual values matter in the end result as 1K & 10K or 1M & 10M give the same gain.

I am doing something wrong but not sure what.

neon · Joined: 25 Feb 2006 Posts: 593

the amp open loop gain is 100 DB one is more then enough to saturate to 5 volts obviously you have no idea on how to setup gain A= input R devided by the feedback R 10kin and 100k f/b out =A=10 gain of ten. and 20DB is =of 100 gain. change the R change the gainn

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