help with lm358 for amplify milivolt

i dont know why that is happening i want to amplify little voltage difference..

by that figure

LM358_piezo_schem | Flickr - Photo Sharing!

i give -2pin 12volt and in +3 pin as 12.3volt logically it has voltage difference .3 volt or 300mV so i will get on output as amplification of 300mV. but i got always 3.67 volt on output pin why????

another pic here


two_op_amps_instrumentation_amplifier | Flickr - Photo Sharing!

i also tried that one with lm358 but still have no hope

i give V2 12.5volt and V1 as 12 volt lm358 have power as 5V

? s the problem i dont know please give me some advice

regards
_misel

---------- Post added at 12:36 ---------- Previous post was at 11:03 ----------

i wanna do that


lm358 | Flickr - Photo Sharing!

i have +48 main supply then i boost it means negative voltage varies with input to keep 36V fixed then i give it to load and wanna measure current by giving a shunt in negative side. froma that shunt i take two point which is the input for 358 .

when is in load theres no voltage difference in those two pin

when theres a load theres 250mV voltage difference

i need to amplify that and measure that voltage by microcontroller..


but still i cant measure that 250mV from Vout

thanks

regards _Misel

Seems to me your summing stage isn't connected as unity-gain, so there is gain in the summing stage. So the difference is amplified by the summing stage. That might be why you are getting 3.67V.

thanks for the reply

but im sorry i cant understand the Solution

_misel

The first circuit isn't a voltage amplifier. It's intended to amplify current (or charge) from a piezo sensor.

The second circuit can work, if the OP input voltage is within the specified common mode range. For LM358, it's 0 to Vb-1.5V. So you need 14 or 15 V Vb to measure input voltages a 12V common mode.

I mean your summing stage should be connected as unity-gain, then the output of the summing stage will be 12.3V-12V=0.3V. Specifically try removing the 100K resistor across the pin 2 and pin 1 and directly connect pin 1 and pin 2.