Help for opamp to adc

[kimmi]

Hello all ..
I have a problem need to make this to work on adc input
my volt is (minus)-10mv to +65mv to smale to feed the adc what to do & howto do it

/Kim

 

[audioguru]

1) You need to use an inverting opamp with a dual supply voltage.
2) You add a +10mV offset voltage to the signal so that it is from 0V to +75mV.
3) You set the opamp for a gain of 5.0V/75mV= 66.67.

 

[kimmi]

is Lm358 ok for this + 5v - 5v
& can i finde a schematic for this here on forum

 

[audioguru]

is Lm358 ok for this + 5v - 5v

Nearly any opamp will work with a +5V and -5V supply. The LM358 is dual, not single, and is pretty slow if speed doesn't matter.

can i finde a schematic for this here on forum

You won't find a schematic, this is your project.
Can't you make a simple inverting opamp circuit?
Can't you add a single resistor for offset voltage?

 

[Analog]

Hello all ..
I have a problem need to make this to work on adc input
my volt is (minus)-10mv to +65mv to smale to feed the adc what to do & howto do it

/Kim

It depends on the ADC. Many ADCs accept bipolar input voltages. You need to specify the A/D that you are using!

Suppose your A/D accepts +/-5vdc. Then you need to "gain-up" the signal to approximately 76.9. Since your input is asymmetric, then your negative side will suffer some resolution degredation (what resolution is your ADC, and what precision do you desire?).. Lots of work here folks!

 

[kimmi]

Yep thats right
adc 0 - vreff (2.56) or 0-5 v in this is a atmel M8 (sorry)
have made some atemt to use a 741 not good then 1458 (dual)gain to smal

now i dont know what to do
I only program software the electronic part in do by a frind but he is on hollyday for 2 weeks and this need to be done now cant wait

the hard part is the (damm) -10mv

kim

 

[audioguru]

A 1458 is a dual 741 opamp and will work fine with positive and negative 10V supplies.
Their open-loop gain at low frequencies is typically about 200,000.

 

[kimmi]

If I use a 741 and add a pot 10k to pin 1 and 5 and to -10v is that not same at zero adj. ?

 

[audioguru]

If I use a 741 and add a pot 10k to pin 1 and 5 and to -10v is that not same at zero adj. ?

The pot is to adjust the zero to cancel the opamp's built-in offset voltage. Your opamp might need the opposite voltage and more offset adjustment for your circuit.

 

[kimmi]

Well I give up dont want to sepnd more time on this
have to wait 2 weeks
Tnx for the help (Analog) and audioguru ..

 

[kimmi]

Hmm !!
now it work by using the LM358 (dual) gain ok offset ok

and using + - 5 volt
( 5/1023) adc is 0 to 20502
at last .... Thats way I dont make electronic but only program software
my skill in electronik is so damm bad ( sorry)