offset circuit

[mvbp]

hi all,
i am using dsPIC30f6010 microchip and i would like to know how to design a offset circuit for voltage signal to be in the range (0-5)v, if before applying it to the adc, voltage is in range -2.5 to 2.5V
thank u

 

[Nigel Goodwin]

An opamp is the obvious way, using an offset to correct the shift.

 

[ericgibbs]

hi mvbp,

Goto the National Semiconductors web page and download
'Operational Amplifier Circuit Collection' AP Note 31.

It covers all the basics of opa's, with lots of diagrams.

EricG

 

[mvbp]

offset

hi all,
i am using dsPIC30f6010 microchip and i would like to know whether the designed offset circuit, (for voltage signal to be in the range (0-5)v if before applying it to the adc, voltage is in range -2.5 to 2.5), is correct and what type of opamp i have to use here?sorry i forgot to put the zener diode at the o/p of opamp.
how should i get the -ve voltage for -2.5V
thank u

 

[ericgibbs]

hi mvbp,

Questions.

What opa are you using and what +V supply ?

Do you have a MAX232 driver i/c in the circuit.

Eric

 

[mvbp]

offset

my question is itself to what opamp i have to use? and i dont have max driver

 

[mvbp]

+&-5v is being used.

 

[ericgibbs]

hi,

If you only have a single power supply rail, eg +5V, then you want an opa that is designed to work with a single supply.

The two opa's that are in common use are the LM358N and the TS358.

The TS358 is designed to enable the Vout to swing to ground.

To get the full Vout swing you should supply the opa with a higher voltage than +5v.

These two opa's are about 25pence in UK. [ they are dual opa devices]

Eric

 

[mvbp]

i am using dual rail supply

 

[ericgibbs]

hi,
What are the supply voltages and what if any opa's do you have?

I maybe able to suggest using one of them.

Eric

 

[mvbp]

+ & - 5v is my supply voltage , and i would like to know the type of opa

 

[ericgibbs]

hi,

EDIT:Use the TS358 opa, the +/-5Volt psu will be OK.

Let me know what you are using the opa for and what is going to the
input device. I maybe able to help you with a drawing.

Eric

If you are in the UK, I can name a supplier of the opa.

 

[mvbp]

i am using dsPIC30f6010 microchip .for voltage signal to be in the range (0-5)v if before applying it to the adc, voltage is in range -2.5 to 2.5V, and what type of opamp i have to use here

 

[ericgibbs]

hi

Checkout the attached drawing, OK as a starting point.

NOTE: the output from 'B' is 0v to -5V. for your reference only, use 'A' for the PIC's ADC.


Work out for yourself, how to use the second opa stage, to get the Vout signal to the correct sense.

Eric

 

[mvbp]

thank u very much for ur support.
i am using dsPIC30f6010 microchip . for voltage signal to be in the range (0-5)v if before applying it to the adc, voltage is in range -2.5 to 2.5V (i.e nearly sinusoidal with peak-peak amplitude of -2.5v to +2.5V). I got this range of voltage after scaling down the sensed voltage( or current) using voltage (or current) sensor. now please tell me whether i can use the proposed circuit by u?

 

[ericgibbs]

hi,

Use the first half of the opa circuit.

The Vout on 'A' will be close to +5V when the input signal is -2.5V and 0V when the input signal is +2.5V.

You do not require the second half of the opa, I have shown it connected in order to identify the pin out for you.

You can use it on some other way, if you have a need for a 2nd opa.

Eric