How to add offset voltage to the circuit?

[tofinhk]

voltage offset circuit

Hi all,

I need to receive the AC signals with ±2.5V. Because of my ADC receives signals from 0V to 5V only, so I need to add 2.5V to the signal so that the signal varies from 0V to 5V. My ADC will then receive all the signals. May I ask how to add this 2.5V to the circuit?

Thanks in advance.

 

[nand_gates]

add voltage to signal

Use opamp to add 2.5v to i/p. Or simple ckt to do this is ....

         + 5v
         | 
         ,   
         \
    R1   /
         \
         /
  to     '
  ADC    |        C
  <------+-------|(---- ac input
         |
         ,
         \
     R2  /
         \
         /
         '
         |
        __
        \/

R1==R2 Here

 

[tofinhk]

add dc offset circuit

But if I add a Cap into the circuit, will the AC signals be blocked?
And also, isn't that the circuit will force the point to the ADC equal to 2.5V?
Thanks in advance.

 

[aNdreiBuuu]

offset voltage circuit

the AC Signal is not blocked by the cap... only the DC component is blocked... If you need a large frequency range biasing circuit (from DC to AC) is better to use an OP amp, a voltage refference and 3 resistors to make your biasing circuit... if you need a schematic please drop me a PM,

hope this helps,
//a

 

[smartsarath2003]

how to add voltages

Hai aNdreiBuuu,

Won't that resister bridge adds some crosover to the signal :?: . I mean the ADC input stays at 2.5V for any voltage less than 2.5 and then fallows the input signal, if it is greater than 2.5V.Is its that actually required?

Thanks

 

[aNdreiBuuu]

adding voltage offset

If I understand corectley you have to translate the -2.5 - +2.5 domain to 0-5 for feeding your ADC. If your signal is AC with medium or high frequency the cap circuit is ok for your application (for audio or video signal, for example). If your voltage is from an industrial transducer (low, very low frequency or pseudo DC) you must use an OP and 3 resistors to do the task.

It would be usefool for us to describe your application for helping you!

//a

 

[tofinhk]

add offset to ac signal

Sorry I haven't give too much information

My project is to receive signals from a transducer. Actually what I concern is 0-80Hz only. So it is really low frequency. The ADC I used can receive 0-5V and I can't change the ADC. The voltage from the transducer is ±7V. But I don't want the higher voltage. What I concern is ±0.1V. So, I amplify the signals by 25 times. The signals I want is now ±2.5V. Due to the limitation of my ADC, I want to add the offset voltage to the AC signal by 3.2V in fact. After that, I will use a diode to block the negative signal and then to the ADC.

Thanks again.

 

[10kangstroms]

adding dc offset circuit

Sorry I haven't give too much information

My project is to receive signals from a transducer. Actually what I concern is 0-80Hz only. So it is really low frequency. The ADC I used can receive 0-5V and I can't change the ADC. The voltage from the transducer is ±7V. But I don't want the higher voltage. What I concern is ±0.1V. So, I amplify the signals by 25 times. The signals I want is now ±2.5V. Due to the limitation of my ADC, I want to add the offset voltage to the AC signal by 3.2V in fact. After that, I will use a diode to block the negative signal and then to the ADC.

Thanks again.

Post a schematic of what you have. The offset can probably be added to the existing circuitry.

 

[Borber]

adding dc offset to a signal

Offset can be added to 25x amplifier if it is OPA.

 

[tofinhk]

add voltage

I use op amp to amplify the signal by 25 times.
May I know how to add the offset voltage in the circuit?

 

[Borber]

adding a dc offset circuit

Set R3 for desired positive offset. At VE=-15V R3 is 150kΩ.

 

[tofinhk]

add offset voltage

But what if I just want to add DC offset voltage but no amplifing the signals?
It is because I finished all the previous parts and I just want to add offset voltage before getting into the ADC.
And also, may I know what if VE is not -15V, but -12V, -9V or even GND(if LM358) ?
Sorry for my poor electronic knowledge. Thanks.

 

[Borber]

add vltage to signal

Perhaps you can use resistor and current generator (source) in series with your voltage source.

 

[lastdance]

adding dc offset to a circuit

what the previous poster do is to take the 0.1V signal, amplify it by 25X and at the same time level shifting it by adjusting Ve. If you do not have a negative supply of -15V, how bout -12V, or...? The amount of DC level shifting is performed by Ve and the resistance values. You should have a negative supply in your system, I guess. Use whatever u have and calculate the R accordingly.
Therefore, the Vin in the diagram is ±0.1V, instead of ±2.5V.
You can use superposition to figureout what's in the circuit.

 

[tofinhk]

add voltage to signal

Yes, I do have -9V. What I have to do is to design a portable system. Therefore, I will bring two 9V portable battary with the system.
Because the signals have to go through the filter, I think it is better to amplify it first.
But using the above method, I have to filter the signal first. I am afraid that the noise with small amplitude will amplified after the amplifier. If I amplify the signals first, the signals become DC instead of AC, and the filter can't work as my expected.
Therefore, I need to add voltage offset after the amplifier and filter.

 

[tofinhk]

add dc offset to ac signal

Sorry, but may anyone can solve my problem?
I am in urgent of my project.
Many thanks.

 

[Borber]

add dc offset to signal

In that case you will have to post your circuit and you will pay a box of bear.

 

[lastdance]

signal amplification+offset adding

If I amplify the signals first, the signals become DC instead of AC, and the filter can't work as my expected.

doesn't make sense..
why?
u r only amplifying, why would an AC become a DC?

Implement an active current source using opamp and a discrete NMOS. Connect the gate(G) of NMOS to output of opamp, the source(S) of NMOS to a resistor, R1(the other end of resistor connect to -9V) and also opamp's inverting input(-). Opamp is powered from ±9V, non-inverting input(+) grounded. Current is given by 9V/R1.
Buffer your signal, if needed, and connect the output to a resistor, R2. The other end of R2 is connected to D, which is connected to your ADC(i assume high impedance input in your ADC). Level shift voltage is 9V X R2/R1.

 

[tofinhk]

add an offset to a operational amplifier

oops...I thought that I posted the circuit already.
Here it is.
I use the first part to amplify the signal by 25 times, and then the signals go through the filter IC by a 4-order butterworth lowpass filter. After that, the signals will go into the ADC.
However, in real practice, I found that the ADC supports only 0-5V, therefore, I want to add 2.5V offset voltage.

 

[Borber]

adding offset to a signal

I don know what IC's are used for filtering but if filter can whithstand +2.5V of input offset you can add a 50kΩ resistor between R14 and +15V to get it.