# Doubt on OPAMP used with Virtual Ground

#### guerrez

Joined Jan 26, 2012
8
I have attached two images, in which I used the method of the virtual ground.
The output of the opamp is exactly what I want in 2 different cases: a sinewwave from 0V to 3V and the other one a sinewave from 0V to 20V, all of them starting with a sinewave from -10V to 10V.

The simulation output is exactly what I want, but I don't understand how does a DC voltage of 5V can move up the wave of 10V and the same for the other case in the other picture!

Do you think that is correct the simulator that I use (Tina TI) ?

Thanks!

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#### hgmjr

Joined Jan 28, 2005
9,027
The voltage gain to signals applied to the positive input on the second diagram is:

$$gain \ =\ 1\ +\ \frac{R10}{R9}$$

In this case R10 = R9 = 10K. That results in a gain of +2.

hgmjr

#### Audioguru

Joined Dec 20, 2007
11,248
You don't need the "virtual earth" opamp because the (+) input of the amplifying opamp has an extremely high impedance and uses an extremely low current.

The two resistors for the voltage divider make a reference voltage.
Add a filter capacitor to the reference voltage.

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#### t_n_k

Joined Mar 6, 2009
5,455
The data sheet for the device suggests the typical output voltage swing at 24V DC supply is 0.075V to 23.86V.

This probably accounts for the bottom end distortion particularly evident in the first case.

#### guerrez

Joined Jan 26, 2012
8
Thank you for the two interessant answers! All of you have right!
Instead of using this circuit that I have made, always for obtain the two types of output waveform, could I use in some way an instrumentation amplifier (with single supply at max 24V), to obtain a more precise measurement? Becouse with only the single supply I haven't found any idea!
Thank you very much!

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#### Audioguru

Joined Dec 20, 2007
11,248
If you need an output voltage down to 0.0V then an instrumentation amplifier might be worse than an ordinary opamp if they do not use a dual polarity supply.

#### guerrez

Joined Jan 26, 2012
8
OK, perfect!
A stupid question: I have link to ground one wire of the secondary winding of the LVDT, I cannot do the measure without this, can I?

#### Audioguru

Joined Dec 20, 2007
11,248
If one wire of a transformer wire connects to ground and the signal is a sine-wave then the other wire goes positive and negative. Then the input must use a coupling capacitor.

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#### guerrez

Joined Jan 26, 2012
8
ah, interessant, but what happen if I don't use the coupling capacitor? And what specification has to have the coupling capacitor? Thank you for the advice!

#### Audioguru

Joined Dec 20, 2007
11,248
If the input coupling capacitor is not used then the opamp has DC voltage gain.
Since your signal is AC then you don't need any DC voltage gain. With the coupling capacitor, the average output is the same as the DC reference voltage on the (+) input.

Your opamp without the coupling capacitor has a voltage gain of 2 so it doubles the +5V reference voltage so the average output voltage of the opamp is +10V.

The value of the coupling capacitor and the input resistor determine the low frequency response. The formula for the -3dB frequency is (1/2 pi RC).

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