# Question about amplitude on OpAmp

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#### Paul98

##### Member level 5
Good morning everyone,
I'm learning to use LtSpice and I'm doing some circuits with opamps. In the example I have below: I apply a 10uV input signal with offset 0. Consequently I have a sine wave which will do + 10uV / 0 / -10uV as in the image at 100Mhz . However when I measure the output of the OpAmp I notice that despite the amplified output signal it has lost the Offset 0. In the example in the photo the blue wave and the input signal while the green one to the amplified output one. The green signal has an amplitude ranging from about + 17uV to + 62uV. Thank you..

#### FvM

##### Super Moderator
Staff member
Not sure what your asking. Compared to AD8099 typical and maximal input offset specification, the offset error is massively underestimated in your simulation. Specified maximal input offset voltage is 0.5 mV, it would generate 1 mV output offset in your G=-2 amplifier.
--- Updated ---

Floating the PD pin disables by the way the input bias current cancellation, and causes additional offset voltage due to asymmetrical input load. Apparently not modelled.

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#### Paul98

##### Member level 5
I did not understand your explanation with the PD pin. The diagram I have attached in fact is related to a G = 2 What I mean is that I was expecting an amplified infusion wave with zero offset. In the example of the first post I talked about an output sine wave (green) which according to the graph has a wave that starts from + 17uV and reaches + 62uV therefore an amplitude of + 45uV which is the gain provided by the OpAmp but this amplitude it is not offset zero. If it were at zero the amplitude would be 45uV / 2 = + 22.5uV / 0 / -22uV. . What I would like to understand is if there is a way to keep the output sine wave with zero offset. Sorry if I can't be clearer.

#### FvM

##### Super Moderator
Staff member
I did not understand your explanation with the PD pin.
PD pin comment is only a side remark. More important, you also don't understand the role of OP input offset voltage.

Please look at the below equivalent circuit from Gray, Hurst, Lewis, Meyer: Analysis and Design of Analog Integrated Circuits
The offset voltage Vos is connected in series with the OP input, it acts like an additional input voltage in case of the non-inverting amplifier circuit. You find the expectable Vos range in the datasheet, for AD8099 the spec is shown in my previous post.

#### Paul98

##### Member level 5
So could I turn down the input voltage? In the schemes proposed on the datasheet there are some resistances on the input.

#### FvM

##### Super Moderator
Staff member
Part of the total offset that is caused by bias current in combination with asymmetrical source resistance at the input terminals can be cancelled by balancing the resistors. The input offset voltage itself can be at best adjusted away by a compensation circuit. Offset voltage drift however remains. An input offset voltage of 20 uV as observed in your simulation will be created by only 8 degree of temperature variation.

### Paul98

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#### stenzer

##### Advanced Member level 2
Hi,

I agree with @FvM, you have to apply external "measures" to get rid of the DC offset. You may have a look at [1]. The easiest way to achieve this is AC coupling the signal by means of a series capacitor connected between the output and the load, if you are not interested in low frequency signals or even DC voltages. A 10 nF or 100 nF capacitor (which are "typical" values used all over a PCB) will do the job, as these values represent a low impedance at 100 MHz.

[1] https://www.analog.com/media/en/training-seminars/tutorials/MT-037.pdf

BR

### Paul98

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#### crutschow

##### Advanced Member level 5
If you need DC response with low offset, then you will need to use an op amp with lower input offset.

### Paul98

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#### FvM

##### Super Moderator
Staff member
You can have "zero offset" OP amps with low or medium bandwidth, but not with 100 MHz or GHz GBW. In case your application needs both features, you have to design a combo amplifier with splitted DC and RF signal.

### Paul98

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#### Paul98

##### Member level 5
In case your application needs both features, you have to design a combo amplifier with splitted DC and RF signal.

Actually I was already thinking about creating a circuit with a stage for the low frequencies and one for the highs but I should also use an output mixer. I don't know which via RLC or active filter. I give it a look.

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