# Single supply chain drift the reference output voltage

#### acheriti

Joined Apr 26, 2020
30
Hi guys,
So I'm simulating a LPF with a non inverting op amp. I am doing a single supply design. My problem is the following : for I reason that I do not understand, my final gain stage (U6) makes the voltage output reference drift. My main supply is 5V, and my reference supply is 1.25V. When using a gain of approx 10, my output voltages is averaged around 1.35V. If using a gain of 50 for example, my new average DC value is 1.75V as I would have expected my signal to be centered around 1.25V.
Can anyone help me with this circuit ? Thank you very much.

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#### crutschow

Joined Mar 14, 2008
30,753
It's not a "drift".
Op amps have an input voltage offset whose value depends upon the particular op amp you are using.
At a gain of 50, the input offset is (1.75V-1.25V) / 50 = 0.5v / 50 = 10mV, which is not an unusual value of op amp input offset.

So, if the output offset is a problem, here are some options:
• Use a lower offset op amp
• Use a DC blocking capacitor at the op amp output.
• Use a DC blocking capacitor at the connection between the 1.25V reference and R11. That will reduce the DC (offset) gain from 50 to 1, so the output offset would now be only 10mV instead of 500mV.

#### AnalogKid

Joined Aug 1, 2013
10,122
There is no firm DC operating point for any of the amplifiers. All of the non-inverting inputs refer back to V9 / V10. Connecting C8 and C9 to the internal reference does not set Vref as the operating point for those stages.

What is the operating frequency range for the circuit?

ak

#### acheriti

Joined Apr 26, 2020
30
It's not a "drift".
Op amps have an input voltage offset whose value depends upon the particular op amp you are using.
At a gain of 50, the input offset is (1.75V-1.25V) / 50 = 0.5v / 50 = 10mV, which is not an unusual value of op amp input offset.

So, if the output offset is a problem, here are some options:
• Use a lower offset op amp
• Use a DC blocking capacitor at the op amp output.
• Use a DC blocking capacitor at the connection between the 1.25V reference and R11. That will reduce the DC (offset) gain from 50 to 1, so the output offset would now be only 10mV instead of 500mV.

#### acheriti

Joined Apr 26, 2020
30
There is no firm DC operating point for any of the amplifiers. All of the non-inverting inputs refer back to V9 / V10. Connecting C8 and C9 to the internal reference does not set Vref as the operating point for those stages.

What is the operating frequency range for the circuit?

ak
My circuit needs to operate in a BW of [25 500]Hz. Would you recommand connecting C8 & C9 to the GND?

#### AnalogKid

Joined Aug 1, 2013
10,122
My circuit needs to operate in a BW of [25 500]Hz. Would you recommand connecting C8 & C9 to the GND?
Yes, but you still need a DC reference for the non-inverting inputs. One option is a resistor to Vref. If the resistor value is much larger than the worst-case filter network impedances, it won't affect the corner frequencies.

#### acheriti

Joined Apr 26, 2020
30
Yes, but you still need a DC reference for the non-inverting inputs. One option is a resistor to Vref. If the resistor value is much larger than the worst-case filter network impedances, it won't affect the corner frequencies.
Thank you very much for your answer. For my understanding why would it be better to connect the capacitors to GND instead of to VREF?

#### Audioguru again

Joined Oct 21, 2019
5,143
V10 is the DC reference voltage. Why do you make another DC reference voltage with U7?
You do not understand that your opamps DO NOT have a DC voltage reference from U7, because C8 and C9 block DC.
Your U6 has DC voltage gain so of course the DC offset voltage changes when you change the gain of U6.

Connect C8 and C9 to ground.
Remove U5 and U7.

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#### acheriti

Joined Apr 26, 2020
30
V10 is the DC reference voltage. Why do you make another DC reference voltage with U7?
You do not understand that your opamps DO NOT have a DC voltage reference from U7, because C8 and C9 block DC.
Your U6 has DC voltage gain so of course the DC offset voltage changes when you change the gain of U6.

Connect C8 and C9 to ground.