# Calculate Decoupling/Bypass Capacitor for a Delta sigma modulator

#### Zouglou LeMagicien

Joined Feb 12, 2019
33
Hey,

For a project I need to have a delta sigma modulator, so I need also to take some capacitor to bypass.

In the delta sigma, I have some OpAmp, so I think, I need to put capacitors at the input and output. But I do not understand how can I choose a value for these capacitors...

Thanks for your response

Chris

#### ericgibbs

Joined Jan 29, 2010
10,845
hi Chris,
Welcome to AAC.
Please post a circuit diagram also details of the signals.
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#### DickCappels

Joined Aug 21, 2008
6,593
...and while you are at it, what frequencies do you want to pass through the capacitors -assuming these are coupling capacitors rather than bypass capacitors which are generally not used on the inputs and outputs of amplifiers.

#### Zouglou LeMagicien

Joined Feb 12, 2019
33
Hey,
Here is my modulator (the 2 OpAmp) :

The input signal is a Pulse.
The frequency that i want to pass into the capacitors is the ripple frequency right ? How can I know it ?

#### ericgibbs

Joined Jan 29, 2010
10,845
hi Z,
Please post your LTSpice asc file, many LTS users here.
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#### Zouglou LeMagicien

Joined Feb 12, 2019
33
Here it is

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#### ericgibbs

Joined Jan 29, 2010
10,845
hi,
Where did you load the AD712 model from.? or did you edit the model.?

I had to replace your AD712 model with the LTS standard AD712 from the LTS OPA selection, now the sim runs.
Ref this error message image.
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#### ericgibbs

Joined Jan 29, 2010
10,845
hi,
This is what LTS shows for your basic circuit.
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Update:
On U4 AD712 the supply is +/-9V but the +/-Vinps are at a possible +35V [ the resistive divider on the +Vin is at +17.5v, is this within spec for the AD712.???

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Last edited:

#### Zouglou LeMagicien

Joined Feb 12, 2019
33
...and while you are at it, what frequencies do you want to pass through the capacitors -assuming these are coupling capacitors rather than bypass capacitors which are generally not used on the inputs and outputs of amplifiers.
Actually what is the difference bewteen a decoupling and a bypass capacitor ?

#### MrChips

Joined Oct 2, 2009
21,900
Actually what is the difference bewteen a decoupling and a bypass capacitor ?
Same difference.
A coupling capacitor refers to a high pass filter whose purpose is to remove low frequencies.
Decoupling or bypass capacitor is meant as a low pass filter whose purpose is to remove high frequencies.

#### DickCappels

Joined Aug 21, 2008
6,593
I think he meant to ask what the difference is between a coupling capacitor and a bypass capacitor. I wrote "...-assuming these are coupling capacitors rather than bypass capacitors which are generally not used on the inputs and outputs of amplifiers."

A bypass capacitor is used to direct signals to ground while a coupling capacitor is used to AC couple signals within a circuit or system.

What frequencies do you want to pass through the capacitors if they are coupling capacitors.

#### ericgibbs

Joined Jan 29, 2010
10,845
hi Z,
What is your response ref posts #7 & #8

On U4 AD712 the supply is +/-9V but the +/-Vinps are at a possible +35V [ the resistive divider on the +Vin is at +17.5v, is this within spec for the AD712.???

Also what will be input frequency range.? and Output Node.?

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