Calculate Decoupling/Bypass Capacitor for a Delta sigma modulator

Thread Starter

Zouglou LeMagicien

Joined Feb 12, 2019
4
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
 

DickCappels

Joined Aug 21, 2008
5,877
...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.
 

Thread Starter

Zouglou LeMagicien

Joined Feb 12, 2019
4
Hey,
Here is my modulator (the 2 OpAmp) :
1573217468361.png
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
8,723
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.
E
 

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ericgibbs

Joined Jan 29, 2010
8,723
hi,
This is what LTS shows for your basic circuit.
E


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:

Thread Starter

Zouglou LeMagicien

Joined Feb 12, 2019
4
...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
19,270
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.
 
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
8,723
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.?

E
 
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