Electronics > Projects, Designs, and Technical Stuff
A type of instability seen in feedback ac-coupling of in-amp
<< < (3/3)
David Hess:

--- Quote from: Zero999 on May 22, 2019, 08:55:55 am ---
--- Quote from: David Hess on May 22, 2019, 01:50:28 am ---The supply splitter will be unhappy driving the decoupling capacitors.  You might get away with that by swamping its output with a 10 to 100 microfarad aluminum electrolytic capacitor.
--- End quote ---

This. One solution is don't connect any capacitors to the output of the rail splitter. Connect the decoupling capacitors across RB1 and RB2.

It's true, the output impedance of the rail splitter will rise, at higher frequencies, but it's the same op-amp as the others in the circuit, and their performance will also degrade at higher frequencies.
--- End quote ---

I have tried that before and removing the decoupling capacitors always caused problems with the rest of the circuit; the AC impedance was just too high.

This article from Analog Devices discusses the problem with driving a capacitve load in detail and shows one way to solve it at the end.  This can work however the impedance rises at medium frequencies so getting it right can be tricky.

A better solution in my experience is to place a bulk decoupling capacitor directly at the output of the operational amplifier to one of the supply rails; 10 to 100 microfarads is typical.  Combined with the output resistance of the operational amplifier, this lowers the bandwidth enough to prevent loop oscillation.  The ESR creates phase lead adding to the stability of this configuration so use a standard aluminum electrolytic or tantalum part.

Some operational amplifiers can drive any capacitive load by lowering their own bandwidth and would not require any additional measures although they would still benefit from a bulk decoupling capacitor at their output.
alishirali:

--- Quote from: dmills on May 21, 2019, 08:16:19 pm ---That still leaves you with all your other decouplers connected to Vgnd, try putting those caps back and sticking 100R or so between the output of U2B (AFTER you tap off the feedback) and the Vgnd node.
 
Opamps in general really dislike capacitive loads.

Regards, Dan.

--- End quote ---

I removed all bypassing capacitors (C_T1, C_T2, C_B_GND, ...) and added 100 and 10  \$\Omega\$ from U2B output to VGND. No significant difference occurred  :(
Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod