Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Cascading Op-Amps suffer from oscillation

Status
Not open for further replies.
B

bleedingwolf

Newbie level 3
Joined
May 27, 2010
Messages
3
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Location
FL
Activity points
1,304
Hi All, I am now using AD8099 for a low noise preamplifier, which is followed by a low pass filter for filtering out low frequency noise and DC created by the unbalance impedences between V+ and V-. For single stage very thing is fine. However when I cascade two stage, oscillation happens. Please see circuit in attachment. Anyone tell me what cause this problem? Thank you.
 

The amplifier has 400 overall gain with about 150 MHz bandwidth. It's very easy to get oscillations by unwanted coupling between input and
output or through power supply. Apparently, you didn't use a suitable "RF" circuit layout. A continuous ground plane and additional supply
decoupling by ferrite beads is suggested, also correctly grounded coaxial connectors for in- and output.

As another point, you don't show source and load connected to the circuit. There's no impedance matching provided. If you connect coaxial
cables, that are unterminated at both ends, they possibly act as resonators, further promoting existing oscillation tendency.

You may want show a photo or layout drawing to discuss the details.
 

bleedingwolf said:
Hi All, I am now using AD8099 for a low noise preamplifier, which is followed by a low pass filter for filtering out low frequency noise and DC created by the unbalance impedences between V+ and V-.
..............
Click to expand...

A "low pass filter" ? I suppose, it's a typographical error.
I think, in addition to to effects as mentioned by FvM the opamps do not like the capacitive load (decrease of stability margin).
 

Hi,
I has attach the PCB layout picture. There are 12 amplifier on it and each single amplifer layout is draw as the evluation board provide by AD. I think it is not very easy for read. The evluation board sheet have been attached. In addition, I terminated the input of the first stage by a RF signal generator with 50ohms output impedance. And terminated the ouput of the second stage by oscillscope(1Mohm input impedance). Please have a shoot at the attachment.

Thank you for your help.


FvM said:
The amplifier has 400 overall gain with about 150 MHz bandwidth. It's very easy to get oscillations by unwanted coupling between input and
output or through power supply. Apparently, you didn't use a suitable "RF" circuit layout. A continuous ground plane and additional supply
decoupling by ferrite beads is suggested, also correctly grounded coaxial connectors for in- and output.

As another point, you don't show source and load connected to the circuit. There's no impedance matching provided. If you connect coaxial
cables, that are unterminated at both ends, they possibly act as resonators, further promoting existing oscillation tendency.

You may want show a photo or layout drawing to discuss the details.
Click to expand...
 

I think it is not very easy for read.
Click to expand...
I agree. You may want to post gerber files in addition to provide a clearer view. I don't agree however, that it's a copy of the Eval-Board
layout. If I understand right, you also have copper pour on both sides, but it's not that continuous. Furthermore, even a continuous ground
plane hasn't zero impedance. It can have "modes", particularly if you inject differential currents between different points at the ground
plane. Having the ground plane partially cut up, as apparently in your layout, reinforces the risk of ground plane resonances.

For the coax output, you should absolutely have a 50 0hm series termination, otherwise it acts as a critical capacitive load respectively
a transmission line resonator. 1 m coax cable with one end open (oscilloscope side) and one end shorted (OP side) is a fine λ/4 resonator
(about 50 MHz resonance). Adding the series resistor gives it sufficient low Q to disable it as a resonator, respectively isolate the
capacitive load from the OP output.
 

Hi,
I finally fixed this oscillation by replacing R1 by a 50ohm resistor. I can not tell why. Do you have only idea about this?

Thank you.

FvM said:
I think it is not very easy for read.
Click to expand...
I agree. You may want to post gerber files in addition to provide a clearer view. I don't agree however, that it's a copy of the Eval-Board
layout. If I understand right, you also have copper pour on both sides, but it's not that continuous. Furthermore, even a continuous ground
plane hasn't zero impedance. It can have "modes", particularly if you inject differential currents between different points at the ground
plane. Having the ground plane partially cut up, as apparently in your layout, reinforces the risk of ground plane resonances.

For the coax output, you should absolutely have a 50 0hm series termination, otherwise it acts as a critical capacitive load respectively
a transmission line resonator. 1 m coax cable with one end open (oscilloscope side) and one end shorted (OP side) is a fine λ/4 resonator
(about 50 MHz resonance). Adding the series resistor gives it sufficient low Q to disable it as a resonator, respectively isolate the
capacitive load from the OP output.
Click to expand...

Added after 31 seconds:

Hi,
I finally fixed this oscillation by replacing R1 by a 50ohm resistor. I can not tell why. Do you have only idea about this?

Thank you.

FvM said:
The amplifier has 400 overall gain with about 150 MHz bandwidth. It's very easy to get oscillations by unwanted coupling between input and
output or through power supply. Apparently, you didn't use a suitable "RF" circuit layout. A continuous ground plane and additional supply
decoupling by ferrite beads is suggested, also correctly grounded coaxial connectors for in- and output.

As another point, you don't show source and load connected to the circuit. There's no impedance matching provided. If you connect coaxial
cables, that are unterminated at both ends, they possibly act as resonators, further promoting existing oscillation tendency.

You may want show a photo or layout drawing to discuss the details.
Click to expand...
 

I can not tell why
Click to expand...
Consider what I said about the transmission line resonator.

Personally I would start with an output series termination instead of and input parallel termination. But the parallel termination also
reduces the loop gain of the driving amplifier and may be more effective to suppress oscillations. Unfortunately is causes more
power dissipation.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top