AD8015 - transimpedance amplifier oscillating

I'm building a optical comm. circuit using an AD8015 transimpedance
amplifier from Analog Devices. Datasheet: http://www.analog.com/UploadedFiles/Data_Sheets/AD8015.pdf

Here is the circuit I'm using: http://gallamine.com/photodiode_tia_schematic.pdf

I'm having trouble with the circuit producing a strong ~1.9MHz
oscillation. This is independent of what the circuit is hooked to. I
have tried it with several different photodiodes (and no photodiode)
and it doesn't seem to have a big effect.

Thoughts on why this is occurring?

On Mar 17, 7:45=A0pm, william...@gmail.com wrote:
> I'm building a optical comm. circuit using an AD8015 transimpedance
> amplifier from Analog Devices. Datasheet:http://www.analog.com/UploadedFil=
es/Data_Sheets/AD8015.pdf
>
> Here is the circuit I'm using:http://gallamine.com/photodiode_tia_schemati=
c.pdf
>
> I'm having trouble with the circuit producing a strong ~1.9MHz
> oscillation. This is independent of what the circuit is hooked to. I
> have tried it with several different photodiodes (and no photodiode)
> and it doesn't seem to have a big effect.
>
> Thoughts on why this is occurring?

It's probably your PCB design.

Leon

On Mar 18, 8:38=A0am, Leon <leon...@btinternet.com> wrote:
> On Mar 17, 7:45=A0pm, william...@gmail.com wrote:
>
> > I'm building a optical comm. circuit using an AD8015 transimpedance
> > amplifier from Analog Devices. Datasheet:http://www.analog.com/UploadedF=
iles/Data_Sheets/AD8015.pdf
>
> > Here is the circuit I'm using:http://gallamine.com/photodiode_tia_schema=
tic.pdf
>
> > I'm having trouble with the circuit producing a strong ~1.9MHz
> > oscillation. This is independent of what the circuit is hooked to. I
> > have tried it with several different photodiodes (and no photodiode)
> > and it doesn't seem to have a big effect.
>
> > Thoughts on why this is occurring?
>
> It's probably your PCB design.
>
> Leon

Try to slow it down if you can.....

williamcox@gmail.com writes:

> I'm building a optical comm. circuit using an AD8015 transimpedance
> amplifier from Analog Devices. Datasheet: http://www.analog.com/UploadedFiles/Data_Sheets/AD8015.pdf
>
> Here is the circuit I'm using: http://gallamine.com/photodiode_tia_schematic.pdf
>
> I'm having trouble with the circuit producing a strong ~1.9MHz
> oscillation. This is independent of what the circuit is hooked to. I
> have tried it with several different photodiodes (and no photodiode)
> and it doesn't seem to have a big effect.
>
> Thoughts on why this is occurring?

Read Jim Williams' AN47:

<http://www.google.com/search?q=an47>

-- 

John Devereux

On Mar 17, 4:13 pm, stp <slave...@gmail.com> wrote:
> On Mar 18, 8:38 am, Leon <leon...@btinternet.com> wrote:
>
>
>
> > On Mar 17, 7:45 pm, william...@gmail.com wrote:
>
> > > I'm building a optical comm. circuit using an AD8015 transimpedance
> > > amplifier from Analog Devices. Datasheet:http://www.analog.com/UploadedFiles/Data_Sheets/AD8015.pdf
>
> > > Here is the circuit I'm using:http://gallamine.com/photodiode_tia_schematic.pdf
>
> > > I'm having trouble with the circuit producing a strong ~1.9MHz
> > > oscillation. This is independent of what the circuit is hooked to. I
> > > have tried it with several different photodiodes (and no photodiode)
> > > and it doesn't seem to have a big effect.
>
> > > Thoughts on why this is occurring?
>
> > It's probably your PCB design.
>
> > Leon
>
> Try to slow it down if you can.....

What do you mean by, "slow it down?"

On Mon, 17 Mar 2008 11:45:22 -0700 (PDT), williamcox@gmail.com wrote:

>I'm building a optical comm. circuit using an AD8015 transimpedance
>amplifier from Analog Devices. Datasheet: http://www.analog.com/UploadedFiles/Data_Sheets/AD8015.pdf
>
>Here is the circuit I'm using: http://gallamine.com/photodiode_tia_schematic.pdf
>
>I'm having trouble with the circuit producing a strong ~1.9MHz
>oscillation. This is independent of what the circuit is hooked to. I
>have tried it with several different photodiodes (and no photodiode)
>and it doesn't seem to have a big effect.
>
>Thoughts on why this is occurring?

Post a pic of the actual layout!

John

williamcox@gmail.com wrote:
> I'm building a optical comm. circuit using an AD8015 transimpedance
> amplifier from Analog Devices. Datasheet: http://www.analog.com/UploadedFiles/Data_Sheets/AD8015.pdf
> 
> Here is the circuit I'm using: http://gallamine.com/photodiode_tia_schematic.pdf
> 
> I'm having trouble with the circuit producing a strong ~1.9MHz
> oscillation. This is independent of what the circuit is hooked to. I
> have tried it with several different photodiodes (and no photodiode)
> and it doesn't seem to have a big effect.
> 
> Thoughts on why this is occurring?


I don't know this TIA chip because I usually roll my own but TIAs do not 
like much capacitance from their input pin to ground. If it's much above 
a pf for a fast TIA they can go berserk. What you could try is to either 
lower the stray capacitance at the input or provide a small capacitance 
(fraction of a pf) from inverting input to output. IIRC the AD8015 is 
diff-out so you'd have to figure out which output to use.

While I am an advocate for a full ground plane you might have to create 
a small void in it around the input pin in order to minimize 
capacitance. If the layout is already done you could try to lift the 
input pin, bend it up and solder the PD to it that way. And no long 
cables or traces there. If you can yield enough stability margin that 
way it would be time for a re-layout.

-- 
Regards, Joerg

http://www.analogconsultants.com/