Re: Photodiode amplifier noise

bill.sloman@xxxxxxxx wrote:
On May 1, 4:25 pm, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Fri, 1 May 2009 00:23:49 -0700 (PDT), bill.slo...@xxxxxxxx wrote:
On May 1, 3:12 am, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Thu, 30 Apr 2009 15:15:46 -0700 (PDT), bill.slo...@xxxxxxxx wrote:
On Apr 30, 4:22 am, Paul Probert <paulprob...@xxxxxxxxxxxxx> wrote:
Nemo wrote:
I'm embarrassed to ask this but -
I have a high gain (transconductance) amp with a photodiode, biased to
be a few hundred pF, on its input. I was tweaking it to optimise the
noise levels and found the noise of the amp was way less than the noise
of (the amp with the diode on the front). So I replaced the diode with a
simple capacitor... and I still get extra noise when I fit it. It does
not appear to be noise from the bias supply; the capacitance can be to
ground and I still see the extra noise appearing.
The texts I've read say there's noise generated by Cin, and it certainly
seems like there is. I can understand it acts as a potential divider on
the amp's gain and attenuates high frequency signals. What I don't get
is why it generates noise without a signal present? What is the physical
mechanism which creates this noise?
The feedback resistor and input capacitance together, besides
attenuating the output signal also cause a 90 degree phase shift, which
isn't a problem except at the high frequency end of the opamp's useful
gain, where it also starts shifting the phase by close to 90 degrees.
The result is a loop phase shift of nearly 180 degrees, or positive
feedback. This results in a big peak in the gain seen by the input noise
voltage at this high frequency.
A good place to look is Phil Hobb's websitehttp://www.electrooptical.net/http://www.electrooptical.net/www/front...
You can also look at the data sheets on TI fast FET-input op amps -
they still seem to be being written by the people who were working for
Burr-Brown back when TI took them over.
http://focus.ti.com/lit/ds/symlink/opa657.pdf
who don't yet seem to have been properly indoctrinated about the TI
corporate policy of ripping off the customers.
--
Bill Sloman, Nijmegen
TI makes a lot of kick-ass parts. Their field guys call on us, and
support is excellent.
They have the best fast 15-volt analog process on the planet; look up
THS3201.
I started using the their analog parts with the TLC2001 for which the
data *** didn't mention the anomalously high inut capacitiance (at
around 15pF). When this reared up and bit me I rang TI support, who
didn't have a clue what the input capacitance might be and showed no
willingness to find out.
If Cin wasn't specified, why did you assume it was suitable for your
application? That's reckless. I always measure stuff like that when it
matters.

It was certainly reckless, but lots of cheap op amps don't specify Cin
and it's usually close enough to a couple of pF. I'd actually checked
to see if I'd need a feedback capacitor for an input capacitance in
that ball-park. Since the TLC2001 was in a non-critical bit of the
circuit, I didn't have any trouble solving the problem with a 4.7pF
capacitor from output to inverting input.

It pissed me off because it was - in fact - quite a hairy circuit
which I'd designed and laid out without any protoptyping, and all the
tricky bits of the circuit worked exactly as I'd intended, except for
this boring little integrator wrapped around the TLC2001. Most of the
components were SMD so the 4.7pF stuck out like a sore thumb.

<snip>

--
Bill Sloman, Nijmegen


The OPA657 has much higher input capacitance than the 656, interestingly, which means that it isn't always the better choice for a TIA.

Cheers

Phil Hobbs
.

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