Re: Photodiode Amplifier Noise Effects

On Feb 16, 6:42 pm, WhiteDog <stoopid.mun...@xxxxxxxxx> wrote:
I have been playing with my shiny new photodiode amplifier (BW=75MHz
A=35k) and my trusty spectrum analyzer set to 'noise' mode. I'm
dissecting the thing to try and find areas where some simple tweaks
might improve the SNR.

The photodiode is a 500um PIN device with a NEP of 0.008pW/RtHz and a
dark current under -5V bias (on the anode) of about 1.25nA.

The amplifier circuit under examination takes it's input from the
photodiode cathode through a common-base amplifier (BFG25A/X) and from
there into a low noise op-amp transimpedance amp. The common-base amp
is biased with 12k resistors to +/-5V to get the required bandwidth
from the circuit. Basically the circuit from Phil Hobbs' book in the
'Photodiode Amplifiers' section.

Could someone please explain the following effect?:

1. When the photodiode is removed (i.e. just a biased cascode into the
TIA) , the output noise is 80nV/RtHz at 50MHz. This matches back-of-
the envelope calculations of what this should be.
2. When the photodiode is connected, I get 160nV/RtHz!! Note that I
have covered the photodiode window with copper tape to remove the
possibility of DC bias, cut down the device leads, etc -- I think all
I should be getting at this point is dark current, correct?

This additional 80nV/RtHz seems excessive -- though the system still
meets spec, I would like some 'margin' in there and wouldn't mind
reducing the noise if possible.

The only source of 'noise' current that I could think of would be from
the bias supply, however, I am using a low-noise LT1964 LDO for this,
and am supplying the bias through two 10k resistors with a 1uF and a
1000pF ceramic cap arranged in a 'pi' configuration. This -ve supply
IS shared with the -ve rail of the TIA.

Before I tear into the bias of the PIN (building a second -ve bias
supply from an LDO), is there any other source I have missed or could
this really be all from the bias voltage?

Thanks for any help you can offer


I don't think a bit of noise on the photodiode bais voltage is much of
a problem. When you remove the photodiode try putting in a
capacitor whose value is equal to the capacitance of the biased
photodiode and meause the noise again.

George Herold
.

Relevant Pages

  • Re: Photodiode Amplifier Noise Effects
    ... A=35k) and my trusty spectrum analyzer set to 'noise' mode. ... dark current under -5V bias of about 1.25nA. ... photodiode cathode through a common-base amplifier and from ... and am supplying the bias through two 10k resistors with a 1uF and a ...
    (sci.electronics.design)
  • Photodiode Amplifier Noise Effects
    ... I have been playing with my shiny new photodiode amplifier (BW=75MHz ... A=35k) and my trusty spectrum analyzer set to 'noise' mode. ... dark current under -5V bias of about 1.25nA. ...
    (sci.electronics.design)
  • Re: Photodiodes
    ... Black silicon is well known from trench DRAM, circa 1992--we used to get spikes that looked a lot like that in the bottoms of trench capacitors, mainly because of contamination. ... The stuff in the puff piece is hilarious--impurity sites actually being so numerous as to form *bands!* Claims of low noise are totally unsubstantiated--all they show is a spectral responsivity curve, with comparisons to silicon and Ge/InGaAs PDs with no gain. ... For practical purposes, all primary photogeneration processes exhibit exactly full shot noise, and photoconductors have twice the noise of photodiodes, because there the recombination is stochastic as well as the generation. ... it really isn't difficult to get to the shot noise with a photodiode if you're willing to wait awhile--Gary Eppeldauer et al at NIST published a heroic study where they got *14 decades* of linear response out of a specially prepared Si photodiode. ...
    (sci.electronics.design)
  • Re: Photodiode Amplifier Noise Effects
    ... Grounding the bias end of the 'fake' photodiode yields ... a similar noise figure. ... isolating it from the TIA via the cascode amplifier. ...
    (sci.electronics.design)
  • Re: Low noise op amp
    ... I am measuring current fluctuations from a photodiode from 1Hz to 10kHz. ... I put a 50 Ohms in transimpedance mode, ... I'll make noise calculation with my two stages. ... and change your readings. ...
    (sci.electronics.design)
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