From: "Bill Sloman"
Subject: Re: photodetector circuit, high speed with good ambient light rejection
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Date: Thu, 28 Nov 2002 23:29:26 GMT
NNTP-Posting-Date: Fri, 29 Nov 2002 10:29:26 EST
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"Winfield Hill" wrote in message
> > Dr C.I. Swift wrote:
> >> Need to make such a circuit good for up to 90MHz. will have a
> >> considerable DC light component so want too reject this.
> The answer to whether to use a transimpedence amplifier or a resistive
> load to ground, etc., depends on several things, the total capacitance
> (photodiode + cable + amplifier), your photodiode signal current, and
> your signal-to-noise ratio requirements.
> The Johnson current-noise density of a resistor (connected to ground or
> used as a feedback element) is sqrt(4kT/R), which means that low resistor
> values create high current noise. If you have high light currents, you
> may escape this issue, otherwise this formula will determine the minimum
> resistor value necessary. Once your minimum R value is known, evaluate
> your total capacitance Cin and see if your needed bandwidth is possible
> using just a resistor to ground, i.e. BW = 1 / 2pi R Cin.
> If your capacitance Cin is too high for enough bandwidth using resistor
> R to ground, you'll need a transimpedence amplifier. It can use the same
> value R, or an even higher Rf value for more signal, if you have enough
> amplifier bandwidth. The required opamp GBW is f_T > fc^2 / (2pi Rf
> where fc is your required system bandwidth. You'll also need a parallel
> feedback capacitor limiting the bandwidth to fc, so Cf = 1 / 2pi Rf fc.
> For example, a 4k resistor has a current-noise density of 2pA/Hz^1/2,
> yielding a Dc-to-90MHz-bandwidth noise of 20nA rms. (We'll ignore
> e_n-Cin noise.) If Cin was 5pF, you'd only get an 8MHz bandwidth with
> 4k connected to ground, so a transimpedence amplifier is required, with
> an opamp f_T >= 1000Mz. Depending on the opamp's f_T and any desired
> response peaking, about 0.44pF of feedback capacitance is also required,
> including the resistor's self capacitance.
> As for using an inductor to eliminate DC light currents, this may be
> reasonable for lower reistance values, and can be used in parallel with
> either R to ground or with the Rf transimpedence feedback resistor.
> However, either way be careful of the inductor's self capacitance. In
> the 4k example above the inductor would have to be say 22uH for a 30MHz
> low-frequency cutoff, and have a self-capacitance below 0.4pF, which is
> a self-resonate frequency of over 53MHz. I don't think such a beast is
> available. A typical 22uH has SRF = 15MHz, so an inductor with Rf = 4k
> would force you into a narrowband resonate system. On the other hand,
> if Rf = 1k or less you might have success.
> Tell us more about what you're working on.
Noise calculations for a virtual ground op-amp circuit for a photodiode
looking at a 90MHz signal have to allow for the increase in noise gain above
the frequency where the diode's capacitive reactance falls below the the
feedback impedance - this make the op amp input noise dominant in very short
The op amp for the job is the Burr-Brown (now Texas Instruments) OPA655, and
the data sheet includes quite a good discussion of this application.
Bill Sloman, Nijmegen