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Re: Why Adding a Voltage Amplifier after a Transimpedance Amplifier reduce BW? Book Claim by 0.64

Started by Felipe Jimenez May 14, 2015
On Thursday, May 14, 2015 at 9:15:16 AM UTC-7, Phil Hobbs wrote:
> On 05/14/2015 11:52 AM, Felipe Jimenez wrote: > > On Thursday, May 14, 2015 at 4:08:04 AM UTC-7, Phil Hobbs wrote: > >>> Oh and I'm fairly certain the non-inverting amplifier is wired up > >>> correctly. Between switching back and forth from the > >>> non-inverting to the inverting configuration several times, I am > >>> fairly certain that both configurations were wired up properly. > >> > >> Hmm... "several times" does make it sound like a solderless > >> breadboard, as John suggested. Those things are slabs of misery, > >> for sure. > >> > >> The AD8066 doesn't come in a DIP package, so I assumed it wasn't > >> one of those things, but having bad layout strays is one of the > >> ways to not build what you think you're building. > >> > >> Oh, one other thing--did you get your parts from an authorized > >> distributor? Otherwise it's possible the op amp is counterfeit. > >> > >> (And do knock off the 'Dr.' bit--that's in my sig for SEO reasons. > >> It's all free-men-and-equals round here.) > >> > >> Cheers > >> > >> Phil Hobbs > > > > Sorry Phil...Still in grad school, so I'm just used to saying "Dr." > > if I don't know the person and I see those letters near their name. > > > > As for all the other replies including your own: > > > > The pnp C-multiplier guy was drawn wrong. Drew it last night. I know > > it was as we measured both +/-10V with a DMM and with a scope to > > check for oscillations. > > > > The whole circuit was built on piece of copper clad board. Made the > > job tight. Lifting input pins of the op amps off of the copper clad > > board. Used 0603 caps and resistors and 30AWG for connections. I will > > tell my co-worker to add another C-multiplier for the photodiode and > > to add some beefy caps on the supply pins. And to consider using > > single package opamps instead of dual package. > > <snip> > > > > As for his specs: 50nW of light, PD: PC5-6-TO5, BW = 50kHz, light > > wavelength = 852nm > > Okay, good. > > As a SWAG, I'd say it's still getting in via the negative supply. As > John says, the fact that inverting the output polarity (and hence the > current required to drive the cable) changes the bandwidth is pretty > diagnostic. > > At 50 kHz, a 6-foot cable (200 pF) will be pulling > > i_out = 2 pi * 50kHz * 200pF = 63 uA per volt of output amplitude, with > a 90-degree phase lead. > > That volt of output is 1V/45M = 22 nA. To get 22 nA through 10 pF at 50 > kHz requires a supply jiggle of > > v_ripple = 22nA/(2 pi * 50kHz * 10e-12) = 7 mV. > > That's a very believable number for a lightly loaded cap multiplier. > Using a separate one for the PD, and loading it down so that it 's > drawing a few milliamps ought to fix it. A 2k resistor from each supply > pin to ground will stiffen the supplies a lot. > > Meanwhile, try measuring the output with a x10 probe! > > Cheers > > Phil Hobbs > > > > > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC > Optics, Electro-optics, Photonics, Analog Electronics > > 160 North State Road #203 > Briarcliff Manor NY 10510 > > hobbs at electrooptical dot net > http://electrooptical.net
Hey Phil...Just to clarify, you're suggesting a 2kohm resistor to gnd on all c-multiplier outputs (3 of them: +/- 10V for the opamps and -10V for the PD), right? we'll try using a x10 scope probe for the non-inverting configuration.