Linear seem to have some new "interesting" high impedance, high-speed, low capacitance opamps LTC6268 and LTC6268-10. "500MHz" http://www.linear.com/product/LTC6268 "4GHz" http://www.linear.com/product/LTC6268-10 (but what's with the current noise plots?) -- John Devereux
wierd opamps
Started by ●May 2, 2015
Reply by ●May 2, 20152015-05-02
John Devereux wrote...> > Linear seem to have some new "interesting" high impedance, > high-speed, low capacitance opamps LTC6268 and LTC6268-10. > "500MHz" http://www.linear.com/product/LTC6268 > "4GHz" http://www.linear.com/product/LTC6268-10 > > (but what's with the current noise plots?)LTC's LTC6268 competes with Burr-Brown / TI OPA656, and the decompensated LTC6268-10 with the OPA657. Those are especially useful to make trans-resistance amplifiers, TIA, used with high-speed photo-current signals, etc. See AoE-III Chapter 8, pages 537-552. Or see Phil Hobbs' book, pages 693-714. The datasheet's current noise plot, which rises with frequency, shows e_n Cin noise: i_n = 2pi f e_n Cin, see AoE-III sect 8.11.3 and E'qn 8.44. The business of voltage-noise turning into a rather nasty current noise surprises people not familiar with the concept. For example, the amplifier's intrinsic low current- noise density of 7 fA/rt-Hz turns into a high noise of 4000 fA at 50MHz. Their graph should have shown the node capacitance value. We can back-calculate Cin = i_n / 2pi f e_n = 4pA / 314M 4.3nV = 3 pF. This is very low, probably without a PD connected. One thing that makes LTC's new amplifier ICs special are very low input capacitance, which is achieved by bootstrapping the protection diodes and other parts of the circuit to follow the input pins. This bootstrap scheme also reduces the input leakage current at high temperatures. There are a few other amplifiers that can also do this, see AoE Figure 5.6. -- Thanks, - Win
Reply by ●May 2, 20152015-05-02
Yup, they've obviously bootstrapped everything in sight. It's a bit of a cheat, really, from a SNR point of view, because the bootstrapped capacitance doesn't load the input but still contributes current noise as if it did. It helps the loop bandwidth and frequency compensation, for sure. TI keeps pitching its OPA657 as a TIA, but its huge input capacitance makes it a fairly poor choice in reality. The LTC part is interesting--I'll have to run some numbers on it. The 1 MHz 1/f corner is pretty horrendous--even a pHEMT's isn't that much higher, and they start from a flatband 20 dB lower, which puts their LF noise in the same ballpark. Thanks for pointing it out. Cheers Phil Hobbs
Reply by ●May 4, 20152015-05-04
On Saturday, May 2, 2015 at 8:42:09 AM UTC-4, Winfield Hill wrote:> John Devereux wrote... > > > > Linear seem to have some new "interesting" high impedance, > > high-speed, low capacitance opamps LTC6268 and LTC6268-10. > > "500MHz" http://www.linear.com/product/LTC6268 > > "4GHz" http://www.linear.com/product/LTC6268-10 > > > > (but what's with the current noise plots?) > > LTC's LTC6268 competes with Burr-Brown / TI OPA656, > and the decompensated LTC6268-10 with the OPA657. > > Those are especially useful to make trans-resistance > amplifiers, TIA, used with high-speed photo-current > signals, etc. See AoE-III Chapter 8, pages 537-552. > Or see Phil Hobbs' book, pages 693-714. > > The datasheet's current noise plot, which rises with > frequency, shows e_n Cin noise: i_n = 2pi f e_n Cin, > see AoE-III sect 8.11.3 and E'qn 8.44. The business > of voltage-noise turning into a rather nasty current > noise surprises people not familiar with the concept. > > For example, the amplifier's intrinsic low current- > noise density of 7 fA/rt-Hz turns into a high noise > of 4000 fA at 50MHz. Their graph should have shown > the node capacitance value. We can back-calculate > Cin = i_n / 2pi f e_n = 4pA / 314M 4.3nV = 3 pF. > This is very low, probably without a PD connected. > > One thing that makes LTC's new amplifier ICs special > are very low input capacitance, which is achieved by > bootstrapping the protection diodes and other parts > of the circuit to follow the input pins. > > This bootstrap scheme also reduces the input leakage > current at high temperatures. There are a few other > amplifiers that can also do this, see AoE Figure 5.6. > > > -- > Thanks, > - WinOK, If the current noise is due to e-sub-n-C, in the LT chip. Then why is there nothing similar in the OPA656 data sheet? http://www.ti.com/lit/ds/symlink/opa656.pdf It shows current noise as being absolutely flat. I must admit the idea that the bias current is a few fA and the current noise at ~1MHz is several pA strikes me as strange. George H.
Reply by ●May 4, 20152015-05-04
On 05/04/2015 11:41 AM, George Herold wrote:> On Saturday, May 2, 2015 at 8:42:09 AM UTC-4, Winfield Hill wrote: >> John Devereux wrote... >>> >>> Linear seem to have some new "interesting" high impedance, >>> high-speed, low capacitance opamps LTC6268 and LTC6268-10. >>> "500MHz" http://www.linear.com/product/LTC6268 >>> "4GHz" http://www.linear.com/product/LTC6268-10 >>> >>> (but what's with the current noise plots?) >> >> LTC's LTC6268 competes with Burr-Brown / TI OPA656, >> and the decompensated LTC6268-10 with the OPA657. >> >> Those are especially useful to make trans-resistance >> amplifiers, TIA, used with high-speed photo-current >> signals, etc. See AoE-III Chapter 8, pages 537-552. >> Or see Phil Hobbs' book, pages 693-714. >> >> The datasheet's current noise plot, which rises with >> frequency, shows e_n Cin noise: i_n = 2pi f e_n Cin, >> see AoE-III sect 8.11.3 and E'qn 8.44. The business >> of voltage-noise turning into a rather nasty current >> noise surprises people not familiar with the concept. >> >> For example, the amplifier's intrinsic low current- >> noise density of 7 fA/rt-Hz turns into a high noise >> of 4000 fA at 50MHz. Their graph should have shown >> the node capacitance value. We can back-calculate >> Cin = i_n / 2pi f e_n = 4pA / 314M 4.3nV = 3 pF. >> This is very low, probably without a PD connected. >> >> One thing that makes LTC's new amplifier ICs special >> are very low input capacitance, which is achieved by >> bootstrapping the protection diodes and other parts >> of the circuit to follow the input pins. >> >> This bootstrap scheme also reduces the input leakage >> current at high temperatures. There are a few other >> amplifiers that can also do this, see AoE Figure 5.6. >> >> >> -- >> Thanks, >> - Win > > OK, If the current noise is due to e-sub-n-C, in the LT chip. > Then why is there nothing similar in the OPA656 data sheet? > http://www.ti.com/lit/ds/symlink/opa656.pdf > It shows current noise as being absolutely flat. > > I must admit the idea that the bias current is a few fA and the current noise at > ~1MHz is several pA strikes me as strange. > > George H. > >The OPA656 just lets its input capacitance rage, instead of bottling it up. ;) It's still lower than the OPA657's, though, which is weird. 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
Reply by ●May 4, 20152015-05-04
On Mon, 4 May 2015 08:41:25 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Saturday, May 2, 2015 at 8:42:09 AM UTC-4, Winfield Hill wrote: >> John Devereux wrote... >> > >> > Linear seem to have some new "interesting" high impedance, >> > high-speed, low capacitance opamps LTC6268 and LTC6268-10. >> > "500MHz" http://www.linear.com/product/LTC6268 >> > "4GHz" http://www.linear.com/product/LTC6268-10 >> > >> > (but what's with the current noise plots?) >> >> LTC's LTC6268 competes with Burr-Brown / TI OPA656, >> and the decompensated LTC6268-10 with the OPA657. >> >> Those are especially useful to make trans-resistance >> amplifiers, TIA, used with high-speed photo-current >> signals, etc. See AoE-III Chapter 8, pages 537-552. >> Or see Phil Hobbs' book, pages 693-714. >> >> The datasheet's current noise plot, which rises with >> frequency, shows e_n Cin noise: i_n = 2pi f e_n Cin, >> see AoE-III sect 8.11.3 and E'qn 8.44. The business >> of voltage-noise turning into a rather nasty current >> noise surprises people not familiar with the concept. >> >> For example, the amplifier's intrinsic low current- >> noise density of 7 fA/rt-Hz turns into a high noise >> of 4000 fA at 50MHz. Their graph should have shown >> the node capacitance value. We can back-calculate >> Cin = i_n / 2pi f e_n = 4pA / 314M 4.3nV = 3 pF. >> This is very low, probably without a PD connected. >> >> One thing that makes LTC's new amplifier ICs special >> are very low input capacitance, which is achieved by >> bootstrapping the protection diodes and other parts >> of the circuit to follow the input pins. >> >> This bootstrap scheme also reduces the input leakage >> current at high temperatures. There are a few other >> amplifiers that can also do this, see AoE Figure 5.6. >> >> >> -- >> Thanks, >> - Win > >OK, If the current noise is due to e-sub-n-C, in the LT chip. >Then why is there nothing similar in the OPA656 data sheet? >http://www.ti.com/lit/ds/symlink/opa656.pdf >It shows current noise as being absolutely flat.No capacitance bootstrapping!> >I must admit the idea that the bias current is a few fA and the current noise at >~1MHz is several pA strikes me as strange. > >George H. >It's a no-free-lunch-ism. An opamp without the bootstrapping can have much lower input current noise, but the higher input capacitance will plop a pole in the TIA feedback loop and make a heap of voltage noise at high frequencies. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●May 4, 20152015-05-04
On Monday, May 4, 2015 at 11:53:36 AM UTC-4, Phil Hobbs wrote:> On 05/04/2015 11:41 AM, George Herold wrote: > > On Saturday, May 2, 2015 at 8:42:09 AM UTC-4, Winfield Hill wrote: > >> John Devereux wrote... > >>> > >>> Linear seem to have some new "interesting" high impedance, > >>> high-speed, low capacitance opamps LTC6268 and LTC6268-10. > >>> "500MHz" http://www.linear.com/product/LTC6268 > >>> "4GHz" http://www.linear.com/product/LTC6268-10 > >>> > >>> (but what's with the current noise plots?) > >> > >> LTC's LTC6268 competes with Burr-Brown / TI OPA656, > >> and the decompensated LTC6268-10 with the OPA657. > >> > >> Those are especially useful to make trans-resistance > >> amplifiers, TIA, used with high-speed photo-current > >> signals, etc. See AoE-III Chapter 8, pages 537-552. > >> Or see Phil Hobbs' book, pages 693-714. > >> > >> The datasheet's current noise plot, which rises with > >> frequency, shows e_n Cin noise: i_n = 2pi f e_n Cin, > >> see AoE-III sect 8.11.3 and E'qn 8.44. The business > >> of voltage-noise turning into a rather nasty current > >> noise surprises people not familiar with the concept. > >> > >> For example, the amplifier's intrinsic low current- > >> noise density of 7 fA/rt-Hz turns into a high noise > >> of 4000 fA at 50MHz. Their graph should have shown > >> the node capacitance value. We can back-calculate > >> Cin = i_n / 2pi f e_n = 4pA / 314M 4.3nV = 3 pF. > >> This is very low, probably without a PD connected. > >> > >> One thing that makes LTC's new amplifier ICs special > >> are very low input capacitance, which is achieved by > >> bootstrapping the protection diodes and other parts > >> of the circuit to follow the input pins. > >> > >> This bootstrap scheme also reduces the input leakage > >> current at high temperatures. There are a few other > >> amplifiers that can also do this, see AoE Figure 5.6. > >> > >> > >> -- > >> Thanks, > >> - Win > > > > OK, If the current noise is due to e-sub-n-C, in the LT chip. > > Then why is there nothing similar in the OPA656 data sheet? > > http://www.ti.com/lit/ds/symlink/opa656.pdf > > It shows current noise as being absolutely flat. > > > > I must admit the idea that the bias current is a few fA and the current noise at > > ~1MHz is several pA strikes me as strange. > > > > George H. > > > > > > > The OPA656 just lets its input capacitance rage, instead of bottling it > up. ;) It's still lower than the OPA657's, though, which is weird. > > > 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.netOK so It's the bootstrap that folds the voltage noise back in? Hey what about the opa655... It's only got 1 pF Cin! http://www.ti.com/lit/ds/sbos048/sbos048.pdf George H.
Reply by ●May 4, 20152015-05-04
On 05/04/2015 12:00 PM, John Larkin wrote:> On Mon, 4 May 2015 08:41:25 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > >> On Saturday, May 2, 2015 at 8:42:09 AM UTC-4, Winfield Hill wrote: >>> John Devereux wrote... >>>> >>>> Linear seem to have some new "interesting" high impedance, >>>> high-speed, low capacitance opamps LTC6268 and LTC6268-10. >>>> "500MHz" http://www.linear.com/product/LTC6268 >>>> "4GHz" http://www.linear.com/product/LTC6268-10 >>>> >>>> (but what's with the current noise plots?) >>> >>> LTC's LTC6268 competes with Burr-Brown / TI OPA656, >>> and the decompensated LTC6268-10 with the OPA657. >>> >>> Those are especially useful to make trans-resistance >>> amplifiers, TIA, used with high-speed photo-current >>> signals, etc. See AoE-III Chapter 8, pages 537-552. >>> Or see Phil Hobbs' book, pages 693-714. >>> >>> The datasheet's current noise plot, which rises with >>> frequency, shows e_n Cin noise: i_n = 2pi f e_n Cin, >>> see AoE-III sect 8.11.3 and E'qn 8.44. The business >>> of voltage-noise turning into a rather nasty current >>> noise surprises people not familiar with the concept. >>> >>> For example, the amplifier's intrinsic low current- >>> noise density of 7 fA/rt-Hz turns into a high noise >>> of 4000 fA at 50MHz. Their graph should have shown >>> the node capacitance value. We can back-calculate >>> Cin = i_n / 2pi f e_n = 4pA / 314M 4.3nV = 3 pF. >>> This is very low, probably without a PD connected. >>> >>> One thing that makes LTC's new amplifier ICs special >>> are very low input capacitance, which is achieved by >>> bootstrapping the protection diodes and other parts >>> of the circuit to follow the input pins. >>> >>> This bootstrap scheme also reduces the input leakage >>> current at high temperatures. There are a few other >>> amplifiers that can also do this, see AoE Figure 5.6. >>> >>> >>> -- >>> Thanks, >>> - Win >> >> OK, If the current noise is due to e-sub-n-C, in the LT chip. >> Then why is there nothing similar in the OPA656 data sheet? >> http://www.ti.com/lit/ds/symlink/opa656.pdf >> It shows current noise as being absolutely flat. > > No capacitance bootstrapping! > > >> >> I must admit the idea that the bias current is a few fA and the current noise at >> ~1MHz is several pA strikes me as strange. >> >> George H. >> > > It's a no-free-lunch-ism. An opamp without the bootstrapping can have > much lower input current noise, but the higher input capacitance will > plop a pole in the TIA feedback loop and make a heap of voltage noise > at high frequencies. > >I think the ease-of-use issue is probably the main motivating factor, and getting rid of most of the input capacitance helps. There's still the photodiode capacitance, of course. I remember, back in the late '80s, being really excited about some new Motorola op amp with high speed and low noise (for the time--probably 30 MHz and 5 nV). Turned out the input capacitance was so horrible that you couldn't use feedback resistors larger than 10k except at really high gain. (I just looked in my 1999 Moto databook, and it wasn't there, so it obviously vanished pretty fast.) It might be possible to externally bootstrap the part of the input capacitance that goes to the rails (i.e. the CM capacitance) but unfortunately you can't bootstrap the differential input capacitance (between the inputs) without also eliminating the signal! The effects of that on op amp stability might be amusing as well. Using a single-ended BF862, with an op amp off to the side forcing it to run at V_GS = 0 (i.e. I_DSS) is a reasonable approach. (We've done that together a few times of course--the JL trademarked name is "snooping the input".) You can use the bootstrap as the input to the next stage, which could be an ADA4899 or something like that. Putting the op amp off to the side as a snooper, isolated by a big resistor, eliminates most of the e_N*C noise, but of course adds the full Johnson noise of the resistor at frequencies within the snooper's bandwidth. To allow the slowest possible snooper, it's good to run the JFET at constant power dissipation, e.g. by DC-coupling the drain bootstrap. I still miss the LF357 for front ends--10 nV and 2 pF was pretty good for 1973. 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
Reply by ●May 4, 20152015-05-04
On 05/04/2015 12:34 PM, George Herold wrote:> On Monday, May 4, 2015 at 11:53:36 AM UTC-4, Phil Hobbs wrote: >> On 05/04/2015 11:41 AM, George Herold wrote: >>> On Saturday, May 2, 2015 at 8:42:09 AM UTC-4, Winfield Hill wrote: >>>> John Devereux wrote... >>>>> >>>>> Linear seem to have some new "interesting" high impedance, >>>>> high-speed, low capacitance opamps LTC6268 and LTC6268-10. >>>>> "500MHz" http://www.linear.com/product/LTC6268 >>>>> "4GHz" http://www.linear.com/product/LTC6268-10 >>>>> >>>>> (but what's with the current noise plots?) >>>> >>>> LTC's LTC6268 competes with Burr-Brown / TI OPA656, >>>> and the decompensated LTC6268-10 with the OPA657. >>>> >>>> Those are especially useful to make trans-resistance >>>> amplifiers, TIA, used with high-speed photo-current >>>> signals, etc. See AoE-III Chapter 8, pages 537-552. >>>> Or see Phil Hobbs' book, pages 693-714. >>>> >>>> The datasheet's current noise plot, which rises with >>>> frequency, shows e_n Cin noise: i_n = 2pi f e_n Cin, >>>> see AoE-III sect 8.11.3 and E'qn 8.44. The business >>>> of voltage-noise turning into a rather nasty current >>>> noise surprises people not familiar with the concept. >>>> >>>> For example, the amplifier's intrinsic low current- >>>> noise density of 7 fA/rt-Hz turns into a high noise >>>> of 4000 fA at 50MHz. Their graph should have shown >>>> the node capacitance value. We can back-calculate >>>> Cin = i_n / 2pi f e_n = 4pA / 314M 4.3nV = 3 pF. >>>> This is very low, probably without a PD connected. >>>> >>>> One thing that makes LTC's new amplifier ICs special >>>> are very low input capacitance, which is achieved by >>>> bootstrapping the protection diodes and other parts >>>> of the circuit to follow the input pins. >>>> >>>> This bootstrap scheme also reduces the input leakage >>>> current at high temperatures. There are a few other >>>> amplifiers that can also do this, see AoE Figure 5.6. >>>> >>>> >>>> -- >>>> Thanks, >>>> - Win >>> >>> OK, If the current noise is due to e-sub-n-C, in the LT chip. >>> Then why is there nothing similar in the OPA656 data sheet? >>> http://www.ti.com/lit/ds/symlink/opa656.pdf >>> It shows current noise as being absolutely flat. >>> >>> I must admit the idea that the bias current is a few fA and the current noise at >>> ~1MHz is several pA strikes me as strange. >>> >>> George H. >>> >>> >> >> >> The OPA656 just lets its input capacitance rage, instead of bottling it >> up. ;) It's still lower than the OPA657's, though, which is weird. >>> > OK so It's the bootstrap that folds the voltage noise back in?Yup.> > Hey what about the opa655... It's only got 1 pF Cin! > http://www.ti.com/lit/ds/sbos048/sbos048.pdfGone, unfortunately. Nice part. 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
Reply by ●May 4, 20152015-05-04
On Mon, 04 May 2015 12:52:59 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 05/04/2015 12:00 PM, John Larkin wrote: >> On Mon, 4 May 2015 08:41:25 -0700 (PDT), George Herold >> <gherold@teachspin.com> wrote: >> >>> On Saturday, May 2, 2015 at 8:42:09 AM UTC-4, Winfield Hill wrote: >>>> John Devereux wrote... >>>>> >>>>> Linear seem to have some new "interesting" high impedance, >>>>> high-speed, low capacitance opamps LTC6268 and LTC6268-10. >>>>> "500MHz" http://www.linear.com/product/LTC6268 >>>>> "4GHz" http://www.linear.com/product/LTC6268-10 >>>>> >>>>> (but what's with the current noise plots?) >>>> >>>> LTC's LTC6268 competes with Burr-Brown / TI OPA656, >>>> and the decompensated LTC6268-10 with the OPA657. >>>> >>>> Those are especially useful to make trans-resistance >>>> amplifiers, TIA, used with high-speed photo-current >>>> signals, etc. See AoE-III Chapter 8, pages 537-552. >>>> Or see Phil Hobbs' book, pages 693-714. >>>> >>>> The datasheet's current noise plot, which rises with >>>> frequency, shows e_n Cin noise: i_n = 2pi f e_n Cin, >>>> see AoE-III sect 8.11.3 and E'qn 8.44. The business >>>> of voltage-noise turning into a rather nasty current >>>> noise surprises people not familiar with the concept. >>>> >>>> For example, the amplifier's intrinsic low current- >>>> noise density of 7 fA/rt-Hz turns into a high noise >>>> of 4000 fA at 50MHz. Their graph should have shown >>>> the node capacitance value. We can back-calculate >>>> Cin = i_n / 2pi f e_n = 4pA / 314M 4.3nV = 3 pF. >>>> This is very low, probably without a PD connected. >>>> >>>> One thing that makes LTC's new amplifier ICs special >>>> are very low input capacitance, which is achieved by >>>> bootstrapping the protection diodes and other parts >>>> of the circuit to follow the input pins. >>>> >>>> This bootstrap scheme also reduces the input leakage >>>> current at high temperatures. There are a few other >>>> amplifiers that can also do this, see AoE Figure 5.6. >>>> >>>> >>>> -- >>>> Thanks, >>>> - Win >>> >>> OK, If the current noise is due to e-sub-n-C, in the LT chip. >>> Then why is there nothing similar in the OPA656 data sheet? >>> http://www.ti.com/lit/ds/symlink/opa656.pdf >>> It shows current noise as being absolutely flat. >> >> No capacitance bootstrapping! >> >> >>> >>> I must admit the idea that the bias current is a few fA and the current noise at >>> ~1MHz is several pA strikes me as strange. >>> >>> George H. >>> >> >> It's a no-free-lunch-ism. An opamp without the bootstrapping can have >> much lower input current noise, but the higher input capacitance will >> plop a pole in the TIA feedback loop and make a heap of voltage noise >> at high frequencies. >> >> > >I think the ease-of-use issue is probably the main motivating factor, >and getting rid of most of the input capacitance helps. There's still >the photodiode capacitance, of course. > >I remember, back in the late '80s, being really excited about some new >Motorola op amp with high speed and low noise (for the time--probably 30 >MHz and 5 nV). Turned out the input capacitance was so horrible that >you couldn't use feedback resistors larger than 10k except at really >high gain. (I just looked in my 1999 Moto databook, and it wasn't >there, so it obviously vanished pretty fast.) > >It might be possible to externally bootstrap the part of the input >capacitance that goes to the rails (i.e. the CM capacitance) but >unfortunately you can't bootstrap the differential input capacitance >(between the inputs) without also eliminating the signal! The effects >of that on op amp stability might be amusing as well. > >Using a single-ended BF862, with an op amp off to the side forcing it to >run at V_GS = 0 (i.e. I_DSS) is a reasonable approach. >(We've done that together a few times of course--the JL trademarked name >is "snooping the input".) You can use the bootstrap as the input to the >next stage, which could be an ADA4899 or something like that.Physicists (excepting you) (see RSI) seem fascinated by differential-pair jfets. Multiply the noise by 40% for no reason!> >Putting the op amp off to the side as a snooper, isolated by a big >resistor, eliminates most of the e_N*C noise, but of course adds the >full Johnson noise of the resistor at frequencies within the snooper's >bandwidth. To allow the slowest possible snooper, it's good to run the >JFET at constant power dissipation, e.g. by DC-coupling the drain bootstrap. > >I still miss the LF357 for front ends--10 nV and 2 pF was pretty good >for 1973.I think opamps inherently lose out to discretes, when making low noise TIAs. The ESD diodes, the substrate connected to V-, the differential input, stuff like that. Johnson noise dominates any reasonable design at low frequencies. It's the high frequency noise that's agonizing. It's amazing how many truly terrible photodiode TIAs are in the literature, and for sale. Bad by factors like 100:1. I've done a few myself, in the ignorance of youth. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
