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discrete JFETs vs CMOS analog switch
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Adam Seychell
Jul 2, 2020, 2:35:33 AM7/2/20
to
I need a switchable 0 /40dB attenuator for a 10Hz-20MHz 1Vp-p signal into a buffer amplifier (10k|3pF input loading). First I thought of using a SPDT analog switch and resistor divider, but even 'low capacitance' devices come with 15~20pF capacitance on the common pin and have poor off isolation for my purpose.
in --.--SW1--.--> to buffer
| |
1kR |
|--SW2--'
10R
|
gnd
When I simulated a 2N5484 J-FET, which have been around for at lease 30 years, they beat the pants of any modern CMOS analog switch for off isolation and shunt capacitance. They only downside is more complex driving circuit because of negative gate drive and should have good AC grounding for best off isolation.
Is the small signal JFET best suited for this application ? There seems very little information on comparing the JEFTs analog switch to the CMOS switch, especially AC performance.
Adam Seychell
in --.--SW1--.--> to buffer
| |
1kR |
|--SW2--'
10R
|
gnd
When I simulated a 2N5484 J-FET, which have been around for at lease 30 years, they beat the pants of any modern CMOS analog switch for off isolation and shunt capacitance. They only downside is more complex driving circuit because of negative gate drive and should have good AC grounding for best off isolation.
Is the small signal JFET best suited for this application ? There seems very little information on comparing the JEFTs analog switch to the CMOS switch, especially AC performance.
Adam Seychell
jurb...@gmail.com
Jul 2, 2020, 3:02:44 AM7/2/20
to
Thanks for the part umber, I might get some. I got JFETs but not for that frequency. (I got complimentaries !)
Anyway, the only reason to use real JFETs is to use the linear range and their unique capability to deal with AC. The other thing about them is they can do it in their linear range. In other words they can be easily used for gain control, there is no need for DC bias on he drain. No other device can do that except for those old light bulb things, opto something but they were a Cds and a light bulb inside, my HP 339 has a couple. But of course the thing dos about dead turtle speed.
If all you need to do is switch, not modulate, mix or any of that other shit you are probably not best off using the JFET. There are so many other options. And if the signal is HF you can bias anything.
The 40dB drop you want should be accomplished with resistors, and the turning on or off of devices. And if you have to match impedance, damn. But assuming you don't, figure just WHY you want those JFETs. There are other devices that can easily do what you want, if I read this right. You said nothing about gain control or any of that. Just attenuation and, well whatever.
If all you want is to switch in attenuation, maybe a 2N7000 ? But I am not sure of the frequency on those.
Still, don't burn up my JFETs, I got a real use for them. }:-)>
Anyway, the only reason to use real JFETs is to use the linear range and their unique capability to deal with AC. The other thing about them is they can do it in their linear range. In other words they can be easily used for gain control, there is no need for DC bias on he drain. No other device can do that except for those old light bulb things, opto something but they were a Cds and a light bulb inside, my HP 339 has a couple. But of course the thing dos about dead turtle speed.
If all you need to do is switch, not modulate, mix or any of that other shit you are probably not best off using the JFET. There are so many other options. And if the signal is HF you can bias anything.
The 40dB drop you want should be accomplished with resistors, and the turning on or off of devices. And if you have to match impedance, damn. But assuming you don't, figure just WHY you want those JFETs. There are other devices that can easily do what you want, if I read this right. You said nothing about gain control or any of that. Just attenuation and, well whatever.
If all you want is to switch in attenuation, maybe a 2N7000 ? But I am not sure of the frequency on those.
Still, don't burn up my JFETs, I got a real use for them. }:-)>
Pimpom
Jul 2, 2020, 3:08:38 AM7/2/20
to
the integral diode prevents blocking of signals more than a few
hundred millivolts in the off state.
Steve Goldstein
Jul 2, 2020, 6:51:57 AM7/2/20
to
to source and using the drains as the input and output. This prevents
body diode conduction.
Tony Stewart
Jul 2, 2020, 8:02:03 AM7/2/20
to
> Is the small signal JFET best suited for this application ? There seems very little information on comparing the JEFTs analog switch to the CMOS switch, especially AC performance.
>
> Adam Seychell
>
Achieving a flat spectral response in an attenuator or amplifier simply
>
> Adam Seychell
>
needs to understand this is done by impedance ratios. So when you have a
load impedance of 10k 2pF ,@ 10 MHz realize that any ratio must use a
ratio that makes the component capacitance negligible. This can be done
by lowering the R impedance load so that 2 pF = 8kOhm at 10Mhz is
negligible.
Chris Jones
Jul 2, 2020, 8:24:54 AM7/2/20
to
discrete MOSFETs the backgate is tied to the source so the diode is from
drain to source, but on a chip (e.g. analog switch chip) the designers
could connect the backgate to something else, (e.g. to the supply rails
in the old 4016), so there is no diode from drain to source. Provided
the signals are with the supply rails, the switch can block conduction
between its terminals in either direction. More recent analog switch
chips try to do things like reduce the on-resistance, in the 4066 this
is done by switching the backgate of one FET to sometimes tie it to the
signal pins, but this has disadvantages for charge injection, if that
matters. Yet more recent ones probably do much more fancy stuff.
Chris Jones
Jul 2, 2020, 8:37:06 AM7/2/20
to
like the FST3125, or one of the similar ones that are specified for
analog operation. NMOS has a better on-resistance vs. capacitance
trade-off than PMOS.
Also, I would suggest considering not trying to achieve >>40dB isolation
in one stage of switching, and for higher frequencies, perhaps not even
in one chip package. How about:
in --.--SW1a--.--SW1c--.--> to buffer
| | |
| S |
| W |
| 1 |
| b |
| | |
| gnd |
| |
| |
| |
1kR |
|--SW2------------'
10R
|
gnd
Pimpom
Jul 2, 2020, 8:52:13 AM7/2/20
to
substantially higher than the signal peak amplitude. That is,
higher by at least the full 'on' gate voltage.
George Herold
Jul 2, 2020, 12:43:30 PM7/2/20
to
or would one of those telecom relays that John L. likes work for you?
George H.
bitrex
Jul 2, 2020, 2:13:06 PM7/2/20
to
gain resistors that can do less-than-unity gain:
<https://wiki.analog.com/_media/university/courses/electronics/var_gain_inv_noninv.png>
John Larkin
Jul 2, 2020, 2:43:46 PM7/2/20
to
spec except switching speed.
Jfets are OK switches, but capacitances can be high and gate drive is
usually a nuisance.
A 2-stage attenuator might be better for a 100:1 ratio. Maybe use one
DPDT cmos switch.
--
John Larkin Highland Technology, Inc trk
The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
Phil Hobbs
Jul 2, 2020, 3:06:17 PM7/2/20
to
input.
There are lots better muxes than the ones you cite--check out the
TMUX1511--it's almost as good as a relay, and 100k times faster.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
Phil Hobbs
Jul 2, 2020, 3:12:32 PM7/2/20
to
dual-gate ones such as the late lamented BF998. Looks as though Toshiba
is the only remaining maker.
Dual-gate FETs make really good medium-speed sampling gates.
bitrex
Jul 2, 2020, 3:23:31 PM7/2/20
to
On 7/2/2020 2:43 PM, John Larkin wrote:
> On Thu, 2 Jul 2020 09:43:24 -0700 (PDT), George Herold
> <gghe...@gmail.com> wrote:
>
>> On Thursday, July 2, 2020 at 2:35:33 AM UTC-4, Adam Seychell wrote:
>>> I need a switchable 0 /40dB attenuator for a 10Hz-20MHz 1Vp-p signal into a buffer amplifier (10k|3pF input loading). First I thought of using a SPDT analog switch and resistor divider, but even 'low capacitance' devices come with 15~20pF capacitance on the common pin and have poor off isolation for my purpose.
>>>
>>> in --.--SW1--.--> to buffer
>>> | |
>>> 1kR |
>>> |--SW2--'
>>> 10R
>>> |
>>> gnd
>>>
>>> When I simulated a 2N5484 J-FET, which have been around for at lease 30 years, they beat the pants of any modern CMOS analog switch for off isolation and shunt capacitance. They only downside is more complex driving circuit because of negative gate drive and should have good AC grounding for best off isolation.
>>>
>>> Is the small signal JFET best suited for this application ? There seems very little information on comparing the JEFTs analog switch to the CMOS switch, especially AC performance.
>>>
>>> Adam Seychell
>>
>> Mechanical switches have ~0.5 - 1.5 pF (NKK are lowest I know)
>> or would one of those telecom relays that John L. likes work for you?
>>
>> George H.
>
> The telecom relays are miles better than most any semiconductor in any
> spec except switching speed.
>
> Jfets are OK switches, but capacitances can be high and gate drive is
> usually a nuisance.
>
> A 2-stage attenuator might be better for a 100:1 ratio. Maybe use one
> DPDT cmos switch.
>
>
>
Don't understand why to use an analog switch in the direct signal path
> On Thu, 2 Jul 2020 09:43:24 -0700 (PDT), George Herold
> <gghe...@gmail.com> wrote:
>
>> On Thursday, July 2, 2020 at 2:35:33 AM UTC-4, Adam Seychell wrote:
>>> I need a switchable 0 /40dB attenuator for a 10Hz-20MHz 1Vp-p signal into a buffer amplifier (10k|3pF input loading). First I thought of using a SPDT analog switch and resistor divider, but even 'low capacitance' devices come with 15~20pF capacitance on the common pin and have poor off isolation for my purpose.
>>>
>>> in --.--SW1--.--> to buffer
>>> | |
>>> 1kR |
>>> |--SW2--'
>>> 10R
>>> |
>>> gnd
>>>
>>> When I simulated a 2N5484 J-FET, which have been around for at lease 30 years, they beat the pants of any modern CMOS analog switch for off isolation and shunt capacitance. They only downside is more complex driving circuit because of negative gate drive and should have good AC grounding for best off isolation.
>>>
>>> Is the small signal JFET best suited for this application ? There seems very little information on comparing the JEFTs analog switch to the CMOS switch, especially AC performance.
>>>
>>> Adam Seychell
>>
>> Mechanical switches have ~0.5 - 1.5 pF (NKK are lowest I know)
>> or would one of those telecom relays that John L. likes work for you?
>>
>> George H.
>
> The telecom relays are miles better than most any semiconductor in any
> spec except switching speed.
>
> Jfets are OK switches, but capacitances can be high and gate drive is
> usually a nuisance.
>
> A 2-stage attenuator might be better for a 100:1 ratio. Maybe use one
> DPDT cmos switch.
>
>
>
here at all, in isolation from un-stated requirements. The signal is AC
coupled as far as I can tell, and isn't that fast or large amplitude. A
switchable VGA eliminates the isolation and loading problems by not
having them. only advantage I can see is budget
whit3rd
Jul 2, 2020, 9:50:21 PM7/2/20
to
On Thursday, July 2, 2020 at 5:02:03 AM UTC-7, Anthony Stewart wrote:
> > Is the small signal JFET best suited for this application ? There seems very little information on comparing the JEFTs analog switch to the CMOS switch, especially AC performance.
> > Is the small signal JFET best suited for this application ? There seems very little information on comparing the JEFTs analog switch to the CMOS switch, especially AC performance.
> Achieving a flat spectral response in an attenuator or amplifier simply
> needs to understand this is done by impedance ratios. So when you have a
> load impedance of 10k 2pF ,@ 10 MHz realize that any ratio must use a
> ratio that makes the component capacitance negligible. This can be done
> by lowering the R impedance load so that 2 pF = 8kOhm at 10Mhz is
> negligible.
Yep, that's the ticket. Amplifiers for good linearity in this range are available
> needs to understand this is done by impedance ratios. So when you have a
> load impedance of 10k 2pF ,@ 10 MHz realize that any ratio must use a
> ratio that makes the component capacitance negligible. This can be done
> by lowering the R impedance load so that 2 pF = 8kOhm at 10Mhz is
> negligible.
for analog video, and are generally 75 ohm items. The good news: there's lots of
em. The bad news: they don't generally use or need 40 dB of attenuation.
Adam Seychell
Jul 2, 2020, 10:27:34 PM7/2/20
to
>
> There aren't any small-signal discrete MOSFETs left, except for a few
> dual-gate ones such as the late lamented BF998. Looks as though Toshiba
> is the only remaining maker.
>
> Dual-gate FETs make really good medium-speed sampling gates.
>
Fortunately there are a number of small N-MOSFETs with ~3pF output capacitance and few ohms on resistance. A quick simulation of back to back N-FETs with schematic in link below. This circuit will have horrendous charge injection and slow switch speed, but ~3pF capacitance loading in off state, even less with on state thanks to low collector capacitance of $0.05 bipolars. The 1nF across the gates add AC grounding when in off state, or the NPN is in full saturation.
> There aren't any small-signal discrete MOSFETs left, except for a few
> dual-gate ones such as the late lamented BF998. Looks as though Toshiba
> is the only remaining maker.
>
> Dual-gate FETs make really good medium-speed sampling gates.
>
Small dual MOSFETs in 6 pin packages NX3020NAKS NXP, VT6K1 Rohm. The NPN/PNP are also readily available as a 6pin SOT363, SOT666 et.
https://www.dropbox.com/s/6g8qvpwa8078b8j/analog%20switch%20dual%20MOSFET.png?dl=0
Phil Hobbs
Jul 2, 2020, 10:36:14 PM7/2/20
to
Clifford Heath
Jul 3, 2020, 2:39:40 AM7/3/20
to
On 3/7/20 12:36 pm, Phil Hobbs wrote:
> On 2020-07-02 22:27, Adam Seychell wrote:
>>>
>>> There aren't any small-signal discrete MOSFETs left, except for a few
>>> dual-gate ones such as the late lamented BF998. Looks as though Toshiba
>>> is the only remaining maker.
>>>
>>> Dual-gate FETs make really good medium-speed sampling gates.
>>>
>>
>> Fortunately there are a number of small N-MOSFETs with ~3pF output
>> capacitance and few ohms on resistance. A quick simulation of back to
>> back N-FETs with schematic in link below. This circuit will have
>> horrendous charge injection and slow switch speed, but ~3pF
>> capacitance loading in off state, even less with on state thanks to
>> low collector capacitance of $0.05 bipolars. The 1nF across the gates
>> add AC grounding when in off state, or the NPN is in full saturation.
>>
>> Small dual MOSFETs in 6 pin packages NX3020NAKS NXP, VT6K1 Rohm. The
>> NPN/PNP are also readily available as a 6pin SOT363, SOT666 et.
>>
>> https://www.dropbox.com/s/6g8qvpwa8078b8j/analog%20switch%20dual%20MOSFET.png?dl=
>>
>
> So where do you buy them?
Digikey claims to have the NX3020NAKS - 23,000 in stock
> On 2020-07-02 22:27, Adam Seychell wrote:
>>>
>>> There aren't any small-signal discrete MOSFETs left, except for a few
>>> dual-gate ones such as the late lamented BF998. Looks as though Toshiba
>>> is the only remaining maker.
>>>
>>> Dual-gate FETs make really good medium-speed sampling gates.
>>>
>>
>> Fortunately there are a number of small N-MOSFETs with ~3pF output
>> capacitance and few ohms on resistance. A quick simulation of back to
>> back N-FETs with schematic in link below. This circuit will have
>> horrendous charge injection and slow switch speed, but ~3pF
>> capacitance loading in off state, even less with on state thanks to
>> low collector capacitance of $0.05 bipolars. The 1nF across the gates
>> add AC grounding when in off state, or the NPN is in full saturation.
>>
>> Small dual MOSFETs in 6 pin packages NX3020NAKS NXP, VT6K1 Rohm. The
>> NPN/PNP are also readily available as a 6pin SOT363, SOT666 et.
>>
>> https://www.dropbox.com/s/6g8qvpwa8078b8j/analog%20switch%20dual%20MOSFET.png?dl=
>>
>
> So where do you buy them?
CH
Gerhard Hoffmann
Jul 3, 2020, 3:05:35 AM7/3/20
to
Am 02.07.20 um 21:12 schrieb Phil Hobbs:
>
> There aren't any small-signal discrete MOSFETs left, except for a few
> dual-gate ones such as the late lamented BF998. Looks as though Toshiba
> is the only remaining maker.
>
> Dual-gate FETs make really good medium-speed sampling gates.
>
what about these?
<
https://epc-co.com/epc/Portals/0/epc/documents/datasheets/EPC2038_datasheet.pdf
>
Gerhard
>
> There aren't any small-signal discrete MOSFETs left, except for a few
> dual-gate ones such as the late lamented BF998. Looks as though Toshiba
> is the only remaining maker.
>
> Dual-gate FETs make really good medium-speed sampling gates.
>
<
https://epc-co.com/epc/Portals/0/epc/documents/datasheets/EPC2038_datasheet.pdf
>
Gerhard
John Larkin
Jul 3, 2020, 9:56:56 AM7/3/20
to
On Thu, 2 Jul 2020 15:06:09 -0400, Phil Hobbs
<pcdhSpamM...@electrooptical.net> wrote:
>On 2020-07-02 02:35, Adam Seychell wrote:
>> I need a switchable 0 /40dB attenuator for a 10Hz-20MHz 1Vp-p signal into a buffer amplifier (10k|3pF input loading). First I thought of using a SPDT analog switch and resistor divider, but even 'low capacitance' devices come with 15~20pF capacitance on the common pin and have poor off isolation for my purpose.
>>
>> in --.--SW1--.--> to buffer
>> | |
>> 1kR |
>> |--SW2--'
>> 10R
>> |
>> gnd
>>
>> When I simulated a 2N5484 J-FET, which have been around for at lease 30 years, they beat the pants of any modern CMOS analog switch for off isolation and shunt capacitance. They only downside is more complex driving circuit because of negative gate drive and should have good AC grounding for best off isolation.
>>
>> Is the small signal JFET best suited for this application ? There seems very little information on comparing the JEFTs analog switch to the CMOS switch, especially AC performance.
>>
>> Adam Seychell
>>
>
>JFETs work fine for that, especially if your buffer has a high-impedance
>input.
>
>There are lots better muxes than the ones you cite--check out the
>TMUX1511--it's almost as good as a relay, and 100k times faster.
>
>Cheers
>
>Phil Hobbs
A BFT25 has about 0.25 pF of collector capacitance and a reverse beta
<pcdhSpamM...@electrooptical.net> wrote:
>On 2020-07-02 02:35, Adam Seychell wrote:
>> I need a switchable 0 /40dB attenuator for a 10Hz-20MHz 1Vp-p signal into a buffer amplifier (10k|3pF input loading). First I thought of using a SPDT analog switch and resistor divider, but even 'low capacitance' devices come with 15~20pF capacitance on the common pin and have poor off isolation for my purpose.
>>
>> in --.--SW1--.--> to buffer
>> | |
>> 1kR |
>> |--SW2--'
>> 10R
>> |
>> gnd
>>
>> When I simulated a 2N5484 J-FET, which have been around for at lease 30 years, they beat the pants of any modern CMOS analog switch for off isolation and shunt capacitance. They only downside is more complex driving circuit because of negative gate drive and should have good AC grounding for best off isolation.
>>
>> Is the small signal JFET best suited for this application ? There seems very little information on comparing the JEFTs analog switch to the CMOS switch, especially AC performance.
>>
>> Adam Seychell
>>
>
>JFETs work fine for that, especially if your buffer has a high-impedance
>input.
>
>There are lots better muxes than the ones you cite--check out the
>TMUX1511--it's almost as good as a relay, and 100k times faster.
>
>Cheers
>
>Phil Hobbs
around 4. Something could be done with that.
Tom Del Rosso
Jul 3, 2020, 10:23:14 AM7/3/20
to
Chris Jones wrote:
>
> There is a diode is from the drain and source to the body/backgate. In
> discrete MOSFETs the backgate is tied to the source so the diode is
> from drain to source, but on a chip (e.g. analog switch chip) the
> designers could connect the backgate to something else, (e.g. to the
> supply rails in the old 4016), so there is no diode from drain to
> source. Provided the signals are with the supply rails, the switch
> can block conduction between its terminals in either direction. More
> recent analog switch chips try to do things like reduce the
> on-resistance, in the 4066 this is done by switching the backgate of
> one FET to sometimes tie it to the signal pins, but this has
> disadvantages for charge injection, if that matters. Yet more recent
> ones probably do much more fancy stuff.
Why the hell don't they make 4-pin MOSFET's? A discreet device should
>
> There is a diode is from the drain and source to the body/backgate. In
> discrete MOSFETs the backgate is tied to the source so the diode is
> from drain to source, but on a chip (e.g. analog switch chip) the
> designers could connect the backgate to something else, (e.g. to the
> supply rails in the old 4016), so there is no diode from drain to
> source. Provided the signals are with the supply rails, the switch
> can block conduction between its terminals in either direction. More
> recent analog switch chips try to do things like reduce the
> on-resistance, in the 4066 this is done by switching the backgate of
> one FET to sometimes tie it to the signal pins, but this has
> disadvantages for charge injection, if that matters. Yet more recent
> ones probably do much more fancy stuff.
have fewer parasitics, not more.
Phil Hobbs
Jul 3, 2020, 2:55:31 PM7/3/20
to
albeit small.
Clifford Heath
Jul 4, 2020, 6:07:44 AM7/4/20
to
CH
Chris Jones
Jul 4, 2020, 9:36:02 AM7/4/20
to
SST213. Also for some uses, the (4000-series CMOS chip) 4007 has MOSFETs
with various terminals brought out, is widely stocked, but poorly
specified for analog use.
omni...@gmail.com
Jul 4, 2020, 3:41:18 PM7/4/20
to
The substrate has its own pin.
BSS83 Datasheet
https://html.alldatasheet.com/html-pdf/16715/PHILIPS/BSS83/493/2/BSS83.html
M Philbrook
Jul 6, 2020, 7:30:51 AM7/6/20
to
Jeroen Belleman
Jul 6, 2020, 9:23:57 AM7/6/20
to
M Philbrook wrote:
> In article <880c04f3-0c33-47a9...@googlegroups.com>,
> adam.s...@gmail.com says...
>> I need a switchable 0 /40dB attenuator for a 10Hz-20MHz 1Vp-p signal
>> into a buffer amplifier (10k|3pF input loading).
[...]
> In article <880c04f3-0c33-47a9...@googlegroups.com>,
> adam.s...@gmail.com says...
>> I need a switchable 0 /40dB attenuator for a 10Hz-20MHz 1Vp-p signal
>> into a buffer amplifier (10k|3pF input loading).
>>
>> Is the small signal JFET best suited for this application ? There
>> seems very little information on comparing the JEFTs analog switch to
>> the CMOS switch, especially AC performance.
>>
>> Adam Seychell
>
> PIN diode switch ?
>
branch, because of their low open inter-contact capacitance.
(Pickering series 103 SPST, 70fF.) For the shunt branch, I
found the series inductance of a reed switch intolerable,
so there I used BAR88 PIN diodes with a lot of forward
current, because they don't otherwise work for such low
frequencies.
Jeroen Belleman
John Larkin
Jul 6, 2020, 10:22:11 AM7/6/20
to
GHz.
https://www.dropbox.com/s/se162xpw86hpmzs/DSC06884.JPG?raw=1
https://www.dropbox.com/s/lpf6t3c6rtw5gid/DSC06873.JPG?raw=1
Jeroen Belleman
Jul 6, 2020, 10:41:51 AM7/6/20
to
there. Do the Fujitsu relays survive being washed?
Jeroen Belleman
John Larkin
Jul 6, 2020, 12:19:23 PM7/6/20
to
On Mon, 06 Jul 2020 16:41:47 +0200, Jeroen Belleman
We have learned to not water-wash them.
https://www.dropbox.com/s/w1l7qcu9m7c2i48/WET.jpg?raw=1
We have two wash systems, one water and one "vapor degreaser" which is
actually a dunk in boiling solvent followed by spray-down. They are
happy with that process.
We used the Omrons once but don't any more for some reason.
https://www.dropbox.com/s/w1l7qcu9m7c2i48/WET.jpg?raw=1
We have two wash systems, one water and one "vapor degreaser" which is
actually a dunk in boiling solvent followed by spray-down. They are
happy with that process.
We used the Omrons once but don't any more for some reason.
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