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On Sunday, 29 January 2023 at 20:11:31 UTC, Fred Bloggs wrote:
That\'s a really weird circuit.
I\'m pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp.
The BC850 clamps in conjunction with the 22k input resistors are almost certainly there
to protect the patient from excessive dc under single fault conditions. Direct current
flowing through electrodes attached to the skin can cause severe burns if it is sustained
for a long time - which could easily happen in some medical situations.
Nobody would put 22k resistors in series with the input of a low-noise amplifier if
they didn\'t need to. Their purpose is to limit the current that can flow when driven by the clamping
voltage. Without the clamps the fault current through the electrodes would be around
10 times greater if, for example, one of the BAV99s developed a short to Vcc.
Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:
That\'s a really weird circuit.
it sorta works like a zener diode
https://youtu.be/BGcKjy_UNQ4?t=18
That circuit has a big advantage. The capacitance of a reverse-biased BE
junction is very low, so it should be much faster than a Zener.
it sorta works like a zener diode
https://youtu.be/BGcKjy_UNQ4?t=18
That circuit has a big advantage. The capacitance of a reverse-biased BE
junction is very low, so it should be much faster than a Zener.
There are USB ESD protectors that are much, much better.
Cheers
Phil Hobbs
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
it sorta works like a zener diode
https://youtu.be/BGcKjy_UNQ4?t=18
That circuit has a big advantage. The capacitance of a reverse-biased BE
junction is very low, so it should be much faster than a Zener.
There are USB ESD protectors that are much, much better.
Cheers
Phil Hobbs
Any links?
The ones we\'ve used are
mandag den 30. januar 2023 kl. 00.34.59 UTC+1 skrev Clifford Heath:
On 30/01/23 08:34, Phil Hobbs wrote:
In one hand-wired proto recently,(*) I was using an LM319N
open-collector dual comparator to drive the !set and !clear inputs of
one section of a 74VHC74A dflop.
The 319 was running off +-15V. By mistake, I wired the pull-up resistor
on one section to +15 rather than +5. The circuit worked perfectly--I
only noticed the blunder when I went to make a slight change to the ramp
current source.
Abs max on both VDD and any input is +7V, which appears to be a fairly
absurd underestimate of what the !clear input can comfortably manage, at
least for a day or two. It didn\'t seem to be drawing any input current,
either--at most a few dozen microamps.
Possibly such high-speed parts dispense with ESD diodes to reduce input
capacitance?
usually so they can be 5V tolerant and/or handle input while not powered
Any links?
The ones we\'ve used are
IC TVS 4X USB ZENER ESD BEEFY TSSOP8 SN75240PWR TI
$0.57815 @ qty 1000
IC TVS 2X USB ESD DIODES+ZENER 5.5V XSON-6 PRTR5V0U2F,115
Nexperia $0.16539 @ qty 1000
Cheers
Phil Hobbs
Mike Monett VE3BTI wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
it sorta works like a zener diode
https://youtu.be/BGcKjy_UNQ4?t=18
That circuit has a big advantage. The capacitance of a
reverse-biased BE
junction is very low, so it should be much faster than a Zener.
There are USB ESD protectors that are much, much better.
Cheers
Phil Hobbs
Any links?
The ones we\'ve used are
IC TVS 4X USB ZENER ESD BEEFY TSSOP8 SN75240PWR TI $0.57815 @ qty 1000
IC TVS 2X USB ESD DIODES+ZENER 5.5V XSON-6 PRTR5V0U2F,115
Nexperia               $0.16539 @ qty 1000
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Any links?
The ones we\'ve used are
IC TVS 4X USB ZENER ESD BEEFY TSSOP8 SN75240PWR TI
$0.57815 @ qty 1000
IC TVS 2X USB ESD DIODES+ZENER 5.5V XSON-6 PRTR5V0U2F,115
Nexperia $0.16539 @ qty 1000
Cheers
Phil Hobbs
Thanks. The datasheet is at
https://www.ti.com/lit/ds/symlink/sn75240.pdf
The circuit is on Page 6, Figure 9-1. TVS Structure and Current â Voltage
Characteristic
Page 1 states:
\"The input capacitance of 35 pF makes it unsuitable for high-speed USB 2.0
applications.\"
The other one (PRTR5V0U2F) is 1 pF from I/O to ground.
The other one (PRTR5V0U2F) is 1 pF from I/O to ground.
Cheers
Phil Hobbs
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
The other one (PRTR5V0U2F) is 1 pF from I/O to ground.
Cheers
Phil Hobbs
That\'s what I wrote. Quote:
So what do IC\'s use for ESD protection? I don\'t know, but I suspect it is
reverse-biased diodes to VCC and GND, similar to the PRTR5V0U2F, at
https://assets.nexperia.com/documents/data-sheet/PRTR5V0U2F_PRTR5V0U2K.pdf
The waveforms on Page 6, Fig 4. ESD clamping test setup and waveforms, show
very significant reduction in ESD amplitudes, from 800V peak to 20V peak.
In addition, the low capacitance of 1 pF enables their use in higher speed
USB applications. Cheaper and much better performance than the 75240.
However, I wonder if it is really needed, since many ICs are already rated
for ESD protection up to 8 kV.
On Sunday, 29 January 2023 at 20:11:31 UTC, Fred Bloggs wrote:
That\'s a really weird circuit.
I\'m pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp.
The BC850 clamps in conjunction with the 22k input resistors are almost certainly there
to protect the patient from excessive dc under single fault conditions. Direct current
flowing through electrodes attached to the skin can cause severe burns if it is sustained
for a long time - which could easily happen in some medical situations.
Nobody would put 22k resistors in series with the input of a low-noise amplifier if
they didn\'t need to. Their purpose is to limit the current that can flow when driven by the clamping
voltage. Without the clamps the fault current through the electrodes would be around
10 times greater if, for example, one of the BAV99s developed a short to Vcc.
John
On Sunday, January 29, 2023 at 6:20:44 PM UTC-5, John Walliker wrote:
On Sunday, 29 January 2023 at 20:11:31 UTC, Fred Bloggs wrote:
That\'s a really weird circuit.
I\'m pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp.
The BC850 clamps in conjunction with the 22k input resistors are almost certainly there
to protect the patient from excessive dc under single fault conditions. Direct current
flowing through electrodes attached to the skin can cause severe burns if it is sustained
for a long time - which could easily happen in some medical situations.
Nobody would put 22k resistors in series with the input of a low-noise amplifier if
they didn\'t need to. Their purpose is to limit the current that can flow when driven by the clamping
voltage. Without the clamps the fault current through the electrodes would be around
10 times greater if, for example, one of the BAV99s developed a short to Vcc.
John
The circuit runs off 1.8V battery power.
On Monday, 30 January 2023 at 17:58:07 UTC, Fred Bloggs wrote:
On Sunday, January 29, 2023 at 6:20:44 PM UTC-5, John Walliker wrote:
On Sunday, 29 January 2023 at 20:11:31 UTC, Fred Bloggs wrote:
That\'s a really weird circuit.
I\'m pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp.
The BC850 clamps in conjunction with the 22k input resistors are almost certainly there
to protect the patient from excessive dc under single fault conditions. Direct current
flowing through electrodes attached to the skin can cause severe burns if it is sustained
for a long time - which could easily happen in some medical situations.
Nobody would put 22k resistors in series with the input of a low-noise amplifier if
they didn\'t need to. Their purpose is to limit the current that can flow when driven by the clamping
voltage. Without the clamps the fault current through the electrodes would be around
10 times greater if, for example, one of the BAV99s developed a short to Vcc.
John
The circuit runs off 1.8V battery power.
Where is that stated on the schematic that Ricky originally posted?
John
On Monday, January 30, 2023 at 1:34:09 PM UTC-5, John Walliker wrote:
On Monday, 30 January 2023 at 17:58:07 UTC, Fred Bloggs wrote:
On Sunday, January 29, 2023 at 6:20:44 PM UTC-5, John Walliker wrote:
On Sunday, 29 January 2023 at 20:11:31 UTC, Fred Bloggs wrote:
That\'s a really weird circuit.
I\'m pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp.
The BC850 clamps in conjunction with the 22k input resistors are almost certainly there
to protect the patient from excessive dc under single fault conditions. Direct current
flowing through electrodes attached to the skin can cause severe burns if it is sustained
for a long time - which could easily happen in some medical situations.
Nobody would put 22k resistors in series with the input of a low-noise amplifier if
they didn\'t need to. Their purpose is to limit the current that can flow when driven by the clamping
voltage. Without the clamps the fault current through the electrodes would be around
10 times greater if, for example, one of the BAV99s developed a short to Vcc.
John
The circuit runs off 1.8V battery power.
Where is that stated on the schematic that Ricky originally posted?
John
Where on that schematic does it say it was anything else? Or even line powered?