On Monday, 30 January 2023 at 18:50:26 UTC, Fred Bloggs wrote:
> 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?
So we are all trying to guess the answer based on insufficient information.
Regardless of that problem, there is a requirement to limit the maximum dc
(and ac, depending on frequency) currents that can flow through electrodes
connected to people or animals both under normal operating conditions and
in single fault conditions.
See IEC 60601 for chapter and verse.
John
Reply by Fred Bloggs●January 30, 20232023-01-30
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?
Reply by John Walliker●January 30, 20232023-01-30
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
Reply by Fred Bloggs●January 30, 20232023-01-30
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.
Reply by John Larkin●January 30, 20232023-01-30
On Mon, 30 Jan 2023 09:22:17 -0000 (UTC), Mike Monett VE3BTI
<spamme@not.com> wrote:
>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.
I've used 8-channel analog multiplexers purely for their ESD diodes.
Reply by Mike Monett VE3BTI●January 30, 20232023-01-30
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.
--
MRM
Reply by Phil Hobbs●January 30, 20232023-01-30
Mike Monett VE3BTI wrote:
> 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 — 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.
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.nethttp://hobbs-eo.com
Reply by Clifford Heath●January 30, 20232023-01-30
On 30/01/23 12:08, Phil Hobbs wrote:
> 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
In-between those two, I have used SRV05-4L.
It looks like it's my news service/client that's wonky; apologies Lasse.
CH
Reply by Mike Monett VE3BTI●January 29, 20232023-01-29
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 � Voltage
Characteristic
Page 1 states:
"The input capacitance of 35 pF makes it unsuitable for high-speed USB 2.0
applications."
This means they they are only suitable for USB version 1.1 at 12 Mbps, but
not many people are using this anymore.
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.
--
MRM
Reply by Clifford Heath●January 29, 20232023-01-29
On 30/01/23 10:56, Lasse Langwadt Christensen wrote:
> 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
>
>
>
>
Right, but it probably helps reduce C too.
Did this response come up in the wrong thread for everyone else too?
Lasse, your news client is weird.
CH.