| Author |
Message |
Jon
Guest
|
 Posted:
Wed Nov 09, 2005 5:35 pm Post subject:
Re: Driving a Discrete Input Having a 50ohm Impedance |
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|
Don,
A back termination resistor is just a resistor in series with the
source signal. It is used to provide impedance matching with the
characteristic impedance of a cable. This minimizes reflections and
ringing.
~
I was assuming that you were concerned with ringing at the load input,
and would, therefore be using cable with a 50 Ohm characteristic
impedance. If this is not the case, then there is no need to provide
the impedance matching provided by the 50 Ohm resistor.
~
By the way, it is very common in video systems (which commonly use 75
Ohm cable) to use video drivers with a voltage gain of 2. This
compensates for the gain of 1/2 that results from the 75 Ohm back
termination resistor that is commonly used.
Regards,
Jon
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Chris Jones
Guest
|
| Posted:
Fri Nov 11, 2005 1:35 am Post subject:
Re: Driving a Discrete Input Having a 50ohm Impedance |
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Don wrote:
| Quote: | Any one know of a good reference design for a driver for an input to
unit with a 50ohm impedance and that's looking for 8 to 11VDC as a
"high" and "-1 to 2VDC for a logic 0? Needs to have a fast (50nsec)
rise time.
I have been pondering just driving a transistor cable of handling about
500mA with the collector ties to the mid-junction of two 50ohm
resistors hooked in series. The high side resistor connected to 20VDC
and the other resistor connected to ground.
Any references or suggestions will be greatly appreciated!
Thanks!
Don
|
Look at the TC4421 and TC4422 gate driver chips, and try to limit the 'high'
time to a microsecond or less, so that the duty cycle is very small, and
therefore you will not need to waste a lot of power.
As long as the load is 50 Ohms, which you say it is, then you don't need a
resistor at the driving end, only at the load end. Those who say otherwise
are mistaken. You can test this all quite nicely if you have a fast scope
and a few tens of metres of 50 Ohm coax cable, this length being desirable
so that you can clearly discern the reflections or absence thereof in the
waveforms. You need to have a low capacitance scope probe or you need to
use the scope in 50 Ohm mode in place of the load termination for testing.
Be careful not to put too much DC for too long a time into a 50 Ohms scope
input as this will fry it.
Chris |
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Fred Bloggs
Guest
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| Posted:
Sat Nov 12, 2005 1:35 am Post subject:
Re: Driving a Discrete Input Having a 50ohm Impedance |
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| Quote: | From the spec for the piece of equipment, here is the description of
the input:
INPUT IMPEDANCE: 50 OHMS +/-5%
LOGIC 1: 8 to 11 VDC
LOGIC 0: -1 to 2 VDC
RISE TIME: < 50nsec
FALL TIME: < 1usec
I guess I don't understand what you mean about "buzzwordese". If you
know and work with the Global Positioning System (GPS), you probably
know what 1PPS means (1 pulse per second - a pretty standard signal in
GPS equipment).
For grins, I measured the resistance from the signal pin to ground, and
guess what? 50 ohms. (Yeah, I know there is other stuff in there, but
it does support the spec.) So. what I described is accurate and I did
not mis-read anything.
|
This one might be too fast, but it's a cheap way of driving that 1PPS
trigger pulse train from something like a PIC. I am assuming that up to
1us pulsewidth is more than adequate- the giveaway is they do not care
much about trailing edge:
View in a fixed-width font such as Courier.
..
..
..
..
.. +5V
.. |
.. +----+--[10K]--|>|---+----------------------+---|<|-----.
.. | | 1N6263 | | 1N5817 |
.. | | | | |
.. | | | ---74AC240 |
.. | | | | | |
.. | | | | |\ | |
.. | | | | +--| o-+ | |
.. === | === | | |/ | | |
.. 0.47u | 0.47u | | . | | |
.. | | | | | . | | |
.. | | | | | |\ | | |
.. | +---------|>|---|---------+------------+--| o-+----+->
.. | | 1N6263 | | | | |/. | | |
.. | | | | | | . | | |
.. | ----74AC241 | | | | |\ | | |
.. | | | | | | +--| o-+ | |
.. | | |\ | | [560] | |/ | |
.. | | +--| >-+ | | | | | |
.. | | | |/ | | | | ---------- |
.. | | | . | | | | | |
.. | | | . | | | | | |
.. | | | |\ | | | | | |
.. IN>--------+--| >-+------+---------+------------+---|>|-----'
.. | | | |/. | | 1N5817
.. | | | . | |
.. | | | |\ | |
.. | | +--| >-+ |
.. | | |/ |
.. | | |
.. | -----------
.. | |
.. | |
.. '----+--------------------------------------------------+
.. | |
.. --- ---
.. /// ///
..
..
..
..
..
..
..
..
..
..
|
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Tony Williams
Guest
|
| Posted:
Sat Nov 12, 2005 10:10 am Post subject:
Re: Driving a Discrete Input Having a 50ohm Impedance |
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|
In article <43753CD4.3070403@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
| Quote: | +5V
|
+----+--[10K]--|>|---+----------------------+---|<|-----.
| | 1N6263 | | 1N5817 |
| | | | |
| | | ---74AC240 |
| | | | | |
| | | | |\ | |
| | | | +--| o-+ | |
=== | === | | |/ | | |
0.47u | 0.47u | | . | | |
| | | | | . | | |
| | | | | |\ | | |
| +---------|>|---|---------+------------+--| o-+----+-
| | 1N6263 | | | | |/. | | |
| | | | | | . | | |
| ----74AC241 | | | | |\ | | |
| | | | | | +--| o-+ | |
| | |\ | | [560] | |/ | |
| | +--| >-+ | | | | | |
| | | |/ | | | | ---------- |
| | | . | | | | | |
| | | . | | | | | |
| | | |\ | | | | | |
IN>--------+--| >-+------+---------+------------+---|>|-----'
| | | |/. | | 1N5817
| | | . | |
| | | |\ | |
| | +--| >-+ |
| | |/ |
| | |
| -----------
| |
| |
'----+--------------------------------------------------+
| |
--- ---
/// ///
|
Perhaps add a Schottky to stop the gates of the 74AC240 from
going above it's supply rail when the input goes down.
Maybe even change the 74AC240 for a mosfet.
Tell you the truth Fred I'm sitting here, buttocks well-
-clenched, waiting for the next edition of this bloody hairy
circuit.
--
Tony Williams. |
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Winfield Hill
Guest
|
| Posted:
Sat Nov 12, 2005 5:35 pm Post subject:
Re: Driving a Discrete Input Having a 50ohm Impedance |
|
|
Tony Williams wrote...
| Quote: |
Fred Bloggs <nospam@nospam.com> wrote:
+5V
|
+----+--[10K]--|>|---+----------------------+---|<|-----.
| | 1N6263 | | 1N5817 |
| | | | |
| | | ---74AC240 |
| | | | | |
| | | | |\ | |
| | | | +--| o-+ | |
=== | === | | |/ | | |
0.47u | 0.47u | | . | | |
| | | | | . | | |
| | | | | |\ | | |
| +---------|>|---|---------+------------+--| o-+----+-
| | 1N6263 | | | | |/. | | |
| | | | | | . | | |
| ----74AC241 | | | | |\ | | |
| | | | | | +--| o-+ | |
| | |\ | | [560] | |/ | |
| | +--| >-+ | | | | | |
| | | |/ | | | | ---------- |
| | | . | | | | | |
| | | . | | | | | |
| | | |\ | | | | | |
IN>--------+--| >-+------+---------+------------+---|>|-----'
| | | |/. | | 1N5817
| | | . | |
| | | |\ | |
| | +--| >-+ |
| | |/ |
| | |
| -----------
| |
| |
'----+--------------------------------------------------+
| |
--- ---
/// ///
Perhaps add a Schottky to stop the gates of the 74AC240 from
going above it's supply rail when the input goes down.
Maybe even change the 74AC240 for a mosfet.
Tell you the truth Fred I'm sitting here, buttocks well-clenched,
waiting for the next edition of this bloody hairy circuit.
|
This is the kind of circuit we think about from time to time,
and quickly reject. For example, a flying opamp with supplies
pinned to the rail of another opamp, to extend ordinary cheap
+/-15V capable opamps to +/-30V operation, rather than using
a HV opamp to begin with. In the case of some steady-state AC
signal applications, the flying opamp's entire power supply can
also be obtained from the first opamp's AC output. Arrggghhh!
--
Thanks,
- Win |
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Fred Bloggs
Guest
|
| Posted:
Sun Nov 13, 2005 1:35 am Post subject:
Re: Driving a Discrete Input Having a 50ohm Impedance |
|
|
Tony Williams wrote:
| Quote: | In article <43753CD4.3070403@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
+5V
|
+----+--[10K]--|>|---+----------------------+---|<|-----.
| | 1N6263 | | 1N5817 |
| | | | |
| | | ---74AC240 |
| | | | | |
| | | | |\ | |
| | | | +--| o-+ | |
=== | === | | |/ | | |
0.47u | 0.47u | | . | | |
| | | | | . | | |
| | | | | |\ | | |
| +---------|>|---|---------+------------+--| o-+----+-
| | 1N6263 | | | | |/. | | |
| | | | | | . | | |
| ----74AC241 | | | | |\ | | |
| | | | | | +--| o-+ | |
| | |\ | | [560] | |/ | |
| | +--| >-+ | | | | | |
| | | |/ | | | | ---------- |
| | | . | | | | | |
| | | . | | | | | |
| | | |\ | | | | | |
IN>--------+--| >-+------+---------+------------+---|>|-----'
| | | |/. | | 1N5817
| | | . | |
| | | |\ | |
| | +--| >-+ |
| | |/ |
| | |
| -----------
| |
| |
'----+--------------------------------------------------+
| |
--- ---
/// ///
Perhaps add a Schottky to stop the gates of the 74AC240 from
going above it's supply rail when the input goes down.
|
Good catch, that is a good safety net.
View in a fixed-width font such as Courier.
.. +5V
.. |
.. +----+--[10K]--|>|---+----------------------+---|<|-----.
.. | | 1N6263 | | 1N5817 |
.. | | | | |
.. | | | ---74AC240 |
.. | | +---|<|---+ | | |
.. | | | 1N6263 | | |\ | |
.. | | | | | +--| o-+ | |
.. === | === | | | |/ | | |
.. 0.47u | 0.47u | | | . | | |
.. | | | | | | . | | |
.. | | | | | | |\ | | |
.. | +---------|>|---|---------+------------+--| o-+----+->
.. | | 1N6263 | | | | |/. | | |
.. | | | | | | . | | |
.. | ----74AC241 | | | | |\ | | |
.. | | | | | | +--| o-+ | |
.. | | |\ | | [560] | |/ | |
.. | | +--| >-+ | | | | | |
.. | | | |/ | | | | ---------- |
.. | | | . | | | | | |
.. | | | . | | | | | |
.. | | | |\ | | | | | |
.. IN>--------+--| >-+------+---------+------------+---|>|-----'
.. | | | |/. | | 1N5817
.. | | | . | |
| Quote: | Maybe even change the 74AC240 for a mosfet.
|
Can you find one that has 20pF input capacitance, handles 200mA at low
threshold, and costs 27 cents? I assume it would be a P-channel. |
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Fred Bloggs
Guest
|
| Posted:
Sun Nov 13, 2005 1:35 am Post subject:
Re: Driving a Discrete Input Having a 50ohm Impedance |
|
|
Winfield Hill wrote:
| Quote: | Tony Williams wrote...
Fred Bloggs <nospam@nospam.com> wrote:
+5V
|
+----+--[10K]--|>|---+----------------------+---|<|-----.
| | 1N6263 | | 1N5817 |
| | | | |
| | | ---74AC240 |
| | | | | |
| | | | |\ | |
| | | | +--| o-+ | |
=== | === | | |/ | | |
0.47u | 0.47u | | . | | |
| | | | | . | | |
| | | | | |\ | | |
| +---------|>|---|---------+------------+--| o-+----+-
| | 1N6263 | | | | |/. | | |
| | | | | | . | | |
| ----74AC241 | | | | |\ | | |
| | | | | | +--| o-+ | |
| | |\ | | [560] | |/ | |
| | +--| >-+ | | | | | |
| | | |/ | | | | ---------- |
| | | . | | | | | |
| | | . | | | | | |
| | | |\ | | | | | |
IN>--------+--| >-+------+---------+------------+---|>|-----'
| | | |/. | | 1N5817
| | | . | |
| | | |\ | |
| | +--| >-+ |
| | |/ |
| | |
| -----------
| |
| |
'----+--------------------------------------------------+
| |
--- ---
/// ///
Perhaps add a Schottky to stop the gates of the 74AC240 from
going above it's supply rail when the input goes down.
Maybe even change the 74AC240 for a mosfet.
Tell you the truth Fred I'm sitting here, buttocks well-clenched,
waiting for the next edition of this bloody hairy circuit.
This is the kind of circuit we think about from time to time,
and quickly reject. For example, a flying opamp with supplies
pinned to the rail of another opamp, to extend ordinary cheap
+/-15V capable opamps to +/-30V operation, rather than using
a HV opamp to begin with. In the case of some steady-state AC
signal applications, the flying opamp's entire power supply can
also be obtained from the first opamp's AC output. Arrggghhh!
|
You can reject it but I won't. You either do 10V @200mA or 5V@400mA. I
don't call a common logic 5V supply, an octal logic driver, standard
decoupling caps, and some Schottky's excessive. If you have no logic in
the system then maybe something else makes sense, but if it's already
there, the additional loading is 200mA E-6 which looks like leakage
current to me. |
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Tony Williams
Guest
|
| Posted:
Mon Nov 14, 2005 7:13 am Post subject:
Re: Driving a Discrete Input Having a 50ohm Impedance |
|
|
In article <dl4qvs02hk9@drn.newsguy.com>,
Winfield Hill <Winfield_member@newsguy.com> wrote:
| Quote: | This is the kind of circuit we think about from time to time,
and quickly reject. For example, a flying opamp with supplies
pinned to the rail of another opamp, to extend ordinary cheap
+/-15V capable opamps to +/-30V operation, rather than using
a HV opamp to begin with. In the case of some steady-state AC
signal applications, the flying opamp's entire power supply can
also be obtained from the first opamp's AC output. Arrggghhh!
|
A novel circuit that is interesting to look at though.
I'd never thought about a single 5V pump of the rail
and a simultaneous 5V swing of the output.
Will the output reliably swing from 0 to 10v in about
50nS? A possible killer might be load capacitance and
the effective transient current load on the 74AC241.
What might be other ambushes?
--
Tony Williams. |
|
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|
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Tony Williams
Guest
|
| Posted:
Mon Nov 14, 2005 7:14 am Post subject:
Re: Driving a Discrete Input Having a 50ohm Impedance |
|
|
In article <43764B93.6070202@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
| Quote: | Maybe even change the 74AC240 for a mosfet.
Can you find one that has 20pF input capacitance, handles 200mA
at low threshold, and costs 27 cents? I assume it would be a
P-channel.
|
No.
--
Tony Williams. |
|
| Back to top |
|
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Winfield Hill
Guest
|
| Posted:
Mon Nov 14, 2005 5:35 pm Post subject:
Re: Driving a Discrete Input Having a 50ohm Impedance |
|
|
Tony Williams wrote...
| Quote: |
Fred Bloggs wrote:
Maybe even change the 74AC240 for a mosfet.
Can you find one that has 20pF input capacitance, handles 200mA
at low threshold, and costs 27 cents? I assume it would be a
P-channel.
No.
|
What about ON Semi's NTA4151P that was just suggested? Only
0.49 ohms with -1.8V drive, and 16 cents each. Oops, 156pF of
input capacitance, well OK, then a similar but smaller MOSFET.
The NTA4151 can handle 760mA, well over the 200mA requirement.
--
Thanks,
- Win |
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|
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Fred Bloggs
Guest
|
| Posted:
Mon Nov 14, 2005 5:35 pm Post subject:
Re: Driving a Discrete Input Having a 50ohm Impedance |
|
|
Tony Williams wrote:
| Quote: | In article <dl4qvs02hk9@drn.newsguy.com>,
Winfield Hill <Winfield_member@newsguy.com> wrote:
This is the kind of circuit we think about from time to time,
and quickly reject. For example, a flying opamp with supplies
pinned to the rail of another opamp, to extend ordinary cheap
+/-15V capable opamps to +/-30V operation, rather than using
a HV opamp to begin with. In the case of some steady-state AC
signal applications, the flying opamp's entire power supply can
also be obtained from the first opamp's AC output. Arrggghhh!
A novel circuit that is interesting to look at though.
I'd never thought about a single 5V pump of the rail
and a simultaneous 5V swing of the output.
Will the output reliably swing from 0 to 10v in about
50nS? A possible killer might be load capacitance and
the effective transient current load on the 74AC241.
What might be other ambushes?
|
The OP said coax- if its RG-58 then the transition times should be <10ns. |
|
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