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What IC drives digital data over coax with 50 ohm load?

J

Jmm39531

Jan 1, 1970
0
Hi,
Can somebody recommend an IC that can drive TTL or LVTTL digital data (1Mbps to
25Mbps) over a coax with a 50 ohm load? At 5V do I need a 100mA output capable
driver? The '244 can only drive about 24mA according to the datasheet I just
read. Do I need to use a high-output-current op-amp or is there a digital
logic IC designed for this?
Thanks for any info.
 
J

Joerg

Jan 1, 1970
0
You can also do that at lower levels (ECL translators). If it has to be TTL there
is a chance to hook up driver stages in parallel but only if they are on the same
chip. Be careful though and check with the maufacturer before doing that.

Regards, Joerg.
 
W

Winfield Hill

Jan 1, 1970
0
Jmm39531 wrote...
Can somebody recommend an IC that can drive TTL or LVTTL digital data
(1Mbps to 25Mbps) over a coax with a 50 ohm load? At 5V do I need a
100mA output capable driver? The '244 can only drive about 24mA
according to the datasheet I just read. Do I need to use a high-
output-current op-amp or is there a digital logic IC designed for this?

We show a surprisingly simple solution to this problem in The Art
of Electronics, page 612, figure 9.42, which consists of a 2n4401
emitter-follower transistor driving a coax cable loaded with 50 ohms
at the far end. The 2n4401 can easily drive 150mA for a 3V TTL swing
into 50 ohms, without excessively loading down the TTL source, and
the 50-ohm termination resistor provides rapid pulldown at the end of
the pulse. I'm not sure if this simple circuit will work to 25MHz.

Thanks,
- Win

whill_at_picovolt-dot-com
 
J

John Fields

Jan 1, 1970
0
Hi,
Can somebody recommend an IC that can drive TTL or LVTTL digital data (1Mbps to
25Mbps) over a coax with a 50 ohm load? At 5V do I need a 100mA output capable
driver? The '244 can only drive about 24mA according to the datasheet I just
read. Do I need to use a high-output-current op-amp or is there a digital
logic IC designed for this?

---
You might want to take a look at

http://www.micrel.com/_PDF/mic4451.pdf

If you drive the cable with 10V through a 50 ohm series resistor and
terminate it with a 50 ohm load, you'll get 5V across the output and
the cable will look entirely resistive to the driver, which looks like
it's capable of <10ns edges with a resistive load.
 
F

Fred Bloggs

Jan 1, 1970
0
Jmm39531 said:
Hi,
Can somebody recommend an IC that can drive TTL or LVTTL digital data (1Mbps to
25Mbps) over a coax with a 50 ohm load? At 5V do I need a 100mA output capable
driver? The '244 can only drive about 24mA according to the datasheet I just
read. Do I need to use a high-output-current op-amp or is there a digital
logic IC designed for this?
Thanks for any info.

Please view in a fixed-width font such as Courier.


R-ARRAY
HC244 +--------+
+---------+ | 75 |
| O0+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O1+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O2+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O3+-|--/\/\--|-+
| | | | | 39
| | | 75 | +---/\/\-->
| O4+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O5+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O6+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O7+-|--/\/\--|-+
+---------+ | |
+--------+
 
F

Fred Bloggs

Jan 1, 1970
0
Winfield said:
Jmm39531 wrote...



We show a surprisingly simple solution to this problem in The Art
of Electronics, page 612, figure 9.42, which consists of a 2n4401
emitter-follower transistor driving a coax cable loaded with 50 ohms
at the far end. The 2n4401 can easily drive 150mA for a 3V TTL swing
into 50 ohms, without excessively loading down the TTL source, and
the 50-ohm termination resistor provides rapid pulldown at the end of
the pulse. I'm not sure if this simple circuit will work to 25MHz.

It may work to 25MHz when driven by F logic, but prone to oscillation
and total destruction if any load reactance at all- may have to add
small collector damping resistor and make it saturate. I continue to
hold out hope that one of these days AoE will contain something I can
actually use though...
 
M

maxfoo

Jan 1, 1970
0
Hi,
Can somebody recommend an IC that can drive TTL or LVTTL digital data (1Mbps to
25Mbps) over a coax with a 50 ohm load? At 5V do I need a 100mA output capable
driver? The '244 can only drive about 24mA according to the datasheet I just
read. Do I need to use a high-output-current op-amp or is there a digital
logic IC designed for this?
Thanks for any info.

Isn't the usual procedure to use twin axial cable and a line driver/receiver
chip? how long a distance are we talking here?






Remove "HeadFromButt", before replying by email.
 
S

Spehro Pefhany

Jan 1, 1970
0
Please view in a fixed-width font such as Courier.


R-ARRAY
HC244 +--------+
+---------+ | 75 |
| O0+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O1+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O2+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O3+-|--/\/\--|-+
| | | | | 39
| | | 75 | +---/\/\-->
| O4+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O5+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O6+-|--/\/\--|-+
| | | | |
| | | 75 | |
| O7+-|--/\/\--|-+
+---------+ | |
+--------+


Absolute maximum Vcc or ground current is +/-70mA or +/-75mA for the
HC244. Maybe the 74AC244 (TI's is rated at 200mA and 50mA/output).

Best regards,
Spehro Pefhany
 
F

Fred Bloggs

Jan 1, 1970
0
Spehro said:
Absolute maximum Vcc or ground current is +/-70mA or +/-75mA for the
HC244. Maybe the 74AC244 (TI's is rated at 200mA and 50mA/output).

Best regards,
Spehro Pefhany

That would be right- the IC delivers 60mA in this circuit- so where is
the problem?
 
S

Spehro Pefhany

Jan 1, 1970
0
That would be right- the IC delivers 60mA in this circuit- so where is
the problem?

Hmm...okay.

Best regards,
Spehro Pefhany
 
F

Fred Bloggs

Jan 1, 1970
0
Spehro said:
Hmm...okay.

Best regards,
Spehro Pefhany

Actually it will never deliver more than 52mA, when the HC244 drain
resistance is assumed zero, and the average should typically be 1/2
that. It will be good for maybe 150' of RG-58 at 40MHz using the typical
transition times and propagation delays of 10ns-otherwise it fails- an
indefinite duration output short gets 80-100mA peak- but even then if
the output is 50%- it is under that nebulous absolute maximum.
 
J

John Larkin

Jan 1, 1970
0
Hi,
Can somebody recommend an IC that can drive TTL or LVTTL digital data (1Mbps to
25Mbps) over a coax with a 50 ohm load? At 5V do I need a 100mA output capable
driver? The '244 can only drive about 24mA according to the datasheet I just
read. Do I need to use a high-output-current op-amp or is there a digital
logic IC designed for this?
Thanks for any info.

If you source-terminate at the driver (put a 50 ohm resistor between
the driver and the coax) and leave the far end open, you get 5 volts
at the end but only need to drive 100 ohms; the problem is cut in
half. A few 'HC or 'AC gates in parallel should work nicely as the
driver.

John
 
J

Joerg

Jan 1, 1970
0
Why can't you use TTL-ECL converters and drive it at lower ECL levels? ECL was
designed to be run across such impedances. Anything you do with TTL chips here will
result in driver devices that become pretty toasty. I have seen 50 ohm cable
drivers designed with TTL where the white lettering on the chips turned amber
within days.

Regards, Joerg.
 
A

Alan Ronemus

Jan 1, 1970
0
The 75ALS174 will do the job. It has a max output of 60 mA, which is sufficient
to make a logic high although it won't give you 5 V. The data sheet from TI
rates it to 20 MHz.
 
M

Marlboro

Jan 1, 1970
0
maxfoo said:
Isn't the usual procedure to use twin axial cable and a line driver/receiver
chip? how long a distance are we talking here?
Yep, forget about your digital stuff. Sorry if I rephrase the question
:
"Can somebody recommend an IC that can drive A SIGNAL (1M HZ to
25MHZ) over a coax with a 50 ohm load?" you are free to go now...
 
M

maxfoo

Jan 1, 1970
0
Yep, forget about your digital stuff. Sorry if I rephrase the question
:
"Can somebody recommend an IC that can drive A SIGNAL (1M HZ to
25MHZ) over a coax with a 50 ohm load?" you are free to go now...

Let me rephrase the question:

"Does anybody have a bullet that will fit into a 357 magnum, so I can blow my
brains out?"

you're free to roam now...




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