Coax cable driving - more drive than LM6181

[Tim Shoppa]

I've been using LM6181's to drive 50 ohm instrumentation interconnects
- typically a few feet of coax but sometimes longer - in the very low
tens of MHz with up to 5V or so swings. Works pretty well at the gains
I need (+2 to +10), is very stable into capacitive loads, and almost
all I know about high-speed medium-current opamps is based around this
chip and the cookbook recipes in National's app notes.

If I wanted to beef these outputs up - maybe up to 20V swing, maybe
better phase response to non-sine waveforms in the 20-30MHz region -
what's the next step up? When I asked this 15 years ago there were
higher-voltage specialty hybrids that did the beefier jobs, but I
honestly don't know if these exist anymore. There are lots of new
video amps that drive a volt or two into coax or twisted-pair into the
100MHz region but I don't see a lot of stuff beyond a few volts. I am
not incredibly price-sensitive (meaning I'm paying $2.00 each for the
6181's but if I found something twice as good at ten times the price
nobody would even blink, they'd just pat me on the back for doubling
the bandwidth and/or output swing).

Clearly I'm asking for twice as much swing, and twice as much
bandwidth, so I think I'm asking about 4 times the slew rate, right?

It's not that I'm unwilling to learn stuff beyond cookbook current-
mode op-amp recipes, but I'd really appreciate it if the cookbook
recipes I've been using in the past are directly applicable.

Tim.

 

[John Larkin]

I've been using LM6181's to drive 50 ohm instrumentation interconnects
- typically a few feet of coax but sometimes longer - in the very low
tens of MHz with up to 5V or so swings. Works pretty well at the gains
I need (+2 to +10), is very stable into capacitive loads, and almost
all I know about high-speed medium-current opamps is based around this
chip and the cookbook recipes in National's app notes.

If I wanted to beef these outputs up - maybe up to 20V swing, maybe
better phase response to non-sine waveforms in the 20-30MHz region -
what's the next step up? When I asked this 15 years ago there were
higher-voltage specialty hybrids that did the beefier jobs, but I
honestly don't know if these exist anymore. There are lots of new
video amps that drive a volt or two into coax or twisted-pair into the
100MHz region but I don't see a lot of stuff beyond a few volts. I am
not incredibly price-sensitive (meaning I'm paying $2.00 each for the
6181's but if I found something twice as good at ten times the price
nobody would even blink, they'd just pat me on the back for doubling
the bandwidth and/or output swing).

Clearly I'm asking for twice as much swing, and twice as much
bandwidth, so I think I'm asking about 4 times the slew rate, right?

It's not that I'm unwilling to learn stuff beyond cookbook current-
mode op-amp recipes, but I'd really appreciate it if the cookbook
recipes I've been using in the past are directly applicable.

Tim.

I like to use two THS3062 sections working in parallel, with each
driving the output pin through 100 ohms. It's one of the few really
fast dual opamps with this much voltage swing. They will get hot, so
use the power-pad heatsinking thing.

John

 

[Clifford Heath]

Clearly I'm asking for twice as much swing, and twice as much
bandwidth, so I think I'm asking about 4 times the slew rate, right?
It's not that I'm unwilling to learn stuff beyond cookbook current-
mode op-amp recipes, but I'd really appreciate it if the cookbook
recipes I've been using in the past are directly applicable.

Google up "White's follower". Three transistors and a few passives,
great for driving heavy capacitive loads.

Clifford Heath.