Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Op-Amp used as comparator --pros and cons

Status
Not open for further replies.

funberry

Junior Member level 1
Joined
Jan 23, 2006
Messages
17
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,547
Hi everyone

I know that traditionally, an op-amp connected as a comparator consistently underperformed dedicated comparators in most respects.
But that was when we were comparing a 741 and a 311.
Op-amp technology has progressed faster than comparator technology.

We have today op-amps with exceptional speed, which I would expect to perform much better as a comparator.

To the point: I need a fast comparator, response time under 40ns. I don't have a fast comp, but I do have an LMH6655 (with 250MHz BW, 200v/us slew rate, 180mA output current). Can I expect this fast Op-amp to carry over his speed qualities to the comparator role?


One thing I'm questioning ( and please correct me on this) is the output topology. In an Op-amp, the output is a totem-pole, with transistors biased for best linearity, symmetrical current sourcing /sinking, and low current consumption. They're not optimized for being constantly slammed against the rails.

In a comparator, no attention is paid to linearity, noise, bias, etc, just to switching and avoiding saturation.

I'm concerned that ANY op-amp, no matter how fast, is susceptible to saturation if driven to the rails, leading to delays in coming out of saturation.
Are there any fast op-amps that have anti-saturation circuitry built-in? Or some for which saturation recovery delays may still be short enough to meet my 40ns response time need?

Thanks
Andy
 

Kral

Advanced Member level 4
Joined
Mar 28, 2005
Messages
1,326
Helped
280
Reputation
558
Reaction score
85
Trophy points
1,328
Location
USA
Activity points
13,417
Funberry,

I'm not familiar with the LM6655, so I don't know what its recommended power supply voltage is. However, with a 200v/uS (0.2v/nS) slew rate, you can expect a total voltage swing of 8V max. If you provide an external clamp to limit the output to say, 5V, the output stage would not saturate. A zener probably won't work, because of capacitance.
~
I don't know of any op-amps with built-in anti-saturation circuitry.

Regards,
Kral
 

    funberry

    points: 2
    Helpful Answer Positive Rating

funberry

Junior Member level 1
Joined
Jan 23, 2006
Messages
17
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,547
Sorry....

TYPO: that was an LMH6655, not an LM6655

Andy
 

Status
Not open for further replies.
Toggle Sidebar

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top