Breaking News
Comments
Newest First | Oldest First | Threaded View
audiopip
User Rank
Rookie
re: Engineer investigates a noisy, high gain amplifier
audiopip   9/2/2010 5:25:43 AM
NO RATINGS
I can add two little additional tales, one.. why does my amplfier oscillate at random, answer when the wife is using the (wireless) phone in the next room, and two, why does my servo controlled DC offset change when the sun comes out answer- photoelectric charge in the bare film integrator capacitors. Obvious when you know, much head scratching till you do! Always expect the unexpected.

bcarso
User Rank
Rookie
re: Engineer investigates a noisy, high gain amplifier
bcarso   8/31/2010 5:00:19 PM
NO RATINGS
AD does mention that the 797 needs a little resistance at the input to avoid oscillation at low gains---an unfortunate thing, as it inevitably adds thermal noise. But they point out that most of the time users will be needing gain, so in most applications it's not that big a deal. As far as keeping the bandwidth as low as possible--that's a half-truth. Given the right high-frequency modulation, you can get local rectification and spurious ~d.c. offsets, right down in your seismic signal passband. It's been noted that JFET input amplifiers are perhaps 100x less susceptible to this rectification than are bipolars. Sadly, most of them also have higher intrinsic voltage noise and higher temperature drift. You can roll your own from selected discretes, but that's anathema for volume production.

_hm
User Rank
CEO
re: Engineer investigates a noisy, high gain amplifier
_hm   8/24/2010 2:14:36 PM
NO RATINGS
This is really good article. James your approach to problem and eventually identifying noise source is commendable. I will remember and keep bandwidth only as much as is required.

Jimelectr
User Rank
Rookie
re: Engineer investigates a noisy, high gain amplifier
Jimelectr   8/21/2010 4:13:03 AM
NO RATINGS
I believe it, having run into similar problems with wideband op amps being used as audio amplifiers. I was working on a home project with a low noise, high bandwidth op amp, an AD797, I think it was, and I started hearing the noise level rise and fall, along with some faint whistling sounds in my headphones, particularly when the gain was turned DOWN. When I looked at the datasheet, I realized that all specs were tested at gains of +5, -4, or higher (in absolute value). The datasheet did not say that the op amp was not unity gain stable, but evidently it wasn't! When I turned the gain down to near unity, it started oscillating at about 200 MHz rail-to-rail, so all bets were off as far as audio specs went! Moral of the story is the same as in James Hahn's story: more bandwidth is not necessarily better. Also, beware of specs that aren't in the datsheet.

W1PK
User Rank
Rookie
re: Engineer investigates a noisy, high gain amplifier
W1PK   8/20/2010 7:23:18 PM
NO RATINGS
That's one way of looking at it. I usually treat this as an EMI problem. Hardening a product against electromagnetic interference requires considering a lot more effects than just radio signals. Of course, this is an entire engineering sub-specialty, but I recommend reading the EN 61000 series of standards as a start, to get a comprehensive look at what effects and levels to protect against in a typical industrial environment.

Dan Mitten
User Rank
Rookie
re: Engineer investigates a noisy, high gain amplifier
Dan Mitten   8/18/2010 7:01:10 PM
NO RATINGS
This is an important reminder to keep all non-signal inputs well by-passed! And, to keep the out of band gain as low as possible.

old account Frank Eory
User Rank
Rookie
re: Engineer investigates a noisy, high gain amplifier
old account Frank Eory   8/18/2010 4:35:00 PM
NO RATINGS
Great article James. I remember as a young engineer working on sensitive RF circuits, I was shocked the first time I ever saw local TV stations on a broadband spectrum analyzer. The signals were so big, how could I keep them from mixing with my UHF local oscillator? I quickly learned that cardinal rule you mentioned -- keep the system bandwidth as small as possible.



EE Life
Frankenstein's Fix, Teardowns, Sideshows, Design Contests, Reader Content & More
Max Maxfield

Synthesize Your Own RTOS for FREE!
Max Maxfield
6 comments
My chum Bob Zeidman, president of Zeidman Technologies, is always involved in something interesting. Some time ago, for example, he told me about SynthOS. This little scamp can ...

EDN Staff

11 Summer Vacation Spots for Engineers
EDN Staff
11 comments
This collection of places from technology history, museums, and modern marvels is a roadmap for an engineering adventure that will take you around the world. Here are just a few spots ...

Glen Chenier

Engineers Solve Analog/Digital Problem, Invent Creative Expletives
Glen Chenier
11 comments
- An analog engineer and a digital engineer join forces, use their respective skills, and pull a few bunnies out of a hat to troubleshoot a system with which they are completely ...

Larry Desjardin

Engineers Should Study Finance: 5 Reasons Why
Larry Desjardin
45 comments
I'm a big proponent of engineers learning financial basics. Why? Because engineers are making decisions all the time, in multiple ways. Having a good financial understanding guides these ...

Flash Poll
Top Comments of the Week
Like Us on Facebook
EE Times on Twitter
EE Times Twitter Feed

Datasheets.com Parts Search

185 million searchable parts
(please enter a part number or hit search to begin)