1. Home
  2. Electronics Design
  3. Differential input and output op-amps. When do you use them?

Differential input and output op-amps. When do you use them?

I tend to keep things single ended and only on rare occasion have I resorted to a differential analog signal.

When interfacing to a 16+ bit ADC with differential inputs. Then I'll generally make the entire circuit differential as far up as practical to the sensor.

Also when the signal has to travel through a wiring harness or pass a noisy part of the board.

I have also played games with psudo-differential with those types of input where a dedicated signal reference trace parallel to the feed trace (or twisted pair) from the "-" input to the local source "ground" when the performance is not THAT critical.

Are there any other places where a differential signal is key to getting the performance and cannot be achieved readily with single ended?

Reply to
mook johnson
Loading thread data ...

It can give you extra voltage swing into floating loads, and can help if even-order harmonics are a particular problem in the application.

Some types of filters (e.g. crystal lattice BP and some types of all-pass) are easier to build with differential signals. Usually I use a transformer for that sort of thing, but some signals have to be DC-coupled.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 USA 
+1 845 480 2058 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

If you are concerned with distortion of signal on long transmission lines, and pickup/radiation, differential is preferred.

Long lines, before 1950, meant a few miles. In 1970, long lines were a few meters. In 2013, long lines are a few centimeters.

Reply to
whit3rd

Except at the DMV or VA.

Reply to
Michael A. Terrell

In leapfrog (ladder) active filter (i.e. continuous time) design, you can use a differential output rather than add inverting stages.

Reply to
miso

there IS something new under the sun...

FULLY differential op amps...

these are cool..

formatting link

Mark

Reply to
Mark

They've been around for 10 or 15 years- useful for driving diff inputs of ADCs if you are not too fussy about DC performance.

Eg.

formatting link

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

More like 50 years old. They had them back in the uA709/uA741 days.

?-)

Reply to
josephkk

Sounds like the uA733 differential-in, differential-out video amplifier :-)

Reply to
upsidedown

Or the uA592, which could work at gains below 10! I used those a fair amount back around 1980.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Very popular in the early 5.25" floppy drives. :)

Reply to
Michael A. Terrell

They're still around, in large quantities.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Yes, almost anything that involves a sensor. Anywhere you need to reduce noise which is just about anywhere. One amp to another etc. Strain guages etc

Reply to
gyansorova

On a sunny day (Thu, 14 Feb 2013 10:23:25 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :

which is just about anywhere. One amp to another

I once designed a system that send audio in a building to many offices and spaces, it used differential drive on some cable. Technician installed it, and they switched it on. I got a call that 'There is a hum in the audio'. Went there, had a look, only one leg of the diff pair was driven. Asked the technician to check the cables for the other signal. 'Oh, he replied', I noticed there were 2 signal leads, we can use that free wire for other things, so I disconnected that." I started to attempt to explain differential signals to him, but somehow the bell did not ring on the other side so to speak. I reconnected it myself.

Reply to
Jan Panteltje

The last I saw were in real cheap DIY C-band sat & 5.25" disk drives. The 3.5" drives I've looked at all had a ASIC chip with the video amp built in. I replaced a handful in Commodore 1541 drives over the years for a variety of failure modes.

Reply to
Michael A. Terrell

There are not many full diferential opamps; so the choice is limited. This reflects the fact that full diferential opamps are not very useful unless for special applications. Typical applications are audiofoolery, and DSL or video line drivers and receivers.

It is hard to find full diff opamp that performs to 16+ bit accuracy, while satisfying power and other requirements. It is easier to build differential drive using traditional opamps.

I tried with differential filters using full differential opamps. It turned out to be better convert the signal from diff to single ended, use traditional filter, and then convert back to differential.

Are you talking about differential signals or differential opamps?

Vladimir Vassilevsky DSP and Mixed Signal Consultant

formatting link

Reply to
Vladimir Vassilevsky

If you use a single-ended amp with the standard 4 resistor network for accepting diff inputs, then the impedance seen at the plus and minus inputs are are different for common-mode signals versus diff signals. A diff amp can solve this

Bob

Reply to
radams2000

The drawbacks of simplest differential amp noted in every textbook; standard cure is 2- or 3- opamp solution. Full diff opamp could be used as well. However there is a lot more choice and flexibility with standard opamps as far as parameters and performance.

I considered full diff opamps for precision ADC application; ended up using traditional schematics.

BTW, measured amazing 130+ dB of CMRR (at 50 Hz) with LT1167.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

formatting link

Reply to
Vladimir Vassilevsky

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.