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[SOLVED] what are the advantages of differential difference amplifier over OPAMPS

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ananthesh bhat

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Hello all,

I'm just analyzing the differential difference amplifier(DDA) circuits and thought DDA will be the good fit for the active filter designs. But I'm not clear with the advantages that I will get with DDAs. Can anybody explain what are the advantages of DDA over OPAMPs?
 

mtwieg

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Well if you need to do a differential difference operation, then that would take at least three op amps, while a DDA circuit could do it with much fewer transistors.
 

hobbyckts

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Is Instrumentation amplifier and differential difference amplifier is same in their functionality?:roll:
 

mtwieg

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Is Instrumentation amplifier and differential difference amplifier is same in their functionality?:roll:
No, an inamp is just a differential amplifier with very good specs.
 

SunnySkyguy

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  1. Differential signals are used when common mode voltage interference causes a differential signal from imbalanced line impedance and ground.
  2. The result can be susceptible (ingress) or cause inteference to others (egress) and be conducted and/or radiated. So Coax is good, differential is good shielded differential is better unless capacitance loading on line length causes current limits. or frequency response issues on high imepdance sources.
  3. In special cases, the common mode high impedance signal is buffered drives the shield to reduce capacitance and improve immunity on a differential pair. ( Guarding)
  4. Single ended line and return or untwisted pairs (UTP) can be prone to CM interference on lengths more than a few feet or less if near current spikes or power lines. UTP like CAT-5 cable in ethernet they use controlled impedance terminations at each end and miniature CM choke and differential transformer splitter for either full, or half duplex, but usually half from crosstalk issues.
  5. For example current mode logic is constant current on the supply so no crosstalk of ground shift.
  6. HIgh Impedance biometric signals are differential IA's because the common mode signals are in 100V range and differential EEG signals are in the microvolt range.
  7. RS485 is differential input to receive higher bandwidth over a longer distance or MHz-meter product limits.
  8. Controlled impedances on Ethernet or other high speed networks are differential for similar reasons.
  9. Slow shared bi-directional signals can be a 1 wire solution with ground only under controlled & protected circumstances used for low cost sensors
  10. whereas current-loop ( e.g. std 4-20mA ) is inherently more immune to high e field common mode (CM) noise. In this case the converts the current to a voltage with a differential input.
  11. Common Mode rejection has bandwidth limitations defined in specs degrading with rising f, so a CM ferrite choke is often used to raise the CM impedance with two CM shunt caps terminating the noise or a differential termination pair of resistors with centre tap to some voltage or ground.
  12. These chokes are also used for egress like all 75 Ohm VGA RGB video cables, since they are only singled ended and appear like plastic cylinder (split or whole) molded around the ferrite around the cable.
 

mtwieg

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Then how about differential difference amplifier
A DDA is just what its name says. It has two differential inputs, and the output is the difference of those two differences. An INAMP just has one differential input, and the output is that amplified difference. Both may have an externally settable common mode output level as well.
 

SunnySkyguy

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  1. All Op Amps are differential. If one side is referenced locally then interface is only single ended.
  2. All Instrument Amps use at least 3 Op Amps to share a common gain setting resistor and buffer each input and then combine the 2 into 1.
    400px-Op-Amp_Instrumentation_Amplifier.svg.png
  3. The result is a significant improvement in inverting input impedance since both inputs use a non-inverting high impedance input AND a significant improvement in Common Mode Rejection ratio (CMRR) AND a much higher impedance possible ( instead of limited by the inverting gain setting resistor.
  4. A DDA is just an academic acronym. What's the difference with a differential input? nothing just words.
    If not differential input it is single ended input with a local reference, that may be inverting or not.
  5. Another term mixed up by this question only used in Acadamia is FDDA
  6. The fully differential difference amplifier is a balanced output differential difference amplifier. But in industry we just say it has differential in and out. However there is no one standard. It could be ECL, Video or an Op Amp with Hi/lo Z for In/out or even open collector out.

    thats all.

But I suppose the term has some benefit to show the difference ( no pun ) with a Multi Input Single-Ended Inverting Summation Amplifier. or if I want ... MISEISA circuit,,no forget that....
2000px-Op-Amp_Summing_Amplifier.svg.png
 

KlausST

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Hi,

i found a document:


Maybe this clarifies things.

Klaus
 

SunnySkyguy

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Just as a I thought a localized acronym only used in very few Academia of very little minor value over the universal IA .

The two-day Microwave Integrated Circuits and System Workshop is a first event ofits kind taking place at the Technion, inspired by an ambition to bring together aca-demic and industrial researchers with shared interests on microwave engineering.

Got any part numbers? :cool:

I dont think high impedance DDAs and Microwave are used in the same circuit.,
 

mtwieg

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Just as a I thought a localized acronym only used in very few Academia of very little minor value over the universal IA .
It's certainly strange to see DDAs mentioned in a microwave context, but they're certainly not rare in signal conditioning applications. I've seen them implemented in receivers for image reject filters and such, though they're not always referred to by that name. I'm really surprised people here aren't familiar with them.

Got any part numbers? :cool:
A wide swing folded cascode?? Heh, got any part numbers?
 

SunnySkyguy

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Of course they exist but then so does my topology in many diverse impedances and arangements.. as voltage or current sources


I prefer Differential In/Out out DIO or for current or voltage out DVICO or DVIVO or DCICO

but I have never seen it used on a datasheet.
 

mtwieg

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My point is basically that DDA isn't just a weird term for a differential amp. It's a unique device with two differential input pairs.

I've seen the symbol referenced on Linear Tech's datasheets in the past, probably in their fancy SMPS controllers. Though it's likely the actual circuit is a cascade of sum and difference amps rather than a "true" DDA in a single stage.
 

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