# fully differential opamp implementation

T

#### tiger66

Jan 1, 1970
0
Hi All
I need a fully differential opamp for my circuit design.
Unfortunately, the simulator I use, multisim, does not have a build in
fully differential opamp.

So, I was wondering if any of you have experience implementing a fully
Any resources (i.e. netlist, circuit diagram) will be really
appreciated

thanks

J

#### John Larkin

Jan 1, 1970
0
Hi All
I need a fully differential opamp for my circuit design.
Unfortunately, the simulator I use, multisim, does not have a build in
fully differential opamp.

So, I was wondering if any of you have experience implementing a fully
Any resources (i.e. netlist, circuit diagram) will be really
appreciated

thanks

google "three opamp differential amplifier". Or cheat and use a Spice
vcvs, which is a perfect diffamp.

John

F

#### Fred Bartoli

Jan 1, 1970
0
John Larkin a écrit :

But the output is not diff.

Or cheat and use a Spice
vcvs, which is a perfect diffamp.

But the output isn't referenced WRT to common mode. You'll have to use 2
VCVS for this, or 3 if you don't want to reflect the output current to
the CM ref input.

J

#### Jim Thompson

Jan 1, 1970
0
Hi All
I need a fully differential opamp for my circuit design.
Unfortunately, the simulator I use, multisim, does not have a build in
fully differential opamp.

So, I was wondering if any of you have experience implementing a fully
Any resources (i.e. netlist, circuit diagram) will be really
appreciated

thanks

I'll knock one out for you for $25K, payable upfront ;-) ...Jim Thompson T #### tiger66 Jan 1, 1970 0 google "three opamp differential amplifier". Or cheat and use a Spice vcvs, which is a perfect diffamp. John Thanks very much John for the response. I was designing my fully differential opamp using "three opamp differential amplifier" concept initially because it only requires two basic opamps and 4 resistors which my simulator has all the components. Unfortunately, after I connect the input of the three opamp differential amplifier to the output of my modulator, I get "timestep too small" error message when simulate. Do you know how to debug this problem? Currently, my circuit can simulate fine using vcvs as you suggested. Do you know if there are any disadvantage for using vcvs instead of normal fully differential opamp? Would it affect my output signal spectral analysis significantly? Thanks again for your assistant T #### tiger66 Jan 1, 1970 0 John Larkin a écrit : But the output is not diff. Or cheat and use a Spice But the output isn't referenced WRT to common mode. You'll have to use 2 VCVS for this, or 3 if you don't want to reflect the output current to the CM ref input. Hi Fred Are you talking about "common mode feedback circuit"? I was reading about that when I was researching about fully differential opamp. Unfortunately, I can't find a detailed explanation about the CMFC(i.e. the articles only show the schematics but not the values I should use for MOSFET, capacitor, resistor and reference voltage). So I really don't know how to integrate CMFC with opamp. Do you have experience implementing the fully differential opamp? Can you send me a netlist of that, so I know what values to put for each components Thanks T #### tiger66 Jan 1, 1970 0 I'll knock one out for you for$25K, payable upfront ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

Thanks for the offer Jim, but I am a poor engineering student. So I
can't afford that kind of money

But I think I can offer you $1 if you can help me design a fully differental opamp.$2 if you can help me design a chopper stabilized
amplifier

J

#### Jim Thompson

Jan 1, 1970
0
Thanks very much John for the response.

I was designing my fully differential opamp using "three opamp
differential amplifier" concept initially because it only requires two
basic opamps and 4 resistors which my simulator has all the
components.

Unfortunately, after I connect the input of the three opamp
differential amplifier to the output of my modulator, I get "timestep
too small" error message when simulate. Do you know how to debug this
problem?

Currently, my circuit can simulate fine using vcvs as you suggested.

Do you know if there are any disadvantage for using vcvs instead of
normal fully differential opamp? Would it affect my output signal
spectral analysis significantly?

"Perfect" VCVS has no delay or phase shift, infinite bandwidth... all
of the constituents of instability.

Back up and study how to model an OpAmp, with at least one pole.

...Jim Thompson

E

#### Eeyore

Jan 1, 1970
0
tiger66 said:
Hi All
I need a fully differential opamp for my circuit design.
Unfortunately, the simulator I use, multisim, does not have a build in
fully differential opamp.

I suspect you don't mean 'op-amp' you probably mean differential amplifier.

Graham

J

#### John Larkin

Jan 1, 1970
0
John Larkin a écrit :

But the output is not diff.

Is that what he meant by "fully differential"?
Or cheat and use a Spice

But the output isn't referenced WRT to common mode. You'll have to use 2
VCVS for this, or 3 if you don't want to reflect the output current to
the CM ref input.

Whatever.

John

J

#### John Larkin

Jan 1, 1970
0
Thanks very much John for the response.

I was designing my fully differential opamp using "three opamp
differential amplifier" concept initially because it only requires two
basic opamps and 4 resistors which my simulator has all the
components.

Unfortunately, after I connect the input of the three opamp
differential amplifier to the output of my modulator, I get "timestep
too small" error message when simulate. Do you know how to debug this
problem?

Currently, my circuit can simulate fine using vcvs as you suggested.

Do you know if there are any disadvantage for using vcvs instead of
normal fully differential opamp? Would it affect my output signal
spectral analysis significantly?

If you are trying to evaluate the spectral properties of a diff-output
mixer, the vcvs is literally ideal. An opamp thing will just add its
own complications and abberations.

If you eventually want to build the circuit, you will need a diffamp,
I suppose. Or a transformer. There are some nice, integrated, very
fast diffamps around these days.

John

M

#### Marte Schwarz

Jan 1, 1970
0
Hi,
If you eventually want to build the circuit, you will need a diffamp,
I suppose. Or a transformer. There are some nice, integrated, very
fast diffamps around these days.

If he search the spice models he should look by the factories...

http://www.ti.com/litv/zip/sloj158

may be a good start.

Marte

T

#### tiger66

Jan 1, 1970
0
Hi,

If he search the spice models he should look by the factories...

http://www.ti.com/litv/zip/sloj158

may be a good start.

Marte

Thanks, I guess I need to learn how to use Pspice now.

Can Pspice generate the Power Spectral Density of a signal? I need to
do some spectral analysis in my circuit

Thanks

B

#### Ban

Jan 1, 1970
0
tiger66 said:
Hi All
I need a fully differential opamp for my circuit design.
Unfortunately, the simulator I use, multisim, does not have a build in
fully differential opamp.

So, I was wondering if any of you have experience implementing a fully
Any resources (i.e. netlist, circuit diagram) will be really
appreciated

thanks

With the 3 opamp IA circuit you can just add an identical output
differential amp stage with the inputs inverted to obtain the other output
signal.

J

#### Jim Thompson

Jan 1, 1970
0
Thanks, I guess I need to learn how to use Pspice now.
Yes.

Can Pspice generate the Power Spectral Density of a signal? I need to
do some spectral analysis in my circuit

Thanks

Yes. You can do a spectral analysis in Probe, or can define
distortion analysis in the setups to get a table.

...Jim Thompson

M

#### Marte Schwarz

Jan 1, 1970
0
Hi tiger,
Thanks, I guess I need to learn how to use Pspice now.

You do not need to switch the program. Multisim can use (p)Spice models as
well as LTSpice for example. The changes that may have to be made are very
little and almost known in the support forums.

Marte

.... who prefers to use LTSpice

V

#### vasile

Jan 1, 1970
0
Thanks, I guess I need to learn how to use Pspice now.

Can Pspice generate the Power Spectral Density of a signal? I need to
do some spectral analysis in my circuit

Even it can, and even if you do it right on simulation, on the real
world will be different
(could be very different at high frequencies)
Focus on fully differential OA datasheet available at Analog Devices,
Texas Instruments and Linear Technology.
There are a lot of articles about those. Some clues: AD8132, THS4121,
LT1994

J

#### John Larkin

Jan 1, 1970
0
With the 3 opamp IA circuit you can just add an identical output
differential amp stage with the inputs inverted to obtain the other output
signal.

The cold end of the resistor going into the + input of the 3rd amp is
the output reference; it need not be ground.

John

T

#### tiger66

Jan 1, 1970
0
Hi tiger,

You do not need to switch the program. Multisim can use (p)Spice models as
well as LTSpice for example. The changes that may have to be made are very
little and almost known in the support forums.

Marte

... who prefers to use LTSpice

Hi All
Does anyone have experience using multisim?

I keeps getting a "timestep too small" error message when I simulate.
I tried all the debugging method I found online, but non of them
solves my problem

Any input/assistant on this would be really appreciated

thanks

E

#### Eeyore

Jan 1, 1970
0
tiger66 said:
Does anyone have experience using multisim?

I keeps getting a "timestep too small" error message when I simulate.
I tried all the debugging method I found online, but non of them
solves my problem

Any input/assistant on this would be really appreciated

I told you before.

It's crap.

Graham

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