fully differential opamp implementation

[tiger66]

Sounds like you have your cap in series with your signal, & your
resistor going to ground, instead of the other way around. Swap them &
it should work. (I'm assuming that your LPF is a simple RC
combination, rather than part of a feedback loop for an opamp.)

PS: When posting in a thread like this, please try to snip out the
quoted material that's unrelated to your own comments. Doing so makes
it a lot easier to understand a person's posts. :^)

--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\|/ \|/ Perna condita delenda est
---^----^---------------------------------------------------------------

Hi Lionel
I think my LPF has the correct configuration as I drew the LPF based
on the wikipedia's LPF layout.

Yes, I am just trying to use the simple RC circuit.

I also tried swap C and R. Unfortunately, my output waveform just flip
vertically. (i.e. it starts from a certain voltage and drops to 0 and
stays at 0 volt)
The waveform looks like a LPF in frequency domain, but I use
oscilloscope to observe my signal. I don't understand why the output
is no longer the shape of my input (a square wave)

Any idea?

Thanks in advance for your help

 

[Lionel]

On 29 Mar 2007 00:13:51 -0700, "tiger66" <[email protected]>
wrote:
[...]

Yes, I am just trying to use the simple RC circuit.

I also tried swap C and R. Unfortunately, my output waveform just flip
vertically. (i.e. it starts from a certain voltage and drops to 0 and
stays at 0 volt)

Well, that's what you'd expect from a high pass filter. And looking
back at your original post, I've just noticed that your description
("it starts from 0 and increases to a steady state value") is actually
what you *should* get from a low pass filter, so it sounds like you
got it right in the first place, but weren't clear on the concept of
what a LP filter is supposed to do.
My apologies for being lazy when I read your description.

The waveform looks like a LPF in frequency domain, but I use
oscilloscope to observe my signal. I don't understand why the output
is no longer the shape of my input (a square wave)

Well no, of course it no longer looks like a square wave! Your filter
is *supposed* to change the shape of the waveform - that's what it's
for. ;^)

Any idea?

Yes. You said that you wanted to get rid of a DC bias (offset
voltage). What you ended up with after my suggestion (voltage drops to
zero) is exactly what you asked for (which is a HP filter, not an LP
filter), because it's getting rid of your DC offset voltage. If your
pulses are too short, that's easy to fix - just increase the value of
your filter capacitor. Try 100nF for starters, & adjust from there,
according to whether you want the pulses shorter or longer.

 

[Lionel]

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

<grin> Good comeback. ;^)

 

[tiger66]

Hi Lionel
Thanks for the clarification about the LPF and HPF.

My chopper stabilized amplifier need a LPF at the end.
The circuit is suppose to modulate the input signal to a higher
frequency. Then insert the noise, which will be at lower frequency.
Finally, a demodulator to move the input signal back to the lower
frequency and the noise to the higher frequency.

I think my demodulator doesn't work, because my noise is still at the
lower frequency(DC offset) than my input signal frequency. So, after
I add the LPF, it just filtered out my input signal at the higher
frequency and I only left with the DC offset.

Do you have any experience designing chopper stabilized amplifier? Can
I send you the circuit that I am trying to simulate and the circuit I
have? Maybe you could spot my problems by comparing the two?

Please let me know

Thanks in advance for your help

On 29 Mar 2007 00:13:51 -0700, "tiger66" <[email protected]>
wrote:
[...]

Yes, I am just trying to use the simple RC circuit.

I also tried swap C and R. Unfortunately, my output waveform just flip
vertically. (i.e. it starts from a certain voltage and drops to 0 and
stays at 0 volt)

Well, that's what you'd expect from a high pass filter. And looking
back at your original post, I've just noticed that your description
("it starts from 0 and increases to a steady state value") is actually
what you *should* get from a low pass filter, so it sounds like you
got it right in the first place, but weren't clear on the concept of
what a LP filter is supposed to do.
My apologies for being lazy when I read your description.

The waveform looks like a LPF in frequency domain, but I use
oscilloscope to observe my signal. I don't understand why the output
is no longer the shape of my input (a square wave)

Well no, of course it no longer looks like a square wave! Your filter
is *supposed* to change the shape of the waveform - that's what it's
for. ;^)

Any idea?

Yes. You said that you wanted to get rid of a DC bias (offset
voltage). What you ended up with after my suggestion (voltage drops to
zero) is exactly what you asked for (which is a HP filter, not an LP
filter), because it's getting rid of your DC offset voltage. If your
pulses are too short, that's easy to fix - just increase the value of
your filter capacitor. Try 100nF for starters, & adjust from there,
according to whether you want the pulses shorter or longer.

--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\|/ \|/ Perna condita delenda est
---^----^---------------------------------------------------------------

 

[tiger66]

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 or anyone
Do you have any article or do you know where I can find information
that explain why VCVS can be used for fully differential amplifier?

If you have any articles, could you please send me a copy?

I would really appreciated

Thanks