Differential to single ended

[Spehro Pefhany]

I read in sci.electronics.design that Spehro Pefhany <speffSNIP@interlog


Because the simpler circuit is quite satisfactory, AIUI.

How's the high frequency common-mode rejection? How's the common mode
rejection when the signal is small compared to the common-mode voltage
and you want to add gain at some point? (eg. Rg << Rf in my
schematic).

Best regards,
Spehro Pefhany

 

[Fred Bloggs]

Fred Bloggs wrote:
[..SNIP..]

If wideband differential to single-ended conversion is required with
minimal input loading, then you have this option in CFB- trim the R's
in-:
Please view in a fixed-width font such as Courier.




R R R R
+---/\/\--+------/\/\---+---/\/\--+------/\/\--+
| | | | |
--- | | | |
COM | |\ | | |\ |
+--|-\ | +--|-\ |
| >-------+ | >------+-> 2x(V2-V1)
+--|+/ +--|+/
| |/ | |/
| |
| |
V1 >--------+ |
|
|
V2 >--------------------------------+

Why would you use that nastly asymmetrical configuration instead of
the below?
Vo = (Vinv- Vnon-inv) * 2 * Rf/Rg
V(inv) |\|
o----------|+\
| >--+ _R_
+-|-/ | +-|___|-+
| |/| | | |
| | | |
| ___ | _R_ | |
+-|___|-+-|___|----| |
| Rf | |\| |
.-. +--|-\ | Vo
| | | >-+---o
Rg | | +--|+/
'-' Rf | |/|
| ___ R__ |
+-|___|-+-|___|----+
| | |
| |\| | .-.
+-|-\ | | |
| >--+ | | R
o---------|+/ '-'
V(non-inv) |/| |
|
===
GND

Thanks both,

I wasn't specific about the application.
The application is an output amplifier for a 300MHz clocked DDS,
with an output signal up to 150MHz or a bit less. It has
differential outputs, great for differential input mixers,
but less usefull for something else.
So the only choice to go single ended is the 4 resistor
subtractor with a damn fast amp such as the THS3001 CFA with
a BW of 420MHz(@gain=1), slewrate 6.5V/ns and 100mA output
current.
There are very few parts available and building my own is
not very rewarding either.

Rene

You do know there are RF transformers available for this?

 

[John Woodgate]

I read in sci.electronics.design that Spehro Pefhany <speffSNIP@interlog

How's the high frequency common-mode rejection? How's the common mode
rejection when the signal is small compared to the common-mode voltage
and you want to add gain at some point? (eg. Rg << Rf in my
schematic).

Nothing is perfect. If the simpler circuit isn't satisfactory, don't use
it. This is just the same as sneering at the 555, 324/358 and 741. If
they aren't good enough, don't use them. If they are good enough, why
not?

 

[Fred Bloggs]

How's the high frequency common-mode rejection? How's the common mode
rejection when the signal is small compared to the common-mode voltage
and you want to add gain at some point? (eg. Rg << Rf in my
schematic).

Best regards,
Spehro Pefhany

You have CM on the input buffers and the DA input- why would the
performance be any better?

 

[Spehro Pefhany]

Nothing is perfect. If the simpler circuit isn't satisfactory, don't use
it. This is just the same as sneering at the 555, 324/358 and 741. If
they aren't good enough, don't use them. If they are good enough, why
not?

If someone suggested an LM741C for a "professional audio" application,
would you speak up?

Best regards,
Spehro Pefhany

 

[John Woodgate]

I read in sci.electronics.design that Spehro Pefhany <speffSNIP@interlog

If someone suggested an LM741C for a "professional audio" application,
would you speak up?

For a new design, no. But there are quite literally millions of 741s
still in service in broadcast-standard equipment now.

These weren't designed by our primitive ancestors (who used AC/SP3RH as
active devices, or the new-fangled EF86 and Z759 later), but by us and
our contemporaries in our youth. Were we, and they, incompetent then?

 

[Rene Tschaggelar]

You do know there are RF transformers available for this?

The amplitude is somewhat low, but it is an idea.
I saw some in some catalogs. A mail is in the queue asking for
delivery conditions. 15weeks and 1000 pieces minimum ?
I hate doing designs where I don't have the parts on my
table before I start with it. Got burnt too often.

Rene

 

[Mac]

The amplitude is somewhat low, but it is an idea.
I saw some in some catalogs. A mail is in the queue asking for
delivery conditions. 15weeks and 1000 pieces minimum ?
I hate doing designs where I don't have the parts on my
table before I start with it. Got burnt too often.

Rene

I recently did a DDS board. I used a part from Coilcraft for the output
transformer. You might take a look at their offerings. Also, minicircuits
has some nice transformers. Once you get the signal single-ended, you can
use your CFA to boost it, if you like.

National has a couple of fast voltage feedback and current feedback amps
that may be worth your consideration. I don't have time to dig up the part
numbers right now, but at least one of them is carried by digikey.

Mac

 

[Fred Bloggs]

The amplitude is somewhat low, but it is an idea.
I saw some in some catalogs. A mail is in the queue asking for
delivery conditions. 15weeks and 1000 pieces minimum ?
I hate doing designs where I don't have the parts on my
table before I start with it. Got burnt too often.

Rene

All of the super-fast pipelined A/D's operating in the GHz range do the
conversion on a differential input at something like 1Vpp or less FS
range. Transformers are widely used to do the 50-ohm single-ended to
differential conversion over very wide bandwidths- you can just turn
this around. MiniCicuits is a major supplier. You can use the much older
line to handle the 150MHz.