Voltage Gain Switch (Design Help)

[Ken Smith]

On 18 Sep 2006 16:23:29 -0700,


No, you need to get LTSpice which is available for free from Linear
Technology. Or if you're on a Unix environment, look around for a
"vanilla" Spice3f5. I've heard that LTSpice runs under wine/Linux, but
I've never tried that.

I have. LTSpice runs quite will under wine. Mike at Linear tweeked it a
bit to make sure it did so.

Don't try LTSpice under "bochs". You will be old and grey before the sim
finishes.

 

[Fred Bartoli]

Ken Smith a écrit :

Ken Smith a écrit :

[.....]
I don`t understand the positive feedback arrangement. If you could work
out an example that would be great.

ASCII art:
R1 R2
10K 10K
---/\/\/------+-------/\/\----------
! !
---!-\ !
! >------------+-+-------
---!+/ !
! \
! / 10K
! \
! !
! A O-----+
------------> !
O \
! / 1K
GND \
!
GND

When the switch is at ground, the circuit is simple. Just ignore the
extra stuff and you have a gain = -1


In the other case, if Vout = 1V, the voltage at A=0.1 so R2 has 0.9V on it
and thus the input must be -0.8V

Should be:

"In the other case, if Vout = 1V, the voltage at A=0.090909... so R2 has
0.9090909...V on it and thus the input must be -0.8181818...V"

or make the divider 9K/1K

You are right. I can only put the error down to hunger. My wife called
me to dinner just as I typed the end of it so I didn't proof read it. I
think you will forgive me when I tell you that it was roast beef, fresh
green beans from the garden, fresh carrots from the garden and potatoes
also from the garden.

This circuit can be improved somewhat by placing a small capacitor from A
to GND. This eats up the switching glitches.

Or replace R1 with a cap, which will give a nice dual functions circuit.

 

[Glen Walpert]

Jim Thompson wrote...

On 18 Sep 2006, [email protected] wrote:

[snip]

So what you are trying to say is that, when the above circuit switches
the feedback, there might be some time of inestability in the circuit.
If that's what you meant there is no problem, there were relays before
that took forever to change the feedback, there's no problem with that
kind of inestability. How's this make before break circuit you mention ?

Did you miss my post...
http://analog-innovations.com/SED/VoltageGainSwitch.pdf

That doesn't Glen's address transient situations, does it? But
yes, I agree there isn't really a problem. The hc405x series
in fact switches so fast that ordinary opamps don't have time
to respond to open-switch situations, as they might with other
slower high-voltage, breakdown-protected CMOS switches. While
Glen's comments are on target for many if not most traditional
analog switch families, the hc405x switch family was first and
foremost a logic switch, with low-resistance low-capacitance
MOSFETs, and it responds rapidly, appropriate to that task.

These are also readily-available low-cost parts, and attractive
if they're buried deep inside a circuit, where static discharge
and low maximum-operating voltages may not be an issue, like
Leo's amplifier.

That said, there's still the issue of charge injection, where an
output pulse appears from the transient current injected into the
opamp's feedback loop from the MOSFET-switch gate-voltage swing.
Using low-value feedback divider resistors minimizes this affect.
Using low-capacitance switches also helps, but these are slow,
and therefore can exacerbate the open-switch problem.

I see the 74hc4052 with the enable tied low will guarantee that one of
the 4 switches will always be on, so that one of the 4 gains will
always be selected, and it handles 2 channels if they both always want
the same gain.

There are some seminar notes at Apex which discuss gain switching with
relays (pp 26-27 in pdf, seminar pp 34-35}:
<http://eportal.apexmicrotech.com/mainsite/apex_download/TechSem/OpAmps.pdf>
They refer to switching the feedback as "bad", but with the hc4052
switch configured so as not to allow open feedback it might be "best".

Glen

 

[Jim Thompson]

Jim Thompson wrote...

On 18 Sep 2006, [email protected] wrote:

[snip]

So what you are trying to say is that, when the above circuit switches
the feedback, there might be some time of inestability in the circuit.
If that's what you meant there is no problem, there were relays before
that took forever to change the feedback, there's no problem with that
kind of inestability. How's this make before break circuit you mention ?

Did you miss my post...
http://analog-innovations.com/SED/VoltageGainSwitch.pdf

That doesn't Glen's address transient situations, does it? But
yes, I agree there isn't really a problem. The hc405x series
in fact switches so fast that ordinary opamps don't have time
to respond to open-switch situations, as they might with other
slower high-voltage, breakdown-protected CMOS switches. While
Glen's comments are on target for many if not most traditional
analog switch families, the hc405x switch family was first and
foremost a logic switch, with low-resistance low-capacitance
MOSFETs, and it responds rapidly, appropriate to that task.

These are also readily-available low-cost parts, and attractive
if they're buried deep inside a circuit, where static discharge
and low maximum-operating voltages may not be an issue, like
Leo's amplifier.

That said, there's still the issue of charge injection, where an
output pulse appears from the transient current injected into the
opamp's feedback loop from the MOSFET-switch gate-voltage swing.
Using low-value feedback divider resistors minimizes this affect.
Using low-capacitance switches also helps, but these are slow,
and therefore can exacerbate the open-switch problem.

I see the 74hc4052 with the enable tied low will guarantee that one of
the 4 switches will always be on, so that one of the 4 gains will
always be selected, and it handles 2 channels if they both always want
the same gain.

There are some seminar notes at Apex which discuss gain switching with
relays (pp 26-27 in pdf, seminar pp 34-35}:
<http://eportal.apexmicrotech.com/mainsite/apex_download/TechSem/OpAmps.pdf>
They refer to switching the feedback as "bad", but with the hc4052
switch configured so as not to allow open feedback it might be "best".

Glen

The 74HC4052 (at least the version I designed for OnSemi) has a
break-before-make circuit built-in... built it's only ~3ns... not many
OpAmps will react to that.

I would expect most of the temporary upset to be due to capacitance of
the nodes of the feedback network.

Charge injection is ~6pC worst case.

...Jim Thompson

 

[[email protected]]

Jim Thompson ha escrito:

See....

http://analog-innovations.com/SED/VoltageGainSwitch.pdf

Took awhile. I had to collect my works for On-Semi's analog switches
into a "blind" part that didn't divulge what I designed for them at
the device-level.

I am sure there are better 1% values, but I just "slung" it together.

The 16-Bit-Step is just my test part to generate digital words.

...Jim Thompson

Where did you get the 4052's SPICE model ? Or did you make it yourself
? I`ve searched for it in the net high and low, and I can`t find it.
PSpice model for 4051 would be great

 

[Jim Thompson]

Jim Thompson ha escrito:


Where did you get the 4052's SPICE model ? Or did you make it yourself
? I`ve searched for it in the net high and low, and I can`t find it.
PSpice model for 4051 would be great

I redesigned ON Semi's whole line of analog switches and a number of
the 'HC' and 'LVX' logic parts as well.

So I have ALL the models _at_the_device_level_, i.e. very accurate ;-)

But I can't share :-( NDA's require my silence.

...Jim Thompson

 

[[email protected]]

Jim Thompson ha escrito:

I redesigned ON Semi's whole line of analog switches and a number of
the 'HC' and 'LVX' logic parts as well.

So I have ALL the models _at_the_device_level_, i.e. very accurate ;-)

But I can't share :-( NDA's require my silence.

...Jim Thompson

I understand.

Still, if anyone could point me in the right direction for a model of
this device, I would be grateful

 

[Jim Thompson]

Jim Thompson ha escrito:
[snip]

I redesigned ON Semi's whole line of analog switches and a number of
the 'HC' and 'LVX' logic parts as well.

So I have ALL the models _at_the_device_level_, i.e. very accurate ;-)

But I can't share :-( NDA's require my silence.

...Jim Thompson

I understand.

Still, if anyone could point me in the right direction for a model of
this device, I would be grateful

I don't know if anyone has spun such a model, but you could probably
make your own using a voltage controlled switch (Spice VSwitch), an
R-C for delay, and a resistor to set the switch on value.

...Jim Thompson

 

[[email protected]]

Jim Thompson ha escrito:

Jim Thompson ha escrito:
[snip]

I redesigned ON Semi's whole line of analog switches and a number of
the 'HC' and 'LVX' logic parts as well.

So I have ALL the models _at_the_device_level_, i.e. very accurate ;-)

But I can't share :-( NDA's require my silence.

...Jim Thompson

I understand.

Still, if anyone could point me in the right direction for a model of
this device, I would be grateful

I don't know if anyone has spun such a model, but you could probably
make your own using a voltage controlled switch (Spice VSwitch), an
R-C for delay, and a resistor to set the switch on value.

...Jim Thompson

Now you can`t throw that off to me like that , you need to get a
little graphical and specific about the analog part. The logic
converter and the decoder are easy to do, but I wonder how to stuff all
those things inside a single part in PSpice (so I only have to place my
4051, like a normal part), I don`t know even if it is possible. If you
could hint how to do it, I`ll search for the rest of the stuff in the
help.

Thanks

 

[Jim Thompson]

Jim Thompson ha escrito:

Jim Thompson ha escrito:
[snip]

I redesigned ON Semi's whole line of analog switches and a number of
the 'HC' and 'LVX' logic parts as well.

So I have ALL the models _at_the_device_level_, i.e. very accurate ;-)

But I can't share :-( NDA's require my silence.

...Jim Thompson

I understand.

Still, if anyone could point me in the right direction for a model of
this device, I would be grateful

I don't know if anyone has spun such a model, but you could probably
make your own using a voltage controlled switch (Spice VSwitch), an
R-C for delay, and a resistor to set the switch on value.

...Jim Thompson

Now you can`t throw that off to me like that , you need to get a
little graphical and specific about the analog part. The logic
converter and the decoder are easy to do, but I wonder how to stuff all
those things inside a single part in PSpice (so I only have to place my
4051, like a normal part), I don`t know even if it is possible. If you
could hint how to do it, I`ll search for the rest of the stuff in the
help.

Thanks

Read thru the help, a "Part" can actually reference a complete
schematic.

The easiest way for novices to make their own part is to copy an
existing part, then change attributes (and "Set Schematic") to match
your needs.

But you're probably using OrCAD Capture, which is a real bear to use
:-(

...Jim Thompson