an IC we need

[John Larkin]

On Thu, 31 Mar 2005 20:20:48 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Hello John,

MOS capacitors are some of the best caps there are. Very low DA, low
TC, very linear, superb on-chip ratio matching (which is why most fast
ADCs now use capacitive ladders.) Their main disadvantage is low
available capacitance and fairly low Q.

And their rather large tolerance. The MAX1474 data sheet states +/-15%.
Not exactly something to write home about.

Vishay makes some surface-mount MOS caps that are fabulous; too bad
they're impossible to solder.

Interesting. Do you have a part number? All I could find were their
thick film arrays and the silicon single caps.

Regards, Joerg

http://www.analogconsultants.com

I was referring to these...

http://www.vishay.com/docs/10137/10137.pdf


a few other people, Metelics and such, make MOS caps.

John

 

[John Larkin]

On Wed, 30 Mar 2005 19:38:13 -0700, Jim Thompson
<thegreatone@example.com> wrote:

On Thu, 31 Mar 2005 00:54:12 GMT, Joerg
notthisjoergsch@removethispacbell.net> wrote:

Hello Jim,

The Maxim thing? No, it's two pins. To set the capacitance to N (N=0
to 31) you...

Raise the EN pin

Pulse the DAT pin N times

Drop EN.


Good thing it's only 31 pulses max ; 65535 would get tiring.

John


Aha! So you don't load a serial word like in SPI?

That's totally weird. That would require decoding counts into binary
for the switches... unless there's actually 32 equal value caps ??

It doesn't have to be weird. You could, for example, have a slow
oscillator with one R, one C and a 1/6th of a Schmitt chip such as the
HC14, then run it until something is "in tune", then stop the oscillator
with another of the Schmitt gates plus a diode from its output to the
oscillator gate input. In terms of simplicity that would be hard to beat.

I'd still use a varicap ;-)

Regards, Joerg

http://www.analogconsultants.com

I used a reference frequency plus a phase detector.

Started at one extreme and swept the tuning.

When phase flipped I stopped.

...Jim Thompson

We're doing something similar. We have an LC vco, with a varicap
driven by a DAC. The Maxim thing is the coarse tune that gets the
varicap within lock range. At powerup, we'll run an algorithm that
finds the best Maxim setting to keep the loop centered. We might do it
periodically afterwards, too, if we can sneak it in somehow without
blipping the loop.

John

 

[Jim Thompson]

On Thu, 31 Mar 2005 19:51:07 -0800, John Larkin
<jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:

On Wed, 30 Mar 2005 19:38:13 -0700, Jim Thompson
thegreatone@example.com> wrote:

On Thu, 31 Mar 2005 00:54:12 GMT, Joerg
notthisjoergsch@removethispacbell.net> wrote:

Hello Jim,

The Maxim thing? No, it's two pins. To set the capacitance to N (N=0
to 31) you...

Raise the EN pin

Pulse the DAT pin N times

Drop EN.


Good thing it's only 31 pulses max ; 65535 would get tiring.

John


Aha! So you don't load a serial word like in SPI?

That's totally weird. That would require decoding counts into binary
for the switches... unless there's actually 32 equal value caps ??

It doesn't have to be weird. You could, for example, have a slow
oscillator with one R, one C and a 1/6th of a Schmitt chip such as the
HC14, then run it until something is "in tune", then stop the oscillator
with another of the Schmitt gates plus a diode from its output to the
oscillator gate input. In terms of simplicity that would be hard to beat.

I'd still use a varicap ;-)

Regards, Joerg

http://www.analogconsultants.com

I used a reference frequency plus a phase detector.

Started at one extreme and swept the tuning.

When phase flipped I stopped.

...Jim Thompson


We're doing something similar. We have an LC vco, with a varicap
driven by a DAC. The Maxim thing is the coarse tune that gets the
varicap within lock range. At powerup, we'll run an algorithm that
finds the best Maxim setting to keep the loop centered. We might do it
periodically afterwards, too, if we can sneak it in somehow without
blipping the loop.

John

If you have DC on the cap it'll be a problem.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| 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.

 

[Jim Thompson]

On Wed, 30 Mar 2005 15:33:45 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

Somebody should make a family of DACs that output capacitance instead
of voltage or current. All sorts of neat stuff would then be possible.

This is a good first start...

http://pdfserv.maxim-ic.com/en/ds/MAX1474.pdf

but it's only 5 bits. The digital interface is of course, in the
tradition of all mixed-signal chips, completely bizarre.

John

I've done that sort of thing to tune on-board-chip filters and
oscillators.

As for the interface... isn't that your typical digital-dork SPI
serial interface ?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| 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.

 

[Joerg]

Hello John,

MOS capacitors are some of the best caps there are. Very low DA, low
TC, very linear, superb on-chip ratio matching (which is why most fast
ADCs now use capacitive ladders.) Their main disadvantage is low
available capacitance and fairly low Q.

And their rather large tolerance. The MAX1474 data sheet states +/-15%.
Not exactly something to write home about.

Vishay makes some surface-mount MOS caps that are fabulous; too bad
they're impossible to solder.

Interesting. Do you have a part number? All I could find were their
thick film arrays and the silicon single caps.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hello John,

I was referring to these...

http://www.vishay.com/docs/10137/10137.pdf

Arrow shows these between 50c and a Dollar. Ouch. I am surprised they
advertise them for cell phones and other very cost sensitive
applications. Even a 2c cap is turned around and around to see if it's
really needed.

a few other people, Metelics and such, make MOS caps.

But not arrays like what this MAX chip provides, AFAIK.

Regards, Joerg

http://www.analogconsultants.com