an IC we need

John Larkin · Mar 30, 2005

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

John Larkin · Mar 30, 2005

On Wed, 30 Mar 2005 16:54:01 -0700, Jim Thompson
<thegreatone@example.com> wrote:

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

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

John Larkin · Mar 30, 2005

On Wed, 30 Mar 2005 17:16:52 -0800, Luhan Monat <x@y.z> wrote:

John Larkin 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

All of those things are possible now: D/A driving a Vari-Cap diode. Two
whole parts.

But that's nonlinear, has limited range, has lots of distortion versus
swing, is useless for real analog signal handling (say, +-5 volt
signal swings) and has a really rotten TC.

Imagine all the slick things you could do with a good programmable
capacitor!

John

Jim Thompson · Mar 30, 2005

On Wed, 30 Mar 2005 16:21:00 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 30 Mar 2005 16:54:01 -0700, Jim Thompson
thegreatone@example.com> wrote:

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

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 ??

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice

480)460-2350 | |
| E-mail Address at Website Fax

Luhan Monat · Mar 31, 2005

John Larkin 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

All of those things are possible now: D/A driving a Vari-Cap diode. Two
whole parts.

--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm

John Larkin · Mar 30, 2005

On Wed, 30 Mar 2005 17:26:44 -0700, Jim Thompson
<thegreatone@example.com> wrote:

On Wed, 30 Mar 2005 16:21:00 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 30 Mar 2005 16:54:01 -0700, Jim Thompson
thegreatone@example.com> wrote:

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

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 ??

...Jim Thompson

I assume the tics from DAT drive a binary counter inside the beast.
The block diagram shows 5 binary-weighted caps. SPI would make more
sense for a part with more steps; I guess they just ran out of pins.

John

Winfield Hill · Mar 30, 2005

Jim Thompson wrote...

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

Monolithic Equal-value caps sounds like a good idea. Of course, then
there's the issue of switch capacitance. These are pretty low-Q caps.
Hmm, for a 13pF cap, a Q of 30 at 100MHz implies 4 ohms of FET series
resistance. But that implies a rather large FET switch, which should
lead to more than 6pF of parasitic capacitance... Hmm...

--
Thanks,
- Win

Jim Thompson · Mar 30, 2005

On Wed, 30 Mar 2005 16:40:40 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 30 Mar 2005 17:26:44 -0700, Jim Thompson
thegreatone@example.com> wrote:

On Wed, 30 Mar 2005 16:21:00 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 30 Mar 2005 16:54:01 -0700, Jim Thompson
thegreatone@example.com> wrote:

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

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 ??

...Jim Thompson

I assume the tics from DAT drive a binary counter inside the beast.
The block diagram shows 5 binary-weighted caps. SPI would make more
sense for a part with more steps; I guess they just ran out of pins.

John

480)460-2350 | |
| E-mail Address at Website Fax

Joerg · Mar 30, 2005

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

John Fields · Mar 30, 2005

On Wed, 30 Mar 2005 17:16:52 -0800, Luhan Monat <x@y.z> wrote:

John Larkin 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

All of those things are possible now: D/A driving a Vari-Cap diode. Two
whole parts.

---
_And_ the LUT to make C/V linear, _and_ the temp sensor and LUT to
cal out the tempco, and...

--
John Fields
Professional Circuit Designer

John Fields · Mar 31, 2005

On Wed, 30 Mar 2005 16:40:40 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 30 Mar 2005 17:26:44 -0700, Jim Thompson
thegreatone@example.com> wrote:

On Wed, 30 Mar 2005 16:21:00 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 30 Mar 2005 16:54:01 -0700, Jim Thompson
thegreatone@example.com> wrote:

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

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 ??

...Jim Thompson

I assume the tics from DAT drive a binary counter inside the beast.
The block diagram shows 5 binary-weighted caps. SPI would make more
sense for a part with more steps; I guess they just ran out of pins.

---
Just one more pin would get them SPI, so my guess is that they ran out
of capacitance. On the low end the limit would be parasitics and on
the high end, area and dielectric thickness would be the killer.

--
John Fields
Professional Circuit Designer

Robert Monsen · Mar 31, 2005

John Larkin wrote:

On Wed, 30 Mar 2005 17:16:52 -0800, Luhan Monat <x@y.z> wrote:

John Larkin 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

All of those things are possible now: D/A driving a Vari-Cap diode. Two
whole parts.

But that's nonlinear, has limited range, has lots of distortion versus
swing, is useless for real analog signal handling (say, +-5 volt
signal swings) and has a really rotten TC.

Drive two identical varicaps with the same voltage, and measure one for
the feedback... measuring the capacitance is probably not going to be
fast enough for what you want, though.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

Joel Kolstad · Mar 31, 2005

"Luhan Monat" <x@y.z> wrote in message
news:gkH2e.26870$Mt5.8195@fed1read01...

All of those things are possible now: D/A driving a Vari-Cap diode. Two
whole parts.

I'd like to see a two part circuit with a D/A and a varicap that works up to
30MHz at +/-10V input levels...

Jim Thompson · Mar 31, 2005

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

480)460-2350 | |
| E-mail Address at Website Fax

Luhan Monat · Mar 31, 2005

Joel Kolstad wrote:

"Luhan Monat" <x@y.z> wrote in message
news:gkH2e.26870$Mt5.8195@fed1read01...

All of those things are possible now: D/A driving a Vari-Cap diode. Two
whole parts.

I'd like to see a two part circuit with a D/A and a varicap that works up to
30MHz at +/-10V input levels...

Yea, so would I. But lots of applications are in the millivolt level
and even less. I rather think about just what *can* be done with the
available technology - not just what *could* be done with hypothetical
components. Hey, but thats just me :>

Joel Kolstad · Mar 31, 2005

"Luhan Monat" <fake@email.xxx> wrote in message
news:baW2e.42897$Mt5.20177@fed1read01...

Joel Kolstad wrote:
I'd like to see a two part circuit with a D/A and a varicap that works up
to 30MHz at +/-10V input levels...

Yea, so would I. But lots of applications are in the millivolt level and
even less. I rather think about just what *can* be done with the
available technology - not just what *could* be done with hypothetical
components. Hey, but thats just me :

I'm just saying that there are plenty of real-world problems where your
proposed architecture doesn't work yet other architectures do...

Frank Miles · Mar 31, 2005

In article <bhdm415kt3p0hv4omc9kahotpfavdrtac9@4ax.com>,
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.

Kinda cute. But such limited range (without serious care, stray wiring
capacitances will be of the same order), limited voltage compliance, and
no spec (at least that I saw on quick scan) for leakage currents.

-frank
--

Joerg · Mar 31, 2005

Hello John,

All of those things are possible now: D/A driving a Vari-Cap diode. Two
whole parts.

_And_ the LUT to make C/V linear, _and_ the temp sensor and LUT to
cal out the tempco, and...

Capacitors on a chip aren't particularly predictable either. Their only
advantage is that they track each other nicely within the same chip.

Regards, Joerg

http://www.analogconsultants.com

John Larkin · Mar 31, 2005

On Thu, 31 Mar 2005 18:14:16 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Hello John,

All of those things are possible now: D/A driving a Vari-Cap diode. Two
whole parts.

_And_ the LUT to make C/V linear, _and_ the temp sensor and LUT to
cal out the tempco, and...

Capacitors on a chip aren't particularly predictable either. Their only
advantage is that they track each other nicely within the same chip.

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.

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

John

Jim Thompson · Mar 31, 2005

On Thu, 31 Mar 2005 10:47:16 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 31 Mar 2005 18:14:16 GMT, Joerg
notthisjoergsch@removethispacbell.net> wrote:

Hello John,

All of those things are possible now: D/A driving a Vari-Cap diode. Two
whole parts.

_And_ the LUT to make C/V linear, _and_ the temp sensor and LUT to
cal out the tempco, and...

Capacitors on a chip aren't particularly predictable either. Their only
advantage is that they track each other nicely within the same chip.

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.

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

John

480)460-2350 | |
| E-mail Address at Website Fax

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