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 Posted: Tue Jul 29, 2008 4:28 pm Post subject: Precision current mirror |
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Most of the fast current-output D/A converters I can find (e.g.
AD9764) can generate only up to 20mA max.
I need to provide up to 40mA, while still maintaining speed (50nS) and
accuracy (>=13 bits).
Any suggestions how can I achieve this? Perhaps a pointer to a
schematic of a an accurate current "amplifier" or mirror?
I have thought of paralleling two D/A converters (and sharing
reference voltages), but perhaps there is a simpler way.
TIA |
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Tim Williams
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| Posted: Wed Jul 30, 2008 1:42 am Post subject: Re: Precision current mirror |
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Well, you can certainly make three transistors into a 1:2 current mirror,
and generic transistors will go that fast. Tweaking the full 13+ bits
accuracy out of it is your problem. ;-)
Tim
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
<edaudio2000@yahoo.co.uk> wrote in message
news:a1f33e87-f7ec-4403-a1f4-f7ada82b375e@56g2000hsm.googlegroups.com...
[quote]Most of the fast current-output D/A converters I can find (e.g.
AD9764) can generate only up to 20mA max.
I need to provide up to 40mA, while still maintaining speed (50nS) and
accuracy (>=13 bits).
Any suggestions how can I achieve this? Perhaps a pointer to a
schematic of a an accurate current "amplifier" or mirror?
I have thought of paralleling two D/A converters (and sharing
reference voltages), but perhaps there is a simpler way.
TIA
[/quote] |
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Guest
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| Posted: Wed Jul 30, 2008 2:34 am Post subject: Re: Precision current mirror |
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On Jul 30, 6:42 am, "Tim Williams" <tmoran...@charter.net> wrote:
[quote]Well, you can certainly make three transistors into a 1:2 current mirror,
and generic transistors will go that fast. Tweaking the full 13+ bits
accuracy out of it is your problem. ;-)
Tim
--
Deep Friar: a very philosophical monk.
Website:http://webpages.charter.net/dawill/tmoranwms
edaudio2...@yahoo.co.uk> wrote in message
news:a1f33e87-f7ec-4403-a1f4-f7ada82b375e@56g2000hsm.googlegroups.com...
Most of the fast current-output D/A converters I can find (e.g.
AD9764) can generate only up to 20mA max.
I need to provide up to 40mA, while still maintaining speed (50nS) and
accuracy (>=13 bits).
Any suggestions how can I achieve this? Perhaps a pointer to a
schematic of a an accurate current "amplifier" or mirror?
I have thought of paralleling two D/A converters (and sharing
reference voltages), but perhaps there is a simpler way.
[/quote]
Tim Williams should know better than to top post.
A Howland current source could provide the accuracy, but you>d need a
very fast amplifier and resistors with very low parallel capacitance
to maintain the bandwidth.
L-trimmed surface mount precision resistors at the resistance levels
you>d need should be good enough, and there are plenty of really fast
op amps around.
Driving the desired current into the source of a fast FET and taking
the constant current from the drain is another popular approach, but
40mA implies a biggish FET, and the capacitances between drain and
source and drain and gate might be a bit too big for comfort at
50nsec. 40mA would not be a problem for most MOSFETs - even the SD214
will take 50mA of drain current, though you might need two in paralle
to keep the gate-to-source voltage down to practical levels.
--
Bill Sloman, Nijmegen |
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Tim Williams
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| Posted: Wed Jul 30, 2008 7:03 am Post subject: Re: Precision current mirror |
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<bill.sloman@ieee.org> wrote in message
news:973131ad-ed83-4095-99aa-b9fe4ac8be5e@u6g2000prc.googlegroups.com...
[quote]Tim Williams should know better than to top post.
[/quote]
What>s that supposed to mean? It was a short (three line) reply to the post
at large, no particular quote needed.
Tim
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms |
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Robert Baer
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| Posted: Wed Jul 30, 2008 7:03 am Post subject: Re: Precision current mirror |
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bill.sloman@ieee.org wrote:
[quote]On Jul 30, 6:42 am, "Tim Williams" <tmoran...@charter.net> wrote:
Well, you can certainly make three transistors into a 1:2 current mirror,
and generic transistors will go that fast. Tweaking the full 13+ bits
accuracy out of it is your problem. ;-)
Tim
--
Deep Friar: a very philosophical monk.
Website:http://webpages.charter.net/dawill/tmoranwms
edaudio2...@yahoo.co.uk> wrote in message
news:a1f33e87-f7ec-4403-a1f4-f7ada82b375e@56g2000hsm.googlegroups.com...
Most of the fast current-output D/A converters I can find (e.g.
AD9764) can generate only up to 20mA max.
I need to provide up to 40mA, while still maintaining speed (50nS) and
accuracy (>=13 bits).
Any suggestions how can I achieve this? Perhaps a pointer to a
schematic of a an accurate current "amplifier" or mirror?
I have thought of paralleling two D/A converters (and sharing
reference voltages), but perhaps there is a simpler way.
Tim Williams should know better than to top post.
A Howland current source could provide the accuracy, but you>d need a
very fast amplifier and resistors with very low parallel capacitance
to maintain the bandwidth.
L-trimmed surface mount precision resistors at the resistance levels
you>d need should be good enough, and there are plenty of really fast
op amps around.
Driving the desired current into the source of a fast FET and taking
the constant current from the drain is another popular approach, but
40mA implies a biggish FET, and the capacitances between drain and
source and drain and gate might be a bit too big for comfort at
50nsec. 40mA would not be a problem for most MOSFETs - even the SD214
will take 50mA of drain current, though you might need two in paralle
to keep the gate-to-source voltage down to practical levels.
--
Bill Sloman, Nijmegen
...err..all of the current going into a FET source would be seen at the[/quote]
drain (assuming zero gate current which in normal operation is valid).
Therefore, it is useless by itself to "amplify" 20mA to 40mA.
Try again. |
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Guest
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| Posted: Thu Jul 31, 2008 3:04 am Post subject: Re: Precision current mirror |
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On Jul 30, 4:33 pm, "Tim Williams" <tmoran...@charter.net> wrote:
[quote]bill.slo...@ieee.org> wrote in message
news:973131ad-ed83-4095-99aa-b9fe4ac8be5e@u6g2000prc.googlegroups.com...
Tim Williams should know better than to top post.
What>s that supposed to mean? It was a short (three line) reply to the post
at large, no particular quote needed.
[/quote]
It was a pretty silly reply which did invite further comment - it
would have been tidier if your comment had ended up below the OP>s
request, rather than above it.
--
Bill Sloman, Nijmegen |
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Guest
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| Posted: Thu Jul 31, 2008 3:04 am Post subject: Re: Precision current mirror |
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On Jul 30, 5:34 pm, Robert Baer <robertb...@localnet.com> wrote:
[quote]bill.slo...@ieee.org wrote:
On Jul 30, 6:42 am, "Tim Williams" <tmoran...@charter.net> wrote:
Well, you can certainly make three transistors into a 1:2 current mirror,
and generic transistors will go that fast. Tweaking the full 13+ bits
accuracy out of it is your problem. ;-)
Tim
--
Deep Friar: a very philosophical monk.
Website:http://webpages.charter.net/dawill/tmoranwms
edaudio2...@yahoo.co.uk> wrote in message
news:a1f33e87-f7ec-4403-a1f4-f7ada82b375e@56g2000hsm.googlegroups.com....
Most of the fast current-output D/A converters I can find (e.g.
AD9764) can generate only up to 20mA max.
I need to provide up to 40mA, while still maintaining speed (50nS) and
accuracy (>=13 bits).
Any suggestions how can I achieve this? Perhaps a pointer to a
schematic of a an accurate current "amplifier" or mirror?
I have thought of paralleling two D/A converters (and sharing
reference voltages), but perhaps there is a simpler way.
Tim Williams should know better than to top post.
A Howland current source could provide the accuracy, but you>d need a
very fast amplifier and resistors with very low parallel capacitance
to maintain the bandwidth.
L-trimmed surface mount precision resistors at the resistance levels
you>d need should be good enough, and there are plenty of really fast
op amps around.
Driving the desired current into the source of a fast FET and taking
the constant current from the drain is another popular approach, but
40mA implies a biggish FET, and the capacitances between drain and
source and drain and gate might be a bit too big for comfort at
50nsec. 40mA would not be a problem for most MOSFETs - even the SD214
will take 50mA of drain current, though you might need two in paralle
to keep the gate-to-source voltage down to practical levels.
--
Bill Sloman, Nijmegen
..err..all of the current going into a FET source would be seen at the
drain (assuming zero gate current which in normal operation is valid).
Therefore, it is useless by itself to "amplify" 20mA to 40mA.
[/quote]
So much should be obvious to the meanest intellect.
I>d started off by recommending the Howland current source, which
allows you to scale a tolerably arbitary input to generate a current
output with close to infinite output impedance. Discussions of the
Howland circuit almost always point out that it is easier to make a
unipolar current source or sink by relying or a FET or a BJT to
isolate the driving amplifier from the load.
The OP>s 13-bit precision requirement more or less cuts out BJT>s -
the base currents are a little too unpredictable and temperature
dependent to make such a circuit practical - but FETs and MOSFET>s are
a real possibility.
I would have thought it was obvious to the meanest intellect that that
the op amp drivng the FET/MOSFET would have to boost the OP>s output
up to the 40mA desired, but you have proved me wrong.
[quote]Try again.
[/quote]
Try thinking about what has been posted before you start trying to
teach your grandmother to suck eggs.
--
Bill Sloman, Nijmegen |
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Guest
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| Posted: Thu Jul 31, 2008 7:05 am Post subject: Re: Precision current mirror |
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On Jul 30, 2:34 am, bill.slo...@ieee.org wrote:
[quote]On Jul 30, 6:42 am, "Tim Williams" <tmoran...@charter.net> wrote:
Well, you can certainly make three transistors into a 1:2 current mirror,
and generic transistors will go that fast. Tweaking the full 13+ bits
accuracy out of it is your problem. ;-)
Tim
--
Deep Friar: a very philosophical monk.
Website:http://webpages.charter.net/dawill/tmoranwms
edaudio2...@yahoo.co.uk> wrote in message
news:a1f33e87-f7ec-4403-a1f4-f7ada82b375e@56g2000hsm.googlegroups.com...
Most of the fast current-output D/A converters I can find (e.g.
AD9764) can generate only up to 20mA max.
I need to provide up to 40mA, while still maintaining speed (50nS) and
accuracy (>=13 bits).
Any suggestions how can I achieve this? Perhaps a pointer to a
schematic of a an accurate current "amplifier" or mirror?
I have thought of paralleling two D/A converters (and sharing
reference voltages), but perhaps there is a simpler way.
Tim Williams should know better than to top post.
A Howland current source could provide the accuracy, but you>d need a
very fast amplifier and resistors with very low parallel capacitance
to maintain the bandwidth.
L-trimmed surface mount precision resistors at the resistance levels
you>d need should be good enough, and there are plenty of really fast
op amps around.
Driving the desired current into the source of a fast FET and taking
the constant current from the drain is another popular approach, but
40mA implies a biggish FET, and the capacitances between drain and
source and drain and gate might be a bit too big for comfort at
50nsec. 40mA would not be a problem for most MOSFETs - even the SD214
will take 50mA of drain current, though you might need two in paralle
to keep the gate-to-source voltage down to practical levels.
--
Bill Sloman, Nijmegen
[/quote]
Bill, Thanks for the suggestion.
Entering "currents sources" in google produced a lot of faff, but
entering "howland" was rather more specific. That was useful!
However, reading about Howland sources (and if I understand it right)
it appears their precision is overly sensitive on component accuracy,
and possibly unable to provide a 13 bit resolution unless I use
incredibly accurate and matched resistors.
Are there any other current source architectures, names or keywords I
could try?
TIA |
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Jim Thompson
Guest
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| Posted: Thu Jul 31, 2008 6:53 pm Post subject: Re: Precision current mirror |
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On Wed, 30 Jul 2008 20:04:53 -0700 (PDT), bill.sloman@ieee.org wrote:
[snip]
[quote]
Try thinking about what has been posted before you start trying to
teach your grandmother to suck eggs.
[/quote]
That>s not what YOUR grandmother sucks ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC>s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Democrats, the party of NO |
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Guest
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| Posted: Fri Aug 01, 2008 6:12 am Post subject: Re: Precision current mirror |
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On Jul 31, 11:53 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@My-
Web-Site.com> wrote:
[quote]On Wed, 30 Jul 2008 20:04:53 -0700 (PDT), bill.slo...@ieee.org wrote:
[snip]
Try thinking about what has been posted before you start trying to
teach your grandmother to suck eggs.
That>s not what YOUR grandmother sucks ;-)
[/quote]
Both of them are dead, as someone with any grasp of reality would
expect, and don>t suck anything any more.
And I did have two of them - odd as this may seem to somebody with
West Virginia hillbilly in his background.
--
Bill Sloman, Nijmegen |
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Guest
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| Posted: Fri Aug 01, 2008 6:12 am Post subject: Re: Precision current mirror |
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On Jul 31, 5:05 pm, edaudio2...@yahoo.co.uk wrote:
[quote]On Jul 30, 2:34 am, bill.slo...@ieee.org wrote:
On Jul 30, 6:42 am, "Tim Williams" <tmoran...@charter.net> wrote:
Well, you can certainly make three transistors into a 1:2 current mirror,
and generic transistors will go that fast. Tweaking the full 13+ bits
accuracy out of it is your problem. ;-)
Tim
--
Deep Friar: a very philosophical monk.
Website:http://webpages.charter.net/dawill/tmoranwms
edaudio2...@yahoo.co.uk> wrote in message
news:a1f33e87-f7ec-4403-a1f4-f7ada82b375e@56g2000hsm.googlegroups.com....
Most of the fast current-output D/A converters I can find (e.g.
AD9764) can generate only up to 20mA max.
I need to provide up to 40mA, while still maintaining speed (50nS) and
accuracy (>=13 bits).
Any suggestions how can I achieve this? Perhaps a pointer to a
schematic of a an accurate current "amplifier" or mirror?
I have thought of paralleling two D/A converters (and sharing
reference voltages), but perhaps there is a simpler way.
Tim Williams should know better than to top post.
A Howland current source could provide the accuracy, but you>d need a
very fast amplifier and resistors with very low parallel capacitance
to maintain the bandwidth.
L-trimmed surface mount precision resistors at the resistance levels
you>d need should be good enough, and there are plenty of really fast
op amps around.
Driving the desired current into the source of a fast FET and taking
the constant current from the drain is another popular approach, but
40mA implies a biggish FET, and the capacitances between drain and
source and drain and gate might be a bit too big for comfort at
50nsec. 40mA would not be a problem for most MOSFETs - even the SD214
will take 50mA of drain current, though you might need two in paralle
to keep the gate-to-source voltage down to practical levels.
--
Bill Sloman, Nijmegen
Bill, Thanks for the suggestion.
Entering "currents sources" in google produced a lot of faff, but
entering "howland" was rather more specific. That was useful!
However, reading about Howland sources (and if I understand it right)
it appears their precision is overly sensitive on component accuracy,
and possibly unable to provide a 13 bit resolution unless I use
incredibly accurate and matched resistors.
Are there any other current source architectures, names or keywords I
could try?
[/quote]
You can buy 0.1% 15ppm resitors off the shelf from Farnell and other
broad-line distributors. 0.1% is 10-bits.
You can trim a Howland circuit with a strategically placed trim-pot to
do better - 13-bits is probably attainable, though soldered-in
parallel trimming resistors would be more stable.
Farnell also stocked really tight-tolerance resistor arrays with very
close tolerances on the initial matching and the temperature matching.
I>ve not got access to a Farnell catalogue at the moment and I can>t
be bothered searching their web-site.
You should have found Bob Pease>s application note
http://www.national.com/an/AN/AN-1515.pdf
http://www.national.com/an/AN/AN-20.pdf#page=1
gives two - very basic - op amp based current sources that don>t
depend on well-matched resistors, both of which use FETs as output
devices. MOSFETs would also serve.
--
Bill Sloman, Nijmegen |
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