| Author |
Message |
John Larkin
Guest
|
 Posted:
Thu Dec 01, 2005 1:35 am Post subject:
HCT4051 leakage |
|
|
Hi,
Has anybody measured typical leakage currents for an HCT4051 analog
mux? I'm wondering about both ESD diode leakage (ie, to rails) and
leakage through open switches. I'm running 0 and +5V rails.
Indications are that everybody's 0.1 uA max spec is wildly
pessimistic, but I was wondering if anybody knows more, before I have
to drag my butt into the lab and make actual measurements. It's a lot
easier to sit here and type and eat bon-bons.
We're scanning eight RTDs. A +2.5 volt reference goes through a
precision 270 ohm resistor and gets mux'd to a selected RTD. The
voltage drop across the RTD gets differentially mux'd, too. A 24-bit
delta-sigma ADC digitizes the voltage drop across the 270, then the
voltage across the RTD, and does the math. We're getting errors in the
tens of PPM, tolerable, but we're curious where they're coming from.
The sensitivity analysis math here is a nuisance.
John
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David L. Jones
Guest
|
| Posted:
Thu Dec 01, 2005 1:35 am Post subject:
Re: HCT4051 leakage |
|
|
John Larkin wrote:
| Quote: | Hi,
Has anybody measured typical leakage currents for an HCT4051 analog
mux? I'm wondering about both ESD diode leakage (ie, to rails) and
leakage through open switches. I'm running 0 and +5V rails.
Indications are that everybody's 0.1 uA max spec is wildly
pessimistic, but I was wondering if anybody knows more, before I have
to drag my butt into the lab and make actual measurements. It's a lot
easier to sit here and type and eat bon-bons.
We're scanning eight RTDs. A +2.5 volt reference goes through a
precision 270 ohm resistor and gets mux'd to a selected RTD. The
voltage drop across the RTD gets differentially mux'd, too. A 24-bit
delta-sigma ADC digitizes the voltage drop across the 270, then the
voltage across the RTD, and does the math. We're getting errors in the
tens of PPM, tolerable, but we're curious where they're coming from.
The sensitivity analysis math here is a nuisance.
John
|
Don't remeber exactly but I vaguely remember these suckers having
different leakages (or some other characteristic) in different
directions too? (or was it the 4066...?)
There was some fine print in the datasheet that admitted this.
Was quite a bad trap for my application at the time.
They can be nasty little suckers.
Dave :) |
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John Larkin
Guest
|
| Posted:
Thu Dec 01, 2005 1:35 am Post subject:
Re: HCT4051 leakage |
|
|
On Wed, 30 Nov 2005 23:56:35 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:
| Quote: | Hello John,
Indications are that everybody's 0.1 uA max spec is wildly
pessimistic, but I was wondering if anybody knows more, before I have
to drag my butt into the lab and make actual measurements. It's a lot
easier to sit here and type and eat bon-bons.
Actually channel-to-channel it's 0.4uA for the '51, less for the '52 and
'53. Sez TI.
However, the difference in RDSon from channel to channel can be up to
10ohms or so. I have never seen that much in practice but if this
matters can't you sink in a reference current instead of a voltage?
|
Putting R(ref) and R(unknown) in series, and measuring their voltage
drops, is a neat way to compute R(unk). The resistance range we can
measure goes from zero to infinity, and it's purely ratiometric on a
single super-precision resistor. Plus, we're summing the two measured
voltages as a sanity check on the entire 4-wire loop.
The small errors we're seeing may be due to cmos switch leakage, but
the error TCs are looking linear enough that leakage is probably not a
big issue... it should be sorta exponential on temperature, and we're
not seeing that.
Switch resistance doesn't matter here as long as it's constant for the
duration of the two measurements. We're taking about 130 millisec for
each measurement, 260 total. It might be that the current (about 6 mA
when we're measuring a 100 ohm RTD) is heating the cmos switch enough
to give it a r-versus-t curve that matters. 6 mA, 75 ohms typ, gives
around 3 milliwatts in the switch. The HCT switch is about 75 ohms and
increases about 0.25 ohms/K. So, what's the thermal coefficient of one
fet in an HCT4051? 1000 k/w maybe?
If it increases 3K as a result of the switched current, we'll have
0.75 ohms increase, serious by our standards. But what's the thermal
tau?
John
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Guest
|
| Posted:
Thu Dec 01, 2005 1:35 am Post subject:
Re: HCT4051 leakage |
|
|
John Larkin wrote:
| Quote: | Hi,
Has anybody measured typical leakage currents for an HCT4051 analog
mux? I'm wondering about both ESD diode leakage (ie, to rails) and
leakage through open switches. I'm running 0 and +5V rails.
Indications are that everybody's 0.1 uA max spec is wildly
pessimistic, but I was wondering if anybody knows more, before I have
to drag my butt into the lab and make actual measurements. It's a lot
easier to sit here and type and eat bon-bons.
We're scanning eight RTDs. A +2.5 volt reference goes through a
precision 270 ohm resistor and gets mux'd to a selected RTD. The
voltage drop across the RTD gets differentially mux'd, too. A 24-bit
delta-sigma ADC digitizes the voltage drop across the 270, then the
voltage across the RTD, and does the math. We're getting errors in the
tens of PPM, tolerable, but we're curious where they're coming from.
The sensitivity analysis math here is a nuisance.
|
The manufacturer's specified leakage current specification used to be
set by the limitiations of their measuring gear and the time available
to take the measurement.
Back in 1979 I got thrown into a project to do a similar job, where the
originator had based the design on measured leakage currents which were
two or three orders of magnitude lower. I rejected the original design
on the basis that the manufacturers were always at liberty to degrade
their silicon to take advantage of the relatively sloppy leakage
current specification, and found a nitride-insulated unprotected
discrete MOSFET that had guaranteed leakage currents comparable with
the measured CMOS leakage currents
I don't know how leaky modern CMOS has become.
------------------
Bill Sloman, Nijmegen |
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Mark
Guest
|
| Posted:
Thu Dec 01, 2005 1:35 am Post subject:
Re: HCT4051 leakage |
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|
consider the capacitance at the input of the A/D.
when the mux changes channels, it has to charge/discharge to the new
value through the series resistance
it might take a long time for the cap to reach final value accurate to
24 bits.
this leads to what appears to be crosstalk between the channels
you can improve the situation by setting the mux to channel with the
fixed ref voltage in bettween reading active channels
it seems hard to belive a small c could do this but if you are going
for 24 bits, it may be part of the problem
Mark |
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john jardine
Guest
|
| Posted:
Thu Dec 01, 2005 1:35 am Post subject:
Re: HCT4051 leakage |
|
|
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:d47so1h0tsdv3lr2jub84772s11n32aa9i@4ax.com...
[...]
| Quote: |
John
A few years ago did a very similar 8 way muxed RTD (4051s), for a motor |
control system. Initially ran at 8ma RTD current but on RTD selection
'noticed' a small frequency shift on the opto isolated V to F output.
Eventually traced to RTD heating, so dropped the constant current to 2ma.
The spec was nowhere near to what you are resolving to so this aspect is
probably of no consequence.
regards
john |
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Joerg
Guest
|
| Posted:
Thu Dec 01, 2005 1:35 am Post subject:
Re: HCT4051 leakage |
|
|
Hello John,
| Quote: | Indications are that everybody's 0.1 uA max spec is wildly
pessimistic, but I was wondering if anybody knows more, before I have
to drag my butt into the lab and make actual measurements. It's a lot
easier to sit here and type and eat bon-bons.
|
Actually channel-to-channel it's 0.4uA for the '51, less for the '52 and
'53. Sez TI.
However, the difference in RDSon from channel to channel can be up to
10ohms or so. I have never seen that much in practice but if this
matters can't you sink in a reference current instead of a voltage?
And watch our for fingerprints, flux and bon-bon residue ;-)
Regards, Joerg
http://www.analogconsultants.com |
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Jim Thompson
Guest
|
| Posted:
Thu Dec 01, 2005 1:35 am Post subject:
Re: HCT4051 leakage |
|
|
On Wed, 30 Nov 2005 14:31:49 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
| Quote: | Hi,
Has anybody measured typical leakage currents for an HCT4051 analog
mux? I'm wondering about both ESD diode leakage (ie, to rails) and
leakage through open switches. I'm running 0 and +5V rails.
Indications are that everybody's 0.1 uA max spec is wildly
pessimistic, but I was wondering if anybody knows more, before I have
to drag my butt into the lab and make actual measurements. It's a lot
easier to sit here and type and eat bon-bons.
We're scanning eight RTDs. A +2.5 volt reference goes through a
precision 270 ohm resistor and gets mux'd to a selected RTD. The
voltage drop across the RTD gets differentially mux'd, too. A 24-bit
delta-sigma ADC digitizes the voltage drop across the 270, then the
voltage across the RTD, and does the math. We're getting errors in the
tens of PPM, tolerable, but we're curious where they're coming from.
The sensitivity analysis math here is a nuisance.
John
|
The ESD diodes are probably in the <1nA range, but the switches are
HUGE (for low on resistance), so they're likely the source of the
leakage.
...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:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food. |
|
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|
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Tony Williams
Guest
|
| Posted:
Thu Dec 01, 2005 8:26 am Post subject:
Re: HCT4051 leakage |
|
|
In article <d47so1h0tsdv3lr2jub84772s11n32aa9i@4ax.com>,
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
| Quote: | We're scanning eight RTDs. A +2.5 volt reference goes through a
precision 270 ohm resistor and gets mux'd to a selected RTD. The
voltage drop across the RTD gets differentially mux'd, too. A
24-bit delta-sigma ADC digitizes the voltage drop across the 270,
then the voltage across the RTD, and does the math. We're getting
errors in the tens of PPM, tolerable, but we're curious where
they're coming from. The sensitivity analysis math here is a
nuisance.
|
I think I'd investigate the possibility (probability?)
that the current is changing during the adc measurements,
due to the temperature coefficient of the switches.
Perhaps modify the adc sequence to take three readings,
V270, Vunknown, V270. See if the two V270 results are
the same.
If they are not, you might get away with using the
average of the two V270 adc readings in the calcs.
--
Tony Williams. |
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John Larkin
Guest
|
| Posted:
Thu Dec 01, 2005 9:35 am Post subject:
Re: HCT4051 leakage |
|
|
On 30 Nov 2005 17:16:24 -0800, "Mark" <makolber@yahoo.com> wrote:
| Quote: | consider the capacitance at the input of the A/D.
when the mux changes channels, it has to charge/discharge to the new
value through the series resistance
it might take a long time for the cap to reach final value accurate to
24 bits.
this leads to what appears to be crosstalk between the channels
you can improve the situation by setting the mux to channel with the
fixed ref voltage in bettween reading active channels
it seems hard to belive a small c could do this but if you are going
for 24 bits, it may be part of the problem
Mark
|
I am letting things settle for about 4 milliseconds before I kick off
the ADC, which should let RC tau's settle pretty well. Impedances are
low here.
Our system spec is 250 PPM, and we're getting 10's, even 1's of PPM
for resistances near what we calibrate with, with a systematic trend
of error versus Rx, peaking at around 100 PPM at our operating
extremes. So we're OK, but I'd like to understand the residual errors
and fix them. We just did a channel-channel crosstalk check, and
changing channel N over its full resistance range affects N+1 about 1
PPM, hard to resolve, so thermals in the cmos switch are looking
unlikely.
We'll build and test a few more boards. If the errors seem systematic,
we can always toss a little software curvature at it, and never really
have to understand the physics.
Y = Y + K * Y^2
has cured more sins than the College of Cardinals.
John |
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Fred Bartoli
Guest
|
| Posted:
Thu Dec 01, 2005 9:35 am Post subject:
Re: HCT4051 leakage |
|
|
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> a écrit dans le
message de news:d47so1h0tsdv3lr2jub84772s11n32aa9i@4ax.com...
| Quote: | Hi,
Has anybody measured typical leakage currents for an HCT4051 analog
mux? I'm wondering about both ESD diode leakage (ie, to rails) and
leakage through open switches. I'm running 0 and +5V rails.
Indications are that everybody's 0.1 uA max spec is wildly
pessimistic, but I was wondering if anybody knows more, before I have
to drag my butt into the lab and make actual measurements. It's a lot
easier to sit here and type and eat bon-bons.
We're scanning eight RTDs. A +2.5 volt reference goes through a
precision 270 ohm resistor and gets mux'd to a selected RTD. The
voltage drop across the RTD gets differentially mux'd, too. A 24-bit
delta-sigma ADC digitizes the voltage drop across the 270, then the
voltage across the RTD, and does the math. We're getting errors in the
tens of PPM, tolerable, but we're curious where they're coming from.
The sensitivity analysis math here is a nuisance.
|
I did measure an old CD4053 about two years ago (sorry did not have 74HC).
It had +/-5V supplies and was arranged for 0V common mode at both switch
ends.
IIRC the leakages were about 1pA at room temperature.
One thing that surpised me was that, while static leakages were that low,
dynamic leakages (100kHz switching) were not negligeable at all.
Quizz: how do you pump fraction of a uA through the resistors with the
switch wired as below?
4053
.------------.
| | ___
| |--|___|- GND
___ | o---/| 50K
GND -|___|--|---o--__ |
50K | o--- | ___
| \|--|___|- GND
'------------' 50K
--
Thanks,
Fred. |
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|
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Jim Thompson
Guest
|
| Posted:
Thu Dec 01, 2005 5:35 pm Post subject:
Re: HCT4051 leakage |
|
|
On Thu, 1 Dec 2005 09:01:04 +0100, "Fred Bartoli"
<fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
| Quote: |
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> a écrit dans le
message de news:d47so1h0tsdv3lr2jub84772s11n32aa9i@4ax.com...
Hi,
Has anybody measured typical leakage currents for an HCT4051 analog
mux? I'm wondering about both ESD diode leakage (ie, to rails) and
leakage through open switches. I'm running 0 and +5V rails.
Indications are that everybody's 0.1 uA max spec is wildly
pessimistic, but I was wondering if anybody knows more, before I have
to drag my butt into the lab and make actual measurements. It's a lot
easier to sit here and type and eat bon-bons.
We're scanning eight RTDs. A +2.5 volt reference goes through a
precision 270 ohm resistor and gets mux'd to a selected RTD. The
voltage drop across the RTD gets differentially mux'd, too. A 24-bit
delta-sigma ADC digitizes the voltage drop across the 270, then the
voltage across the RTD, and does the math. We're getting errors in the
tens of PPM, tolerable, but we're curious where they're coming from.
The sensitivity analysis math here is a nuisance.
I did measure an old CD4053 about two years ago (sorry did not have 74HC).
It had +/-5V supplies and was arranged for 0V common mode at both switch
ends.
IIRC the leakages were about 1pA at room temperature.
One thing that surpised me was that, while static leakages were that low,
dynamic leakages (100kHz switching) were not negligeable at all.
Quizz: how do you pump fraction of a uA through the resistors with the
switch wired as below?
4053
.------------.
| | ___
| |--|___|- GND
___ | o---/| 50K
GND -|___|--|---o--__ |
50K | o--- | ___
| \|--|___|- GND
'------------' 50K
|
Good point, it probably IS charge injection. The measured effect of
that WOULD vary with R value, as John seems to be seeing.
...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:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food. |
|
| Back to top |
|
|
Jim Thompson
Guest
|
| Posted:
Thu Dec 01, 2005 5:35 pm Post subject:
Re: HCT4051 leakage |
|
|
On Wed, 30 Nov 2005 19:37:17 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
[snip]
| Quote: |
We'll build and test a few more boards. If the errors seem systematic,
we can always toss a little software curvature at it, and never really
have to understand the physics.
Y = Y + K * Y^2
has cured more sins than the College of Cardinals.
John
|
Bwahahahaha! Until you get a new batch of parts.
...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:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food. |
|
| Back to top |
|
|
Fred Bartoli
Guest
|
| Posted:
Thu Dec 01, 2005 5:35 pm Post subject:
Re: HCT4051 leakage |
|
|
"Jim Thompson" <To-Email-Use-The-Envelope-Icon@My-Web-Site.com> a écrit dans
le message de news:724uo19cmc1n0lh2n7pueh1iav389rtrg7@4ax.com...
| Quote: | On Thu, 1 Dec 2005 09:01:04 +0100, "Fred Bartoli"
fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
I did measure an old CD4053 about two years ago (sorry did not have
74HC).
It had +/-5V supplies and was arranged for 0V common mode at both switch
ends.
IIRC the leakages were about 1pA at room temperature.
One thing that surpised me was that, while static leakages were that low,
dynamic leakages (100kHz switching) were not negligeable at all.
Quizz: how do you pump fraction of a uA through the resistors with the
switch wired as below?
4053
.------------.
| | ___
| |--|___|- GND
___ | o---/| 50K
GND -|___|--|---o--__ |
50K | o--- | ___
| \|--|___|- GND
'------------' 50K
Good point, it probably IS charge injection. The measured effect of
that WOULD vary with R value, as John seems to be seeing.
|
But charge injection _alone_ can't do that. It will eventually reach
equilibrium but it can't _alone_ have the switch 3 pins negative or positive
all at once.
Have a look at (or remember :-) how switches are designed...
That was surprising a first sight, but then natural, looking a bit further.
I think John's switching frequency is way much lower, so no pb.
--
Thanks,
Fred. |
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Phil Hobbs
Guest
|
| Posted:
Thu Dec 01, 2005 5:35 pm Post subject:
Re: HCT4051 leakage |
|
|
David L. Jones wrote:
| Quote: |
Don't remeber exactly but I vaguely remember these suckers having
different leakages (or some other characteristic) in different
directions too? (or was it the 4066...?)
There was some fine print in the datasheet that admitted this.
Was quite a bad trap for my application at the time.
They can be nasty little suckers.
Dave :)
|
The other thing is that if any of the analogue lines goes outside the
power supplies, all the analogue lines get connected together. Very
confusing.
Cheers,
Phil Hobbs |
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