# HCT4051 leakage

J

#### John Larkin

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

J

#### Jim Thompson

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

J

#### Joerg

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?

And watch our for fingerprints, flux and bon-bon residue ;-)

Regards, Joerg

J

#### John Larkin

Hello John,

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

B

#### [email protected]

John said:
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.

M

#### Mark

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

J

#### john jardine

[...]
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

D

#### David L. Jones

John said:
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

J

#### John Larkin

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

F

#### Fred Bartoli

John Larkin said:
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

T

#### Tony Williams

John Larkin said:
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.

V270, Vunknown, V270. See if the two V270 results are
the same.

If they are not, you might get away with using the

J

#### Jim Thompson

On Wed, 30 Nov 2005 19:37:17 -0800, John Larkin

[snip]
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

J

#### Jim Thompson

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

F

#### Fred Bartoli

Jim Thompson said:
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.

P

#### Phil Hobbs

David said:
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

A

#### Arie de Muynck

"John Larkin" ...
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?

Have you considered the thermal gradient in the chip may cause offset
voltages from the thermocouple like junctions (also Al-Si bonding points
etc) in the chip? I've seen 10..100 uV offsets when a 4051 chip (turned ON)
was next to a dissipating element. What's the equivalent offset voltage you
see?

Arie de Muynck

J

#### John Larkin

"John Larkin" ...

Have you considered the thermal gradient in the chip may cause offset
voltages from the thermocouple like junctions (also Al-Si bonding points
etc) in the chip? I've seen 10..100 uV offsets when a 4051 chip (turned ON)
was next to a dissipating element. What's the equivalent offset voltage you
see?

Arie de Muynck

Yikes, that is a real possibility. I did note in this ng, recently,

Assuming a couple K of self-heating in the 4051, and a few uV/K
thermoelectric effect, we could have very roughly 10-50 (?) microvolts
of error injected into the middle of the series resistor pair, in the
ballpark of the error we're seeing. The error should go as i^2, which
we can analyze to see if this is an actual candidate effect to explain
(some of) the residual errors.

But wait...

vref-------+------------
|
|
+------------
|
|
|
4051a
|
|
+------4051b------
|
|
+------4051c------
|
|
gnd

We're doing a 4-wire voltage-drop measurement on each of the two
resistors. So adding a potential internal to the analog switch 4051a
in effect only changes the overall reference voltage. And the ohms
calculation is ratiometric, so the actual voltage doesn't matter. So
what's left is a time-domain *change* in this error component during
the measurement process.

This precision stuff is interesting. Very low-order effects become
important, and they are the devil to find and quantify.

John

J

#### Joerg

Hello John,
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.

I don't know what the leakage mechanism is, that would be Jim's domain.
However, unless I misunderstood something then 0.4uA out of 6mA is
already 66ppm. So a few ten ppm seems like you are getting a good deal
from those '51 chips. If they spec them at 0.4uA max versus 0.1uA for
the other muxes then it's unlikely you'd be getting 0.1uA from a '51.

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?

Maybe one way to find out is to scoot the measurement window back and
forth a few tens of milliseconds.

Whenever I wanted to know those kind of things from app engineers I was
often told that such data is not available and that I should use this
newer xyz gizmo chip. Which, of course, was an order of magnitude more
expensive.

Regards, Joerg

P

#### Phil Hobbs

John said:
Assuming a couple K of self-heating in the 4051, and a few uV/K
thermoelectric effect, we could have very roughly 10-50 (?) microvolts
of error injected into the middle of the series resistor pair, in the
ballpark of the error we're seeing. The error should go as i^2, which
we can analyze to see if this is an actual candidate effect to explain
(some of) the residual errors.

John,

TC coefficient for silicon to any metal is about _700_uv/K_. Ugly ugly
ugly.

From your schematic, it looks as though TC offsets of sections B and C
are not cancelled by the 4-wire ratiometric approach.

Cheers,

Phil Hobbs

J

#### John Larkin

John,

TC coefficient for silicon to any metal is about _700_uv/K_. Ugly ugly
ugly.

Ghastly. Look at the graph at the end!

http://www.uni-konstanz.de/physik/Jaeckle/papers/thermopower/node1.html

Are p-n couples used as thermal imagers?

From your schematic, it looks as though TC offsets of sections B and C
are not cancelled by the 4-wire ratiometric approach.

Yeah, but there's no current in those switches. There are three
separate 4051's, separate packages.

John