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how to read value into ADC from variable resistor?

M

Michael Noone

Jan 1, 1970
0
Hi - I'm going to be using a CUI pressure sensor
(http://www.cui.com/pdffiles/SF-X.pdf). It acts as a variable resistor. I
want to connect it to an ADC (fairly high impedance - I think about 10M).
How does one do this? My only idea was to make a sort of voltage divider -
with one pin of the pressure sensor held high, the other connected to the
ADC. Then have a resistor between the ADC and ground. I suppose I'd have to
have a large resistor in front of the ADC to raise the impedance as the
pressure sensor's resistance goes up to 100M or so. Is this the best way to
do it? How does one choose a value for the resistor going to ground?

Thanks!

-MJ Noone
 
A

Andrew Holme

Jan 1, 1970
0
Michael said:
Hi - I'm going to be using a CUI pressure sensor
(http://www.cui.com/pdffiles/SF-X.pdf). It acts as a variable resistor. I
want to connect it to an ADC (fairly high impedance - I think about 10M).
How does one do this? My only idea was to make a sort of voltage divider -
with one pin of the pressure sensor held high, the other connected to the
ADC. Then have a resistor between the ADC and ground. I suppose I'd have to
have a large resistor in front of the ADC to raise the impedance as the
pressure sensor's resistance goes up to 100M or so. Is this the best way to
do it? How does one choose a value for the resistor going to ground?

Thanks!

-MJ Noone

What pressure range do you want to measure? The recommended load for
that sensor is 1.5kgf, at which its resistance is in the region of 2k
ohm. The specification section on the datasheet quotes maximum
allowable voltages and currents. It doesn't matter how you use it, as
long as you stick within the limits. You could either feed it with a
constant current, or make it part of a voltage divider. The datasheet
suggests an operating current of 5mA which implies the former.
 
R

Rene Tschaggelar

Jan 1, 1970
0
Michael said:
Hi - I'm going to be using a CUI pressure sensor
(http://www.cui.com/pdffiles/SF-X.pdf). It acts as a variable resistor. I
want to connect it to an ADC (fairly high impedance - I think about 10M).
How does one do this? My only idea was to make a sort of voltage divider -
with one pin of the pressure sensor held high, the other connected to the
ADC. Then have a resistor between the ADC and ground. I suppose I'd have to
have a large resistor in front of the ADC to raise the impedance as the
pressure sensor's resistance goes up to 100M or so. Is this the best way to
do it? How does one choose a value for the resistor going to ground?

With a bridge, the best results are achieved
when both resistors are about the same. Then
use a high impedance buffer amplifier in front
of the ADC.

Rene
 
J

Jonathan Westhues

Jan 1, 1970
0
Michael Noone said:
Hi - I'm going to be using a CUI pressure sensor
(http://www.cui.com/pdffiles/SF-X.pdf). It acts as a variable resistor. I
want to connect it to an ADC (fairly high impedance - I think about 10M).
How does one do this? My only idea was to make a sort of voltage divider -
with one pin of the pressure sensor held high, the other connected to the
ADC. Then have a resistor between the ADC and ground.

Yes, you can do this. The only problem is that the relationship between the
unknown (pressure sensor) resistance and the voltage applied to the ADC is
not linear. That means that you need to do some math to get the resistance
from the ADC reading. It also means that the resolution is not constant with
resistance: for some pressure regions, a small change in resistance will
cause a large change in the ADC reading, but it in others it will take a
large change in resistance to produce any change in the ADC reading.

This might actually be good though, because your sensor is non-linear, so
you might be able to use this non-linearity to make the sensor a bit more
linear in the regions where you care about that.
I suppose I'd have to
have a large resistor in front of the ADC to raise the impedance as the
pressure sensor's resistance goes up to 100M or so.

I don't know anything about pressure sensors, but look at the figure at the
bottom left of the first page. If I am interpreting it correctly then it
shows data for four different samples of the same model of sensor. The
response is all over the place below 250 gf: a 50 gf weight produced a
little less than a 1meg resistance on one sensor, but more than 10meg on two
others. I don't think that the sensor is intended for use in those regions.

Also, impedance does not work like that. If you put a 90meg resistor in
front of an ADC with an input resistance of 10meg, then you are right that
the input resistance increases to 100meg. However, you have also thrown away
nine tenths of your signal; more voltage will be dropped across the 90meg
resistor than across the ADC's input resistance.

In any case, are you sure that your ADC input looks like a resistance? In
many types of A/D, the input looks like the sample and hold capacitor.

Jonathan
 
A

Andrew Holme

Jan 1, 1970
0
Andrew Holme wrote:
[snip]
You could either feed it with a
constant current, or make it part of a voltage divider. The datasheet
suggests an operating current of 5mA which implies the former.

Or you could design your potential divider for 5mA.
 
M

Meindert Sprang

Jan 1, 1970
0
Andrew Holme said:
Andrew Holme wrote:
[snip]
You could either feed it with a
constant current, or make it part of a voltage divider. The datasheet
suggests an operating current of 5mA which implies the former.

Or you could design your potential divider for 5mA.

Now go back to the drawingboard and calculate the needed voltage to 5mA
through a sensor wit 1 to 10MOhm resistor.....

My suggestion: use a decent pressure sensor instead of this crap.
Decent pressure sensors use strain gauges which can be prefectly mearured
using a wheatstone brigde.

Meindert
 
M

Michael Noone

Jan 1, 1970
0
Andrew Holme said:
Andrew Holme wrote:
[snip]
You could either feed it with a
constant current, or make it part of a voltage divider. The datasheet
suggests an operating current of 5mA which implies the former.

Or you could design your potential divider for 5mA.

Now go back to the drawingboard and calculate the needed voltage to 5mA
through a sensor wit 1 to 10MOhm resistor.....

My suggestion: use a decent pressure sensor instead of this crap.
Decent pressure sensors use strain gauges which can be prefectly mearured
using a wheatstone brigde.

Meindert

Can you suggest a different pressure sensor of similar dimensions? (it
needs to be about 3mm square/round) The only other similar sensors I've
been able to find are made by IEE (http://www.iee.lu/nav04/lusense.htm) who
don't have any distributors in the USA. Their parts do look quite nice
though - take a look at Conrad (http://www.conrad.de) 182519-62, datasheet
here: http://www.produktinfo.conrad.com/datenblaetter/175000-199999/182519-
an-01-de-Drucksensor_FSR-149AS.pdf.

-Michael
 
M

Michael Noone

Jan 1, 1970
0
What pressure range do you want to measure? The recommended load for
that sensor is 1.5kgf, at which its resistance is in the region of 2k
ohm. The specification section on the datasheet quotes maximum
allowable voltages and currents. It doesn't matter how you use it, as
long as you stick within the limits. You could either feed it with a
constant current, or make it part of a voltage divider. The datasheet
suggests an operating current of 5mA which implies the former.

Ideally I'd like to be able to measure about 0-1kgf, which unfortunately
is in the least accurate region of the sensor. These are for a one off
application and a total of 6 will be used - so I was planning on
individually calibrating each to compensate for this inaccuracy.

-MJ Noone
 
M

Meindert Sprang

Jan 1, 1970
0
Michael Noone said:
Can you suggest a different pressure sensor of similar dimensions? (it
needs to be about 3mm square/round)

No I can't Have you searched google for "miniature load cells" ?

Meindert
 
T

Tim Wescott

Jan 1, 1970
0
Michael said:
Ideally I'd like to be able to measure about 0-1kgf, which unfortunately
is in the least accurate region of the sensor. These are for a one off
application and a total of 6 will be used - so I was planning on
individually calibrating each to compensate for this inaccuracy.

-MJ Noone

You're asking to be able to read a tremendously large resistance span
there. I would investigate the possibility of preloading the thing with
a few Newtons (what serious engineer uses "kgf" for crying out loud!).
This preloading has the potential to wreck your DC accuracy if you can't
do it with a weight, so be ready to jettison the idea fast.

If you can't preload and you need anything resembling accuracy then
you're stuck with what you've got. While the resistance of the thing
varies wildly with force, it appears that the conductance vs. force
relationship may be more well behaved. If you could design a little
circuit that would hold the sensor voltage constant and monitor the
current you may do well:


|\
2mV <-------|+\
| >---------o----------->
.--|-/ | To ADC
| |/ .-. .--->
| | | |
| | | GND
| '-'
| ___ |
'-----|___|-----o----->
To Sensor
.----->
|
|
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Note that this circuit, with it's 2mV input to the opamp, requires
either an ultra-low offset opamp or an AC coupled system. Since the DC
coupled circuit would be incredibly sensitive to ground loops I'd
probably do it AC coupled: I'd add appropriate blocking caps and drive a
square wave from a microprocessor into the opamp. I'd use the same
microprocessor to read the ADC at both the high and low point of the
square wave, subtract the two values and average the hell out of the
result -- and I'd still expect to have difficulties.
 
M

Michael Noone

Jan 1, 1970
0
You're asking to be able to read a tremendously large resistance span
there. I would investigate the possibility of preloading the thing
with a few Newtons (what serious engineer uses "kgf" for crying out
loud!). This preloading has the potential to wreck your DC accuracy if
you can't do it with a weight, so be ready to jettison the idea fast.

gf and kgf threw me off too. I had never heard of that term before - so I
had to look it up. Preloading would probabaly be possible, though it'll be
hard to do accurately. Today I have been in communication with a couple
other FSR (force sensing resistor) manufacturers, and they look to be more
accurate in the low pressure region, so I'll probabaly end up going with
one of those.

If you can't preload and you need anything resembling accuracy then
you're stuck with what you've got. While the resistance of the thing
varies wildly with force, it appears that the conductance vs. force
relationship may be more well behaved. If you could design a little
circuit that would hold the sensor voltage constant and monitor the
current you may do well:


|\
2mV <-------|+\
| >---------o----------->
.--|-/ | To ADC
| |/ .-. .--->
| | | |
| | | GND
| '-'
| ___ |
'-----|___|-----o----->
To Sensor
.----->
|
|
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Note that this circuit, with it's 2mV input to the opamp, requires
either an ultra-low offset opamp or an AC coupled system. Since the
DC coupled circuit would be incredibly sensitive to ground loops I'd
probably do it AC coupled: I'd add appropriate blocking caps and drive
a square wave from a microprocessor into the opamp. I'd use the same
microprocessor to read the ADC at both the high and low point of the
square wave, subtract the two values and average the hell out of the
result -- and I'd still expect to have difficulties.

If I do end up using these CUI sensors your idea definitely sounds
interesting. Complicated, but interesting!

-Michael
 
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