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Interesting sensor design problem

C

Charles Edmondson

Jan 1, 1970
0
Ok, I am trying to do a little sensor interface design, and running into
an interesting problem.

I am interfacing some of the Freescale pressure sensors to read
barometric pressure. The are basically bridge devices, so I get a
differential voltage on top of a offset.

Now here is my problem. I want to take the range of values for normal
atmosphere, 6.68mV to 13.7mV (I am using 3Vdc) and amp that range to
something like 0 to 3Vdc. So, how do I design a circuit that basically has:
out = 0 for in < 6.68Vdc
out = 300*Vin for 6.68mVdc < Vin < 13.6mVdc
out = 3 for Vin > 13.6mVdc

I am realizing that just a simple instrumentation amplifier ain't gonna
do it this time!
 
J

Jim Thompson

Jan 1, 1970
0
Ok, I am trying to do a little sensor interface design, and running into
an interesting problem.

I am interfacing some of the Freescale pressure sensors to read
barometric pressure. The are basically bridge devices, so I get a
differential voltage on top of a offset.

Now here is my problem. I want to take the range of values for normal
atmosphere, 6.68mV to 13.7mV (I am using 3Vdc) and amp that range to
something like 0 to 3Vdc. So, how do I design a circuit that basically has:
out = 0 for in < 6.68Vdc
out = 300*Vin for 6.68mVdc < Vin < 13.6mVdc
out = 3 for Vin > 13.6mVdc

I am realizing that just a simple instrumentation amplifier ain't gonna
do it this time!

Charlie,

Differential centered at +1.5V?

Rail-to-rail OpAmps?

Or is the 3VDC just the total excitation? And the power supplies
available are??

...Jim Thompson
 
C

Charles Edmondson

Jan 1, 1970
0
Jim said:
Charlie,

Differential centered at +1.5V?

Rail-to-rail OpAmps?

Or is the 3VDC just the total excitation? And the power supplies
available are??

...Jim Thompson
Hi Jim,
Differential is centered at 1.5Vdc. Don't mind using rail-to-rails, as
this is a contest entry. I do have 5Vdc available if I really need it,
but most stuff is running at 3Vdc.

So, I need to ignore the common mode offset, and also ignore part of the
range of the differential. keep thinking I have to do some sort of two
stage thing, like one amp that ignores the lower part of the offset, and
then a second stage that ignores the upper part of the offset...
 
K

Ken Smith

Jan 1, 1970
0
Ok, I am trying to do a little sensor interface design, and running into
an interesting problem.

I am interfacing some of the Freescale pressure sensors to read
barometric pressure. The are basically bridge devices, so I get a
differential voltage on top of a offset.

Now here is my problem. I want to take the range of values for normal
atmosphere, 6.68mV to 13.7mV (I am using 3Vdc) and amp that range to
something like 0 to 3Vdc. So, how do I design a circuit that basically has:
out = 0 for in < 6.68Vdc
out = 300*Vin for 6.68mVdc < Vin < 13.6mVdc
out = 3 for Vin > 13.6mVdc

I am realizing that just a simple instrumentation amplifier ain't gonna
do it this time!

A normal 3 op-amp dif-amp can shift the bridge voltage down to ground as
it amplifies it. To subtract away a voltage, you need a reverence of some
kind.

--/\/\/--- +Vref
!
(-)---!+\ !
! >--+-/\/\/-+--+----/\/\/---
--!-/ ! ! !
! ! ! !
+-/\/\/-- --!-\ !
! ! >------+---
\ --!+/
/ !
\ !
/ !
! !
+-/\/\/-- !
! ! !
--!-\ ! !
! >--+-/\/\/----+
(+)---!+/ !
/
\
/
\
!
GND
 
J

Jim Thompson

Jan 1, 1970
0
A normal 3 op-amp dif-amp can shift the bridge voltage down to ground as
it amplifies it. To subtract away a voltage, you need a reverence of some
kind.

--/\/\/--- +Vref
!
(-)---!+\ !
! >--+-/\/\/-+--+----/\/\/---
--!-/ ! ! !
! ! ! !
+-/\/\/-- --!-\ !
! ! >------+---
\ --!+/
/ !
\ !
/ !
! !
+-/\/\/-- !
! ! !
--!-\ ! !
! >--+-/\/\/----+
(+)---!+/ !
/
\
/
\
!
GND
--

I refuse to provide "reverence" ;-)

...Jim Thompson
 
S

Spehro Pefhany

Jan 1, 1970
0
On Sat, 12 Feb 2005 02:49:23 +0000 (UTC), the renowned
A normal 3 op-amp dif-amp can shift the bridge voltage down to ground as
it amplifies it. To subtract away a voltage, you need a reverence of some
kind.

--/\/\/--- +Vref
!
(-)---!+\ !
! >--+-/\/\/-+--+----/\/\/---
--!-/ ! ! !
! ! ! !
+-/\/\/-- --!-\ !
! ! >------+---
\ --!+/
/ !
\ !
/ !
! !
+-/\/\/-- !
! ! !
--!-\ ! !
! >--+-/\/\/----+
(+)---!+/ !
/
\
/
\
!
GND


Just drop the resistor to +Vref and replace GND with Vref (needs to be
stiff). Maximize the swing at the first stage without risking hitting
the rails.



Best regards,
Spehro Pefhany
 
A

Active8

Jan 1, 1970
0
A normal 3 op-amp dif-amp can shift the bridge voltage down to ground as
it amplifies it. To subtract away a voltage, you need a reverence of some
kind.
Look at the specs. Subtraction isn't the answer.
 
T

Tony Williams

Jan 1, 1970
0
Charles Edmondson said:
Now here is my problem. I want to take the range of values for
normal atmosphere, 6.68mV to 13.7mV (I am using 3Vdc) and amp
that range to something like 0 to 3Vdc. So, how do I design a
circuit that basically has:
out = 0 for in < 6.68Vdc
out = 300*Vin for 6.68mVdc < Vin < 13.6mVdc
out = 3 for Vin > 13.6mVdc

Could you clarify the numbers?

Vout = 0V at Vin = 6.68mV. A zero offset, no prob.

300*(13.6mV - 6.68mV) = 2.076V output swing.

It doesn't reach 3V?
 
C

Charles Edmondson

Jan 1, 1970
0
Rich said:
...

Do your contributors get a piece of the action?
It depends on if there is any prize money worth piecing out... my last
try got me $300 (after spending about $400...)
 
C

Charles Edmondson

Jan 1, 1970
0
Tony said:
Could you clarify the numbers?

Vout = 0V at Vin = 6.68mV. A zero offset, no prob.

300*(13.6mV - 6.68mV) = 2.076V output swing.

It doesn't reach 3V?

Hi Tony,
I would prefer that it did. Was just spewing numbers to get the general
idea across. Wanted to reject values outside that range, and then amp
that range to my full voltage swing...
 
J

Jim Thompson

Jan 1, 1970
0
Hi Tony,
I would prefer that it did. Was just spewing numbers to get the general
idea across. Wanted to reject values outside that range, and then amp
that range to my full voltage swing...

Why bother with "reject"? Just negative rail for VIN<6.68mV and
positive rail for VIN>13.6mV.

...Jim Thompson
 
F

Fred Bloggs

Jan 1, 1970
0
I am interfacing some of the Freescale pressure sensors to read
barometric pressure. The are basically bridge devices, so I get a
differential voltage on top of a offset.

I want to take the range of values for normal
atmosphere, 6.68mV to 13.7mV (I am using 3Vdc) and amp that range to
something like 0 to 3Vdc. So, how do I design a circuit that basically
has:
out = 0 for in < 6.68Vdc
out = 300*Vin for 6.68mVdc < Vin < 13.6mVdc
out = 3 for Vin > 13.6mVdc

You want rectify the output around a reference corresponding to the
output for a 6.7mV positive differential. So something like this works
well if the input CM is known to be Vbatt/2 +/- a few mV:

View in a fixed-width font such as Courier.





INA122
+------------------+
| |
+ | |\ |
V -------|--------|+\ | + -
in | | >---+--|----> V =G x (V -V ) + V
| ,---|-/ | | out in in ref
| | |/ | |
| | | |
| | | | 200K
| +---100K----- | G= (5 + ----)
| | | RG
| | |
,----|---+---25K-----+ | 3.0-1.5
| | | | | Make G= ------- =214
(957)RG | | | | 7m
| | | | |
'----|---+---25K ----+ | 200K
| | | | RG= ---- = 957 ohms
| | | | 209
| | | |
| | | |
| | | | Select
| | | |
| | |\ | | V = 1.5V-G x 6.7m=66.2mV
| +-----|-\ | | ref
- | | | >-- |
V ------|---|-----|+/ | Then
in | | |/ | + -
| 100K | V = Gx(V -V )+ 1.5V-G x 6.7m
| | | out in in
| | |
+------------------+ + -
| = Gx(V -V -6.7m) + 1.5V
| in in
V
ref 1.5V<V < 3.0V
out

as
+ -
6.7m<V -V <13.7m
in in





Final CKT:




gnd Vbatt
| |
INA122--------------+
| | RRIO
+ | |\ | +-R-+
V -------|--------|+\ | | | |\
in | | >---+--|-+-R-+------------|+\
| ,---|-/ | | | | >--+-> 0<V <3V
| | |/ | | R +----|-/ | out
| | | | | | |/ |
| | | | | | |
| +---100K----- | +---R---+-----R----+
| | | | |
| | | | R
,----|---+---25K-----+ | | |+ |
| | | | | +--||---+--Vbatt
(957)RG | | | | | | |
| | | | | | | (3V)
'----|---+---25K ----+ | | (1880)
| | | | | |
| | | | +--42---+
| | | | | |
| | | | gnd |
| | | | |
| | | | |
| | |\ | | |
| +-----|-\ | | |
- | | | >-- | |
V ------|---|-----|+/ | |
in | | |/ | |
| 100K | |
| | | |
| | | |
+------------------+ |
| |
V '----------------------------'
ref
 
F

Fred Bloggs

Jan 1, 1970
0
Shorting RG doesn't help a bit:

View in a fixed-width font such as Courier.





INA122
+------------------+
| |
+ | |\ |
V -------|--------|+\ | + -
in | | >---+--|----> V =G x (V -V ) + V
| ,---|-/ | | out in in ref
| | |/ | |
| | | |
| | | | 200K
| +---100K----- | G= (5 + ----)
| | | RG
| | |
,----|---+---25K-----+ | 3.0-1.5
| | | | Make G= ------- =214
(957)RG | | | 7m
| | | |
'----|---+---25K ----+ | 200K
| | | | RG= ---- = 957 ohms
| | | | 209
| | | |
| | | |
| | | | Select
| | | |
| | |\ | | V = 1.5V-G x 6.7m=66.2mV
| +-----|-\ | | ref
- | | | >-- |
V ------|---|-----|+/ | Then
in | | |/ | + -
| 100K | V = Gx(V -V )+ 1.5V-G x 6.7m
| | | out in in
| | |
+------------------+ + -
| = Gx(V -V -6.7m) + 1.5V
| in in
V
ref 1.5V<V < 3.0V
out

as
+ -
6.7m<V -V <13.7m
in in





Final CKT:




gnd Vbatt
| |
INA122--------------+
| | RRIO
+ | |\ | +-R-+
V -------|--------|+\ | | | |\
in | | >---+--|-+-R-+------------|+\
| ,---|-/ | | | | >--+-> 0<V <3V
| | |/ | | R +----|-/ | out
| | | | | | |/ |
| | | | | | |
| +---100K----- | +---R---+-----R----+
| | | | |
| | | | R
,----|---+---25K-----+ | | |+ |
| | | | +--||---+--Vbatt
(957)RG | | | | | |
| | | | | | (3V)
'----|---+---25K ----+ | | (1880)
| | | | | |
| | | | +--42---+
| | | | | |
| | | | gnd |
| | | | |
| | | | |
| | |\ | | |
| +-----|-\ | | |
- | | | >-- | |
V ------|---|-----|+/ | |
in | | |/ | |
| 100K | |
| | | |
| | | |
+------------------+ |
| |
V '----------------------------'
ref
 
F

Frank Bemelman

Jan 1, 1970
0
Fred Bloggs said:
You want rectify the output around a reference corresponding to the
output for a 6.7mV positive differential. So something like this works
well if the input CM is known to be Vbatt/2 +/- a few mV:

[snip]

I was thinking about using an opamp follower for the 'high' side
of the bridge, and then use a resistive divider to get rid of
the 6.68mV without loading the bridge.

+ |\
V ----------------|+\
in | >---+--.
,----|-/ | |
| |/ | |
| | |
| | |
'---100K----' |
|
44R
|
|
+------------> to classic
instrumentation amplifier
|
|
10K
|
|
GND
-
V ---------------------------------------> to classic
instrumentation amplifier
in


Is that stupid?
 
F

Fred Bloggs

Jan 1, 1970
0
Frank said:
You want rectify the output around a reference corresponding to the
output for a 6.7mV positive differential. So something like this works
well if the input CM is known to be Vbatt/2 +/- a few mV:


[snip]

I was thinking about using an opamp follower for the 'high' side
of the bridge, and then use a resistive divider to get rid of
the 6.68mV without loading the bridge.

+ |\
V ----------------|+\
in | >---+--.
,----|-/ | |
| |/ | |
| | |
| | |
'---100K----' |
|
44R
|
|
+------------> to classic
instrumentation amplifier
|
|
10K
|
|
GND
-
V ---------------------------------------> to classic
instrumentation amplifier
in

Now you have the output of the IA=G*(V(+)-V(-)-44/10.044K*V(+)) and this
is IA=G*(Vdiff-44/10.044K*(Vcm+Vdiff/2))=G*(0.9978*Vdiff-0.004*Vcm), and
G=3V/(.9978*7m)=430 -> d(IA)/d(Vcm)= 430*0.004=1.72V/V, this is a not so
good sensitivity to Vcm. Compare this to the INA122 ckt with
d(Vout)/d(Vcm)=840uV/V (even with the crummy R-divider Vref ckt). You
would have servo 0.004*Vcm out of your classic IA.
 
F

Fred Bloggs

Jan 1, 1970
0
Fred said:
Now you have the output of the IA=G*(V(+)-V(-)-44/10.044K*V(+)) and this
is IA=G*(Vdiff-44/10.044K*(Vcm+Vdiff/2))=G*(0.9978*Vdiff-0.004*Vcm), and
G=3V/(.9978*7m)=430 -> d(IA)/d(Vcm)= 430*0.004=1.72V/V, this is a not so
good sensitivity to Vcm. Compare this to the INA122 ckt with
d(Vout)/d(Vcm)=840uV/V (even with the crummy R-divider Vref ckt). You
would have servo 0.004*Vcm out of your classic IA.

And when you do that, you lose your offset.
 
F

Frank Bemelman

Jan 1, 1970
0
Fred Bloggs said:
Frank said:
I am interfacing some of the Freescale pressure sensors to read
barometric pressure. The are basically bridge devices, so I get a
differential voltage on top of a offset.

I want to take the range of values for normal
atmosphere, 6.68mV to 13.7mV (I am using 3Vdc) and amp that range to
something like 0 to 3Vdc. So, how do I design a circuit that basically
has:
out = 0 for in < 6.68Vdc
out = 300*Vin for 6.68mVdc < Vin < 13.6mVdc
out = 3 for Vin > 13.6mVdc


You want rectify the output around a reference corresponding to the
output for a 6.7mV positive differential. So something like this works
well if the input CM is known to be Vbatt/2 +/- a few mV:


[snip]

I was thinking about using an opamp follower for the 'high' side
of the bridge, and then use a resistive divider to get rid of
the 6.68mV without loading the bridge.

+ |\
V ----------------|+\
in | >---+--.
,----|-/ | |
| |/ | |
| | |
| | |
'---100K----' |
|
44R
|
|
+------------> to classic
instrumentation amplifier
|
|
10K
|
|
GND
-
V ---------------------------------------> to classic
instrumentation amplifier
in

Now you have the output of the IA=G*(V(+)-V(-)-44/10.044K*V(+)) and this
is IA=G*(Vdiff-44/10.044K*(Vcm+Vdiff/2))=G*(0.9978*Vdiff-0.004*Vcm), and
G=3V/(.9978*7m)=430 -> d(IA)/d(Vcm)= 430*0.004=1.72V/V, this is a not so
good sensitivity to Vcm. Compare this to the INA122 ckt with
d(Vout)/d(Vcm)=840uV/V (even with the crummy R-divider Vref ckt). You
would have servo 0.004*Vcm out of your classic IA.

Well, my intuition <g> told me that the common-mode voltage was perhaps
not a problem. The bridge is sitting practically still at 1.5V all the time,
looking at it from a distance.
 
H

hamilton

Jan 1, 1970
0
Charles said:
Ok, I am trying to do a little sensor interface design, and running into
an interesting problem.

I am interfacing some of the Freescale pressure sensors to read
barometric pressure. The are basically bridge devices, so I get a
differential voltage on top of a offset.

Now here is my problem. I want to take the range of values for normal
atmosphere, 6.68mV to 13.7mV (I am using 3Vdc) and amp that range to
something like 0 to 3Vdc. So, how do I design a circuit that basically
has:
out = 0 for in < 6.68Vdc
out = 300*Vin for 6.68mVdc < Vin < 13.6mVdc
out = 3 for Vin > 13.6mVdc

I am realizing that just a simple instrumentation amplifier ain't gonna
do it this time!


Why not? I just did this with :
http://www.analog.com/en/prod/0,2877,AD623,00.html
 
K

Ken Smith

Jan 1, 1970
0
[...]
A normal 3 op-amp dif-amp can shift the bridge voltage down to ground as
it amplifies it. To subtract away a voltage, you need a reverence of some
kind.

--/\/\/--- +Vref
!
(-)---!+\ !
! >--+-/\/\/-+--+----/\/\/---
--!-/ ! ! !
! ! ! !
+-/\/\/-- --!-\ !
! ! >------+---
\ --!+/
/ !
\ !
/ !
! !
+-/\/\/-- !
! ! !
--!-\ ! !
! >--+-/\/\/----+
(+)---!+/ !
/
\
/
\
!
GND
--

I refuse to provide "reverence" ;-)

I don't see why not, its a pretty good circuit. So what if it needs a
little reverence to make it work?
 
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