Interesting sensor design problem
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Charles Edmondson
Guest
Posted: Sat Feb 12, 2005 5:28 am    Post subject: Interesting sensor design problem Reply with quote
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
--
Edmondson Engineering
Unique Solutions to Unusual Problems

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Ken Smith
Guest
Posted: Sat Feb 12, 2005 5:28 am    Post subject: Re: Interesting sensor design problem Reply with quote
In article <420d3fa7$1@news.cadence.com>,
Charles Edmondson <edmondson@ieee.org> wrote:
Quote:
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
--
--
kensmith@rahul.net forging knowledge
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Jim Thompson
Guest
Posted: Sat Feb 12, 2005 5:39 am    Post subject: Re: Interesting sensor design problem Reply with quote
On Fri, 11 Feb 2005 15:28:39 -0800, Charles Edmondson
<edmondson@ieee.org> wrote:

Quote:
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

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
--
| 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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Charles Edmondson
Guest
Posted: Sat Feb 12, 2005 5:58 am    Post subject: Re: Interesting sensor design problem Reply with quote
Jim Thompson wrote:

Quote:
On Fri, 11 Feb 2005 15:28:39 -0800, Charles Edmondson
edmondson@ieee.org> wrote:


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


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...

--
Charlie
--
Edmondson Engineering
Unique Solutions to Unusual Problems
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Rich The Newsgropup Wacko
Guest
Posted: Sat Feb 12, 2005 6:11 am    Post subject: Re: Interesting sensor design problem Reply with quote
On Fri, 11 Feb 2005 15:58:21 -0800, Charles Edmondson wrote:

Quote:
...
this is a contest entry.
....


Do your contributors get a piece of the action?
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Spehro Pefhany
Guest
Posted: Sat Feb 12, 2005 6:11 am    Post subject: Re: Interesting sensor design problem Reply with quote
On Sat, 12 Feb 2005 02:49:23 +0000 (UTC), the renowned
kensmith@green.rahul.net (Ken Smith) wrote:

Quote:
In article <420d3fa7$1@news.cadence.com>,
Charles Edmondson <edmondson@ieee.org> wrote:
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


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
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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Jim Thompson
Guest
Posted: Sat Feb 12, 2005 6:11 am    Post subject: Re: Interesting sensor design problem Reply with quote
On Sat, 12 Feb 2005 02:49:23 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:

Quote:
In article <420d3fa7$1@news.cadence.com>,
Charles Edmondson <edmondson@ieee.org> wrote:
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
--

I refuse to provide "reverence" ;-)

...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
Tony Williams
Guest
Posted: Sat Feb 12, 2005 11:39 am    Post subject: Re: Interesting sensor design problem Reply with quote
In article <420d3fa7$1@news.cadence.com>,
Charles Edmondson <edmondson@ieee.org> wrote:

Quote:
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?

--
Tony Williams.
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Active8
Guest
Posted: Sat Feb 12, 2005 12:23 pm    Post subject: Re: Interesting sensor design problem Reply with quote
On Sat, 12 Feb 2005 02:49:23 +0000 (UTC), Ken Smith wrote:

Quote:
In article <420d3fa7$1@news.cadence.com>,
Charles Edmondson <edmondson@ieee.org> wrote:
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.

Look at the specs. Subtraction isn't the answer.

--
Best Regards,
Mike
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Ken Smith
Guest
Posted: Sat Feb 12, 2005 10:33 pm    Post subject: Re: Interesting sensor design problem Reply with quote
In article <4csq019hd12rk0f7fpibg0p1ls775v5e6b@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:
Quote:
On Sat, 12 Feb 2005 02:49:23 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:

[...]
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?

--
--
kensmith@rahul.net forging knowledge
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Ken Smith
Guest
Posted: Sat Feb 12, 2005 10:36 pm    Post subject: Re: Interesting sensor design problem Reply with quote
In article <6fsq01tl96kbjuqnq9n47c995tr78v25c5@4ax.com>,
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:
Quote:
On Sat, 12 Feb 2005 02:49:23 +0000 (UTC), the renowned
kensmith@green.rahul.net (Ken Smith) wrote:
[...]
--/\/\/--- +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.

But then you need a (-)Vref. Since we only have +5 and GND, why add a
supply voltage just to make it?
--
--
kensmith@rahul.net forging knowledge
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Ken Smith
Guest
Posted: Sat Feb 12, 2005 10:39 pm    Post subject: Re: Interesting sensor design problem Reply with quote
In article <n3k1ftlwvqmt$.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:
[...]
Quote:
out = 0 for in < 6.68Vdc
out = 300*Vin for 6.68mVdc < Vin < 13.6mVdc
out = 3 for Vin > 13.6mVdc
[...]
Look at the specs. Subtraction isn't the answer.


With r-r op-amps running on GND and +3, it looks like subtraction is just
right. What did I miss?

--
--
kensmith@rahul.net forging knowledge
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Charles Edmondson
Guest
Posted: Sat Feb 12, 2005 11:05 pm    Post subject: Re: Interesting sensor design problem Reply with quote
Rich The Newsgropup Wacko wrote:
Quote:
On Fri, 11 Feb 2005 15:58:21 -0800, Charles Edmondson wrote:


...
this is a contest entry.

...

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...)

--
Charlie
--
Edmondson Engineering
Unique Solutions to Unusual Problems
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Charles Edmondson
Guest
Posted: Sat Feb 12, 2005 11:08 pm    Post subject: Re: Interesting sensor design problem Reply with quote
Tony Williams wrote:

Quote:
In article <420d3fa7$1@news.cadence.com>,
Charles Edmondson <edmondson@ieee.org> wrote:


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?


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...

--
Charlie
--
Edmondson Engineering
Unique Solutions to Unusual Problems
Back to top
Jim Thompson
Guest
Posted: Sat Feb 12, 2005 11:23 pm    Post subject: Re: Interesting sensor design problem Reply with quote
On Sat, 12 Feb 2005 09:08:21 -0800, Charles Edmondson
<edmondson@ieee.org> wrote:

Quote:
Tony Williams wrote:

In article <420d3fa7$1@news.cadence.com>,
Charles Edmondson <edmondson@ieee.org> wrote:


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?


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...

--
Charlie

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

...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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