Interesting sensor design problem
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Interesting sensor design problem
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Ken Smith
Guest
Posted: Sun Feb 13, 2005 1:11 am    Post subject: Re: Interesting sensor design problem Reply with quote
In article <19tokk68vtff$.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:
Quote:
On Sat, 12 Feb 2005 22:39:11 +0000 (UTC), Ken Smith wrote:

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

Mentioning r-rs in the first place?

I thought someone else had already introduced them but you are right I
should have specified. The circuit assumes r-r.


Quote:
13.6 mV - 6.68 mV = 6.92 mV which *300 is 2.076 which is a volt shy
of the specified output range.

Yes this flew right by me. I didn't do any math for my posting. This is
why I left all the values unspecified.


We could use the 4th section of a quad op-amp to make a Mickey Mouse
comparitor and flip an analog switch to the +3V at the right point.

[...]
Quote:
How come I could see it wouldn't work without doing the math?

When I do this sort of thing I often try to suggest that it is because I'm
very smart. About 1 in 3 times my cow-orkers let me get away with it.

--
--
kensmith@rahul.net forging knowledge

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Ken Smith
Guest
Posted: Sun Feb 13, 2005 1:19 am    Post subject: Re: Interesting sensor design problem Reply with quote
In article <17d43hzuhe9th.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:
[...]
Quote:
But will r-r OAs mess with the accuracy near the ends of the input
range? Not that we know the required accuracy... just a thought.

Lightly loaded R-R op-amps will swing within a few mV of the rail. I
remember a cute app-note where a CMOS pair was used as the output of a r-r
op-amp making it very good indeed.

I bet we could come up with a way to use two sections of the r-r op-amp
and a couple of largish transistors to make an almost perfect r-r op-amp.
If the supply voltage was more, I'd suggest MOSFETs. As it is a TIP-35 ad
TIP-36 running in inverted mode may be just the ticket.




--
--
kensmith@rahul.net forging knowledge
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Fred Bloggs
Guest
Posted: Sun Feb 13, 2005 1:53 am    Post subject: Re: Interesting sensor design problem Reply with quote
Quote:
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

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Fred Bloggs
Guest
Posted: Sun Feb 13, 2005 1:56 am    Post subject: Re: Interesting sensor design problem Reply with quote
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
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Frank Bemelman
Guest
Posted: Sun Feb 13, 2005 2:58 am    Post subject: Re: Interesting sensor design problem Reply with quote
"Fred Bloggs" <nospam@nospam.com> schreef in bericht
news:420E5EC1.2000504@nospam.com...
Quote:



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


Is that stupid?

--
Thanks, Frank.
(remove 'q' and 'invalid' when replying by email)
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Fred Bloggs
Guest
Posted: Sun Feb 13, 2005 3:34 am    Post subject: Re: Interesting sensor design problem Reply with quote
Frank Bemelman wrote:
Quote:
"Fred Bloggs" <nospam@nospam.com> schreef in bericht
news:420E5EC1.2000504@nospam.com...



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.
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Fred Bloggs
Guest
Posted: Sun Feb 13, 2005 3:39 am    Post subject: Re: Interesting sensor design problem Reply with quote
Fred Bloggs wrote:

Quote:
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.
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Frank Bemelman
Guest
Posted: Sun Feb 13, 2005 4:00 am    Post subject: Re: Interesting sensor design problem Reply with quote
"Fred Bloggs" <nospam@nospam.com> schreef in bericht
news:420E7671.1000206@nospam.com...
Quote:


Frank Bemelman wrote:
"Fred Bloggs" <nospam@nospam.com> schreef in bericht
news:420E5EC1.2000504@nospam.com...



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.

--
Thanks, Frank.
(remove 'q' and 'invalid' when replying by email)
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hamilton
Guest
Posted: Sun Feb 13, 2005 4:28 am    Post subject: Re: Interesting sensor design problem Reply with quote
Charles Edmondson 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!



Why not? I just did this with :
http://www.analog.com/en/prod/0%2C2877%2CAD623%2C00.html
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Active8
Guest
Posted: Sun Feb 13, 2005 5:33 am    Post subject: Re: Interesting sensor design problem Reply with quote
On Sat, 12 Feb 2005 22:39:11 +0000 (UTC), Ken Smith wrote:

Quote:
In article <n3k1ftlwvqmt$.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:
[...]
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?

Mentioning r-rs in the first place?

0 <= Vin <= 6.68 mV

Vin - Vref < 0 and 300*Vin < 0 but output *is* 0 for a r-r

13.6 mV - 6.68 mV = 6.92 mV which *300 is 2.076 which is a volt shy
of the specified output range.

You might be on the right track, but I think this prob is similar to
that r-r OA/PWL arrangement they were playing with a month ago.

try 13.6 - 3.6 = 10 and 10 mV * 300 = 3 which is ok, but

6.68 - 3.6 = 3.08 and 3.08 mV * 300 = 924 mV which is not the
specified 0 V out.

How come I could see it wouldn't work without doing the math?

We need a word explaination of how, from insection, one can conclude
true or false. Sometimes my brain just sees it without courtesy of
explaination.

--
Best Regards,
Mike
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Active8
Guest
Posted: Sun Feb 13, 2005 6:11 am    Post subject: Re: Interesting sensor design problem Reply with quote
On Sun, 13 Feb 2005 01:11:42 +0000 (UTC), Ken Smith wrote:

Quote:
In article <19tokk68vtff$.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:

How come I could see it wouldn't work without doing the math?

When I do this sort of thing I often try to suggest that it is because I'm
very smart. About 1 in 3 times my cow-orkers let me get away with it.

Sometimes when I pull a fast one I'll say, "Good thing I'm smart."
That usually elicites a chuckle.

I don't recall seeing any circuits using transistors to make a
better r-r, like you suggest doing with the TIPs. Again, we don't
know what accuracy the OP needs at the endpoints, but how perfect
can we get?
--
Best Regards,
Mike
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Active8
Guest
Posted: Sun Feb 13, 2005 6:11 am    Post subject: Re: Interesting sensor design problem Reply with quote
On Sat, 12 Feb 2005 15:28:12 -0700, hamilton wrote:

Quote:
Why not? I just did this with :
http://www.analog.com/en/prod/0%2C2877%2CAD623%2C00.html

How much do they cost?

--
Best Regards,
Mike
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Active8
Guest
Posted: Sun Feb 13, 2005 6:11 am    Post subject: Re: Interesting sensor design problem Reply with quote
On Sat, 12 Feb 2005 09:08:21 -0800, Charles Edmondson 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...

Change the gain to 433.

--
Best Regards,
Mike
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Active8
Guest
Posted: Sun Feb 13, 2005 6:11 am    Post subject: Re: Interesting sensor design problem Reply with quote
On Sat, 12 Feb 2005 18:33:59 -0500, Active8 wrote:

Quote:
On Sat, 12 Feb 2005 22:39:11 +0000 (UTC), Ken Smith wrote:

In article <n3k1ftlwvqmt$.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:
[...]
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?

Mentioning r-rs in the first place?


How come I could see it wouldn't work without doing the math

We need a word explaination of how, from insection, one can conclude
true or false. Sometimes my brain just sees it without courtesy of
explaination.

I think what was running through my mind was that subtracting a ref
wouldn't work because of the excursions outside the desired output
range, but I didn't see how easily an r-r would fit the bill. For
that matter, you can clamp the outputs.

As for the amth, it turns out Chas. messed up the spec with an
incorrect gain factor. Indeed, if you change it to 433, we can get a
0 - 3 V range over the spec'd input by subtracting 6.68 mV from the
input.

But will r-r OAs mess with the accuracy near the ends of the input
range? Not that we know the required accuracy... just a thought.
--
Best Regards,
Mike
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Charles Edmondson
Guest
Posted: Sun Feb 13, 2005 6:11 am    Post subject: Re: Interesting sensor design problem Reply with quote
Fred Bloggs wrote:
Quote:

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

Thanks, Fred,

That was what I was trying to get my head around. I just need to use a
Vref to get rid of the lower rejection range, since the output can't go
negative. And it just hits the stop at the top of the range, so who
cares. I knew it shouldn't be that hard, but I have been playing too
little analog recently...

The fun part is that the Freescale appnotes has a 4 opamp circuit that
is supposed to do this, but whenever I try to simulate it, I just get a
pegged output...

--
Charlie
--
Edmondson Engineering
Unique Solutions to Unusual Problems
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