| Author |
Message |
Tony Williams
Guest
|
 Posted:
Sun Feb 13, 2005 11:49 am Post subject:
Re: Interesting sensor design problem |
|
|
In article <420e3808$1@news.cadence.com>,
Charles Edmondson <edmondson@ieee.org> wrote:
| Quote: | Hi Tony, I would prefer that it did. Was just spewing numbers to
get the general idea across. Wanted to reject values outside
that range, and then amp that range to my full voltage swing...
|
Ok, thanks. The 3-opamp diff amp can be offset by
injecting current into the shared gain resistor.
--+-----------------+--3V
| |
[8k2] [6k8]
| _|_
e\| \_/
pnp|---------------+
/| |
| _ |
+Vbridge--------|+ \ | 10k 43k
| | >----+------/\/\--+-/\/\--+
+----|-_/ | | | |
| | | | |
+-------/\/\--+ + | _ |
\ 3k3 \ +|- \ |
/68R 5k/Pot | >--+-Vout
\ 3k3 \ +|+_/
+-------/\/\--+ + |
| _ | | |
+----|- \ | | |
| | >----+------/\/\--+-/\/\--0V
-Vbridge--------|+_/ | 10k 43k
| |
\| |
npn|---------------+
e/| _|_
| \_/
| |
[8k2] [6k8]
| |
--+----------------+--0V
The two transistors inject about 100uA through the
68R resistor. 100uA through 68R is 6.8mV. The two
opamps are forced to adopt a reverse-offset in order
bring things back again.
The 5k Pot adjusts the value of the injection current.
--
Tony Williams.
|
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|
 |
Ken Smith
Guest
|
| Posted:
Sun Feb 13, 2005 5:25 pm Post subject:
Re: Interesting sensor design problem |
|
|
In article <luc04q468arn$.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:
[...]
| Quote: | 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?
|
How much money do you have?
I haven't actually tried the TIP35 but I suspect that you could easily get
to within 1mV of the rail at a few mA of load current. Using a transistor
inverted like that gives a quite low saturation voltage.
--
--
kensmith@rahul.net forging knowledge |
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|
|
Ken Smith
Guest
|
| Posted:
Sun Feb 13, 2005 5:27 pm Post subject:
Re: Interesting sensor design problem |
|
|
In article <4d3c8dee3etonyw@ledelec.demon.co.uk>,
Tony Williams <tonyw@ledelec.demon.co.uk> wrote:
| Quote: | | |
\| |
npn|---------------+
e/| _|_
| \_/
| |
[8k2] [6k8]
| |
--+----------------+--0V
The two transistors inject about 100uA through the
68R resistor. 100uA through 68R is 6.8mV. The two
opamps are forced to adopt a reverse-offset in order
bring things back again.
|
Use 1% resistors please. :) The OP posted 3 digit numbers.
--
--
kensmith@rahul.net forging knowledge
|
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|
|
Ken Smith
Guest
|
| Posted:
Sun Feb 13, 2005 10:21 pm Post subject:
Re: Interesting sensor design problem |
|
|
In article <ebxwf24j2ue2.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:
| Quote: | On Sun, 13 Feb 2005 17:25:07 +0000 (UTC), Ken Smith wrote:
In article <luc04q468arn$.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:
[...]
I don't recall seeing any circuits using transistors to make a
better r-r, like you suggest doing with the TIPs. Again, we don't
know what accuracy the OP needs at the endpoints, but how perfect
can we get?
How much money do you have?
I haven't actually tried the TIP35 but I suspect that you could easily get
to within 1mV of the rail at a few mA of load current. Using a transistor
inverted like that gives a quite low saturation voltage.
Where can I find an example of this circuit?
|
Here's an ascii art example of the output portion of a bipolar one:
Vcc Vcc
! !
/ !
\ !
/ !
\ !
! !/ PNP
+----!
NPN ! !\e
!/ Vcc !
--! ! !
! !\e / !
! ! \ !
! ! / !
A ! ! ! ! B
-----+ +--+ +-------
! ! ! !
! ! \ !
! ! / !
! ! \ !
! ! / !
! ! ! !
! !/e GND !
--! !
!\ !
PNP ! !/ e
+----!
! !\ NPN
\ !
/ !
\ !
/ !
! !
GND GND
There usually has to be a series RC from the point A to B and some local
feedback at the driving op-map to prevent oscillations. The resistor fed
emitters of the driver stage set the bias current of the output devices
when we are in saturation.
The CMOS buffer version is much simpler but I don't think we can use it at
only 3V:
-------/\/\/-------+--------
! !
----/\/\-+-!+\ !\ !
! >---+-----! >O-
----/\/\-+-!-/ ! !/CMOS unbuffered inverter
! !
--!!----
The local feedback on the op-amp is needed if the op-amp is anything but
the slowest type. The extra delay in the CMOS section will take you out
of stability.
--
--
kensmith@rahul.net forging knowledge |
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Active8
Guest
|
| Posted:
Sun Feb 13, 2005 11:50 pm Post subject:
Re: Interesting sensor design problem |
|
|
On Sun, 13 Feb 2005 17:25:07 +0000 (UTC), Ken Smith wrote:
| Quote: | In article <luc04q468arn$.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:
[...]
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?
How much money do you have?
I haven't actually tried the TIP35 but I suspect that you could easily get
to within 1mV of the rail at a few mA of load current. Using a transistor
inverted like that gives a quite low saturation voltage.
|
Where can I find an example of this circuit?
--
Best Regards,
Mike |
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|
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Fred Bloggs
Guest
|
| Posted:
Mon Feb 14, 2005 12:56 am Post subject:
Re: Interesting sensor design problem |
|
|
Charles Edmondson wrote:
| Quote: | Fred Bloggs wrote:
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...
|
Tony Williams' circuit is an absolute best ( uninvert that output
differential amp in his diagram). For something like this using a single
INA122, available from DigiKey for $5.50 in unit quantity, to perform
the whole reject/amplify process:
View in a fixed-width font such as Courier.
.
. Final CKT: Vbatt
. |
. +--------+-----------+----------------------.
. | | |
. [8k2] [6k8] |
. | _|_ INA122--------------+
. e\| \_/ | |
. pnp|------+ + | |\ |
. /| | V -------|--------|+\ |
. | | in | | >---+--|--> Vout
. | | | ,---|-/ | |
. | | | | |/ | |
. | | | | | |
. | | | | | |
. '----------------. | +---100K----- |
. | | | | |
. | | | | |
. + ,---200----|---+---25K-----+ |
. \ | | | |
. 5k/Pot 68 RG| | |
. \ | | | |
. + '---200----|---+---25K ----+ |
. | | | | | |
. | | | | | |
. ,----------------' | | | |
. | | | | | |
. | | | | | |
. | | | | | |
. | | | | |\ | |
. | | | +-----|-\ | |
. | | - | | | >-- |
. | | V ------|---|-----|+/ |
. \| | in | | |/ |
. npn|------+ | 100K |
. e/| _|_ | | |
. | \_/ | | |
. | | +------------------+
. [8k2] [6k8] |
. | | |
. +-------+----------------------+------
.
. gnd
.
.
.
.
. .. |
|
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|
|
Tony Williams
Guest
|
| Posted:
Mon Feb 14, 2005 9:35 am Post subject:
Re: Interesting sensor design problem |
|
|
In article <4d3c8dee3etonyw@ledelec.demon.co.uk>,
Tony Williams <tonyw@ledelec.demon.co.uk> wrote:
| Quote: | Ok, thanks. The 3-opamp diff amp can be offset by
injecting current into the shared gain resistor.
|
Polarity of last stage swapped, as per Fred's correction.
| Quote: | --+-----------------+--3V
| |
[8k2] [6k8]
| _|_
e\| \_/
pnp|---------------+
/| |
| _ |
+Vbridge--------|+ \ | 10k 43k
| | >----+------/\/\--+-/\/\---0V
+----|-_/ | | |
| | | |
+-------/\/\--+ + | _
\ 3k3 \ +|+ \
/68R 5k/Pot | >--+-Vout
\ 3k3 \ +|-_/ |
+-------/\/\--+ + | |
| _ | | | |
+----|- \ | | | |
| | >----+------/\/\--+-/\/\--+
-Vbridge--------|+_/ | 10k 43k
| |
\| |
npn|---------------+
e/| _|_
| \_/
| |
[8k2] [6k8]
| |
--+----------------+--0V
|
--
Tony Williams. |
|
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|
|
Charles Edmondson
Guest
|
| Posted:
Mon Feb 14, 2005 11:13 pm Post subject:
Re: Interesting sensor design problem |
|
|
Fred Bloggs wrote:
| Quote: |
Charles Edmondson wrote:
Fred Bloggs wrote:
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...
Tony Williams' circuit is an absolute best ( uninvert that output
differential amp in his diagram). For something like this using a single
INA122, available from DigiKey for $5.50 in unit quantity, to perform
the whole reject/amplify process:
View in a fixed-width font such as Courier.
.
. Final CKT: Vbatt
. |
. +--------+-----------+----------------------.
. | | |
. [8k2] [6k8] |
. | _|_ INA122--------------+
. e\| \_/ | |
. pnp|------+ + | |\ |
. /| | V -------|--------|+\ |
. | | in | | >---+--|--> Vout
. | | | ,---|-/ | |
. | | | | |/ | |
. | | | | | |
. | | | | | |
. '----------------. | +---100K----- |
. | | | | |
. | | | | |
. + ,---200----|---+---25K-----+ |
. \ | | | |
. 5k/Pot 68 RG| | |
. \ | | | |
. + '---200----|---+---25K ----+ |
. | | | | | |
. | | | | | |
. ,----------------' | | | |
. | | | | | |
. | | | | | |
. | | | | | |
. | | | | |\ | |
. | | | +-----|-\ | |
. | | - | | | >-- |
. | | V ------|---|-----|+/ |
. \| | in | | |/ |
. npn|------+ | 100K |
. e/| _|_ | | |
. | \_/ | | |
. | | +------------------+
. [8k2] [6k8] |
. | | |
. +-------+----------------------+------
.
. gnd
.
.
.
.
. ..
You know, the fun thing is, one of the tasks I decided to do was |
simulate this thing, and see what helped. I got the latest INA122 spice
model from TI/BB, and tried this. Finally figured out that none of the
reference voltage changed did any good, got just about the same results
every time, or results that didn't make any sense. Ralized that they
modeled it as a single opamp, not as an instrumentation amp.
Finally, went to AD and got the AD627 model, and put that in. Got a
great family of curves that do exactly what I want it to do.
Now I realized I don't have any matching PNP/NPN pairs in the junk box,
just a pile of 2N2222's, so will have to run by RatShack later and pick
some up.
BTW, is that a diode in the emitter bias circuit? Or something else....
Thanks for all the help! I figured an actual DESIGN question wouldn't
be TOO off topic... 8-)
--
Charlie
--
Edmondson Engineering
Unique Solutions to Unusual Problems |
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Active8
Guest
|
| Posted:
Tue Feb 15, 2005 12:38 am Post subject:
Re: Interesting sensor design problem |
|
|
On Sun, 13 Feb 2005 22:21:22 +0000 (UTC), Ken Smith wrote:
| Quote: | In article <ebxwf24j2ue2.dlg@ID-222894.news.individual.net>,
Here's an ascii art example of the output portion of a bipolar one:
Vcc Vcc
! !
/ !
\ !
/ !
\ !
! !/ PNP
+----!
NPN ! !\e
!/ Vcc !
--! ! !
! !\e / !
! ! \ !
! ! / !
A ! ! ! ! B
-----+ +--+ +-------
! ! ! !
! ! \ !
! ! / !
! ! \ !
! ! / !
! ! ! !
! !/e GND !
--! !
!\ !
PNP ! !/ e
+----!
! !\ NPN
\ !
/ !
\ !
/ !
! !
GND GND
|
I wondered if you were talking about a current booster. This was the
only way I could figure to do what you said.
Best Regards,
Mike |
|
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|
|
Ken Smith
Guest
|
| Posted:
Tue Feb 15, 2005 3:27 am Post subject:
Re: Interesting sensor design problem |
|
|
In article <1fw409negpd5x.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:
| Quote: | On Sun, 13 Feb 2005 22:21:22 +0000 (UTC), Ken Smith wrote:
In article <ebxwf24j2ue2.dlg@ID-222894.news.individual.net>,
Here's an ascii art example of the output portion of a bipolar one:
Vcc Vcc
! !
/ !
\ !
/ !
\ !
! !/ PNP
+----!
NPN ! !\e
!/ Vcc !
--! ! !
! !\e / !
! ! \ !
! ! / !
A ! ! ! ! B
-----+ +--+ +-------
! ! ! !
! ! \ !
! ! / !
! ! \ !
! ! / !
! ! ! !
! !/e GND !
--! !
!\ !
PNP ! !/ e
+----!
! !\ NPN
\ !
/ !
\ !
/ !
! !
GND GND
I wondered if you were talking about a current booster. This was the
only way I could figure to do what you said.
|
It really doesn't need to boost current if it has a voltage gain a little
over unity, the op-amps swing is less than the outputs. In this case
there is a significant current boost along with the improved saturation
voltage.
--
--
kensmith@rahul.net forging knowledge |
|
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Tony Williams
Guest
|
| Posted:
Tue Feb 15, 2005 7:09 am Post subject:
Re: Interesting sensor design problem |
|
|
In article <4210dc4c$1@news.cadence.com>,
Charles Edmondson <edmondson@ieee.org> wrote:
| Quote: | BTW, is that a diode in the emitter bias circuit?
Or something else....
|
They are diodes, compensating for the dVbe/dT of
their respective transistors. Diode-connected
transistors work slightly better, as below.
| |
| +----+
\| | |/c
npn1|--+--|npn2
e/| b |\e
| |
| |
[8k2] [6k8]
| |
+--------+----
Connect npn2's base to collector.
Good thermal coupling is required to avoid drifts
due to stray breezes. Mount the two transistors
alongside each other and glue them together. A
dollop of hot-melt glue over the pair is useful.
--
Tony Williams. |
|
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Tony Williams
Guest
|
| Posted:
Tue Feb 15, 2005 8:33 pm Post subject:
Re: Interesting sensor design problem |
|
|
In article <42121DD7.1000109@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
| Quote: | On second thought, the transistor current source offset is not so
good in this application for two reasons: 1) it is not
ratiometric to the battery voltage, and 2) it takes a lot
overhead ~ 1.3V to each rail which does not leave a lot of room
for VCM variation- especially when you consider battery drain
down.
|
That particular transistor current source has a poor
temperature stability. Suggestion withdrawn.
--
Tony Williams. |
|
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|
|
Fred Bloggs
Guest
|
| Posted:
Tue Feb 15, 2005 10:05 pm Post subject:
Re: Interesting sensor design problem |
|
|
Charles Edmondson wrote:
| Quote: | You know, the fun thing is, one of the tasks I decided to do was
simulate this thing, and see what helped. I got the latest INA122 spice
model from TI/BB, and tried this. Finally figured out that none of the
reference voltage changed did any good, got just about the same results
every time, or results that didn't make any sense. Ralized that they
modeled it as a single opamp, not as an instrumentation amp.
|
Right- forget their dumb SPICE model. Throw the equivalent circuit into
SPICE using ideal OA's, then do a DC sweep on Vdiff from say -15mV to
+15mV, stepping VCM as a parameter from 0.5V to 2.5V, and plot the two
OA outputs. This will tell you how the INA122 will do.
| Quote: | Now I realized I don't have any matching PNP/NPN pairs in the junk box,
just a pile of 2N2222's, so will have to run by RatShack later and pick
some up.
BTW, is that a diode in the emitter bias circuit? Or something else....
Thanks for all the help! I figured an actual DESIGN question wouldn't
be TOO off topic... 8-)
|
On second thought, the transistor current source offset is not so good
in this application for two reasons: 1) it is not ratiometric to the
battery voltage, and 2) it takes a lot overhead ~ 1.3V to each rail
which does not leave a lot of room for VCM variation- especially when
you consider battery drain down. It is better to go with the INA122
followed by an OA with Vo=2(Vin-Vbatt/2) as in my original circuit- this
is ratiometric and has no CM limitations. You can conserve current by
going with a dual RRIO OA and using an inverted mode transistor like so:
View in a fixed-width font such as Courier.
gnd Vbatt
| |
INA122--------------+
| | RRIO
+ | |\ | +-R-+
V -------|--------|+\ | | | |\
in | | >---+--|-+-R-+------------|+\
| ,---|-/ | | | | >--+-> 0<V <3V
| | |/ | | R +----|-/ | out
| | | | | | |/ |
| | | | | | |
| +---100K----- | +---R---+-----R----+
| | | | |
| | | | 3V R R=10K
,----|---+---25K-----+ | | |+ |
| | | | +--||---+--Vbatt
(957)RG | | | | | |
| | | | | |
'----|---+---25K ----+ | | 443K
| | | | | |
| | | | | |
| | | | | |
| | | | +--10K--+
| | | | | |
| | | | gnd |
| | |\ | | |
| +-----|-\ | | |
- | | | >-- | |
V ------|---|-----|+/ | |
in | | |/ | |
| 100K | |
| | | |
| | | |
+------------------+ |
V | |
ref +----10K----+-------. |
| |100n | |
(66.2mV) e === /| | |
\| | /-|--' |
|--6.8K---+-< | |
/| \+|-----------+
| 2N3904 \|
gnd |
|
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Fred Bloggs
Guest
|
| Posted:
Wed Feb 16, 2005 5:39 pm Post subject:
Re: Interesting sensor design problem |
|
|
Fred Bloggs wrote:
| Quote: | in | | |/ | |
| 100K | |
| | | |
| | | |
+------------------+ |
V | |
ref +----10K----+-------. |
| |100n | |
(66.2mV) e === /| | |
\| | /-|--' |
|--6.8K---+-< | |
/| \+|-----------+
| 2N3904 \|
gnd
|
That should be changed to below for 120dB attenuation of common mode
noise on the sensor at line frequency with ultimate attenuation of 60dB
at high frequency, using something like an OP291:
View in a fixed-width font such as Courier.
| | | | | |
| | | | | 3V |
| | | | | |+ |
| | | | +--||---+--Vbatt
| | | | | | |
| | |\ | | | |
| +-----|-\ | | | 443K
- | | | >-- | | |
V ------|---|-----|+/ | | |
in | | |/ | | |
| 100K | +--10K--+
| | | | |
| | | gnd |
+------------------+ |
V | |
ref +------+------------10K----. |
(66.2mV) |680n | | |
=== e 2N3904 /| | |
| \| /+|--' |
100 |---6.8K----< | |
| /| \-|----'
| | \|
gnd gnd |
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Fred Bloggs
Guest
|
| Posted:
Wed Feb 16, 2005 5:58 pm Post subject:
Re: Interesting sensor design problem |
|
|
Tony Williams wrote:
| Quote: | In article <42121DD7.1000109@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
On second thought, the transistor current source offset is not so
good in this application for two reasons: 1) it is not
ratiometric to the battery voltage, and 2) it takes a lot
overhead ~ 1.3V to each rail which does not leave a lot of room
for VCM variation- especially when you consider battery drain
down.
That particular transistor current source has a poor
temperature stability. Suggestion withdrawn.
|
You can still do it with two Howland type current sources, but by the
time you add in all those components, it may be simpler to just leave
that alone and use the Vref method. |
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