reading a small voltage
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I am trying to get the Stamp to read a very small voltage(.6 mv to neg.
voltage) and I believe Tracy Allen's(www.emesystems.com) fine explanation on
"Measuring small voltages with RCtime" (p. 13-15) will work. The problem I
am having is with the values for R1 and C1 on p. 12. Is anyone familiar with
this site and the 2 values?
Are there any other suggestions for getting a voltage to the Stamp pin that
it can read? Perhaps an op-amplifier circuit?
Thanks in advance!
_________________________________________________________________
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voltage) and I believe Tracy Allen's(www.emesystems.com) fine explanation on
"Measuring small voltages with RCtime" (p. 13-15) will work. The problem I
am having is with the values for R1 and C1 on p. 12. Is anyone familiar with
this site and the 2 values?
Are there any other suggestions for getting a voltage to the Stamp pin that
it can read? Perhaps an op-amplifier circuit?
Thanks in advance!
_________________________________________________________________
Join the world’s largest e-mail service with MSN Hotmail.
http://www.hotmail.com
Comments
>I am trying to get the Stamp to read a very small voltage(.6 mv to neg.
I think that your best solution is to amplify the signal using a DC
accurate precision op-amp, then try Tracey's RC time circuit and code.
How far negative does your input signal go? Or do you mean 0 to 0.6 mV?
As far as choice of op-amps goes, I'd look at a cheap CAZ such as the
ICL7650 - they are DC accurate down to the microvolt region. Otherwise
look at something like an OP-07. You have to pick an op-amp that has much
less input offset voltage than the 600 uV signal you want to measure.
dwayne
--
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
Celebrating 18 years of Engineering Innovation (1984 - 2002)
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Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.
>voltage) and I believe Tracy Allen's(www.emesystems.com) fine explanation on
>"Measuring small voltages with RCtime" (p. 13-15) will work. The problem I
>am having is with the values for R1 and C1 on p. 12. Is anyone familiar with
>this site and the 2 values?
>
>Are there any other suggestions for getting a voltage to the Stamp pin that
>it can read? Perhaps an op-amplifier circuit?
>
>Thanks in advance!
>
0.6 mV is small, yes. In the first circuit (non-inverting)
R1*C1 *1.3/Vin =~ t
R1*C1*1.3/0.0006 = t ~ 2160 microseconds.
So if R1=100 ohms and C1=0.01 uf , then t=~2160 microseconds, or a
BS2 count of half that. Approximately. It is late and I have not
checked the calcs! As Wayne noted, you would have to use an accurate
op-amp (CAZ) to get the best results. Input offset voltage of the
op-amp will add to your signal.
I am not sure what you mean by ".6 mv to neg. voltage".
The RCtime value is inversely proportional to the input voltage and
it blows up as the voltage decreases. That could be inconvenient, if
you need a continuous reading as your input voltage goes through
zero. The circuit can measure both positive and negative voltages,
but the integration capacitor has to be initialized differently for
the two polarities, which complicates the program if you need to
measure both.
It is probably better to use an analog to digital converter unless
this curious circuit can serve within its limitations.
-- Tracy
have a better idea witch way to go?
norm
>From: "JOHN BELL" <johnabell8@h...>
>Reply-To: basicstamps@yahoogroups.com
>To: basicstamps@yahoogroups.com
>Subject: [noparse][[/noparse]basicstamps] reading a small voltage
>Date: Tue, 11 Jun 2002 21:38:31 -0400
>
>
>I am trying to get the Stamp to read a very small voltage(.6 mv to neg.
>voltage) and I believe Tracy Allen's(www.emesystems.com) fine explanation
>on
>"Measuring small voltages with RCtime" (p. 13-15) will work. The problem I
>am having is with the values for R1 and C1 on p. 12. Is anyone familiar
>with
>this site and the 2 values?
>
>Are there any other suggestions for getting a voltage to the Stamp pin that
>it can read? Perhaps an op-amplifier circuit?
>
>Thanks in advance!
>
>_________________________________________________________________
>Join the world’s largest e-mail service with MSN Hotmail.
>http://www.hotmail.com
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>
>To UNSUBSCRIBE, just send mail to:
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>
>
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I am attempting to use a suggested op-amp but being a non hardware/software
person (but a Stamp hobbbyist) am having additional trouble with negative
voltage: in the book "Electronics for Entrepreneurs" on p. 220 it shows an
op-amp with "negative voltage" going into the "negative supply"(-Vs); What
is a "negative supply voltage"? Would I hook the -5v of a 5v power supply
to this; and if so where would the +5v be attatched? Would this be
"Ground"(the schematic on the page shows ground going to the non-inverting
input-prob. pin 3 on the 741)? I am using a 741 op-amp so the neg. voltage
goes to pin 4.
To further explain the input voltage: the input voltage can vary from .6mv
to -30mv. I wish the .6mv to be boosted to a high enough voltage so that
the stamp will recognize it as "high" on the pin i.e. prob. boost to 3v or
higher. That way if the voltage goes neg. the stamp will recognize as a
"low" and my prog. can tell the diff. between .6mv and a -mv.
In addition What does "CAZ" stand for?
Thanks again in advance!
>From: Tracy Allen <tracy@e...>
>Reply-To: basicstamps@yahoogroups.com
>To: basicstamps@yahoogroups.com
>Subject: Re: [noparse][[/noparse]basicstamps] reading a small voltage
>Date: Wed, 12 Jun 2002 00:57:14 -0700
>
> >I am trying to get the Stamp to read a very small voltage(.6 mv to neg.
> >voltage) and I believe Tracy Allen's(www.emesystems.com) fine explanation
>on
> >"Measuring small voltages with RCtime" (p. 13-15) will work. The problem
>I
> >am having is with the values for R1 and C1 on p. 12. Is anyone familiar
>with
> >this site and the 2 values?
> >
> >Are there any other suggestions for getting a voltage to the Stamp pin
>that
> >it can read? Perhaps an op-amplifier circuit?
> >
> >Thanks in advance!
> >
>
>0.6 mV is small, yes. In the first circuit (non-inverting)
>
> R1*C1 *1.3/Vin =~ t
> R1*C1*1.3/0.0006 = t ~ 2160 microseconds.
>
>So if R1=100 ohms and C1=0.01 uf , then t=~2160 microseconds, or a
>BS2 count of half that. Approximately. It is late and I have not
>checked the calcs! As Wayne noted, you would have to use an accurate
>op-amp (CAZ) to get the best results. Input offset voltage of the
>op-amp will add to your signal.
>
>I am not sure what you mean by ".6 mv to neg. voltage".
>
>The RCtime value is inversely proportional to the input voltage and
>it blows up as the voltage decreases. That could be inconvenient, if
>you need a continuous reading as your input voltage goes through
>zero. The circuit can measure both positive and negative voltages,
>but the integration capacitor has to be initialized differently for
>the two polarities, which complicates the program if you need to
>measure both.
>
>It is probably better to use an analog to digital converter unless
>this curious circuit can serve within its limitations.
>
>
> -- Tracy
>
>
>To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
>from the same email address that you subscribed. Text in the Subject and
>Body of the message will be ignored.
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
_________________________________________________________________
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Read: http://www.wd5gnr.com/opampneg.htm
Note that you can't really use some of the techniques there because you
have a negative input voltage, but it might give you some insight into
how this stuff works and there are a few ideas for negative voltage
there. If you have a - supply, you want to hook it up. Keep in mind, the
op amp supply does not need to be regulated and it should be a few volts
above (and below) the maximum (and minimum) voltages you want to output.
So don't put 5V on the V+ rail and expect to get 5V out unless you have
a special kind of op amp.
CAZ stands for communtating auto zero. This is a technique used to
correct offset voltage problems when making differential measurements.
The amplifier uses a chopping technique to alternately charge a
capacitor to some input value, and then switch it so that it is
measured. The Art of Electronics has a good write up on this.
Al Williams
AWC
* Easy RS-232 Prototyping
http://www.al-williams.com/awce/rs1.htm
>
Original Message
> From: JOHN BELL [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=w80ZknpgAI8yLf5vEzn7BCYTUCuF05WWugm_uEHva9nTQRnyY1fQksjAcxmzSSva5duao5kzjmpnhCh7]johnabell8@h...[/url
> Sent: Wednesday, June 12, 2002 9:01 AM
> To: basicstamps@yahoogroups.com
> Subject: Re: [noparse][[/noparse]basicstamps] reading a small voltage
>
>
>
>
> Thanks to All; esp. the mid nite respondents(Dwayne and Tracy).
>
> I am attempting to use a suggested op-amp but being a non
> hardware/software
> person (but a Stamp hobbbyist) am having additional trouble
> with negative
> voltage: in the book "Electronics for Entrepreneurs" on p.
> 220 it shows an
> op-amp with "negative voltage" going into the "negative
> supply"(-Vs); What
> is a "negative supply voltage"? Would I hook the -5v of a
> 5v power supply
> to this; and if so where would the +5v be attatched? Would this be
> "Ground"(the schematic on the page shows ground going to the
> non-inverting
> input-prob. pin 3 on the 741)? I am using a 741 op-amp so
> the neg. voltage
> goes to pin 4.
>
> To further explain the input voltage: the input voltage can
> vary from .6mv
> to -30mv. I wish the .6mv to be boosted to a high enough
> voltage so that
> the stamp will recognize it as "high" on the pin i.e. prob.
> boost to 3v or
> higher. That way if the voltage goes neg. the stamp will
> recognize as a
> "low" and my prog. can tell the diff. between .6mv and a -mv.
>
> In addition What does "CAZ" stand for?
>
> Thanks again in advance!
>
>
>
>
>
> >From: Tracy Allen <tracy@e...>
> >Reply-To: basicstamps@yahoogroups.com
> >To: basicstamps@yahoogroups.com
> >Subject: Re: [noparse][[/noparse]basicstamps] reading a small voltage
> >Date: Wed, 12 Jun 2002 00:57:14 -0700
> >
> > >I am trying to get the Stamp to read a very small voltage(.6 mv to
> > >neg.
> > >voltage) and I believe Tracy Allen's(www.emesystems.com)
> fine explanation
> >on
> > >"Measuring small voltages with RCtime" (p. 13-15) will work. The
> > >problem
> >I
> > >am having is with the values for R1 and C1 on p. 12. Is anyone
> > >familiar
> >with
> > >this site and the 2 values?
> > >
> > >Are there any other suggestions for getting a voltage to the Stamp
> > >pin
> >that
> > >it can read? Perhaps an op-amplifier circuit?
> > >
> > >Thanks in advance!
> > >
> >
> >0.6 mV is small, yes. In the first circuit (non-inverting)
> >
> > R1*C1 *1.3/Vin =~ t
> > R1*C1*1.3/0.0006 = t ~ 2160 microseconds.
> >
> >So if R1=100 ohms and C1=0.01 uf , then t=~2160
> microseconds, or a BS2
> >count of half that. Approximately. It is late and I have
> not checked
> >the calcs! As Wayne noted, you would have to use an accurate op-amp
> >(CAZ) to get the best results. Input offset voltage of the
> op-amp will
> >add to your signal.
> >
> >I am not sure what you mean by ".6 mv to neg. voltage".
> >
> >The RCtime value is inversely proportional to the input
> voltage and it
> >blows up as the voltage decreases. That could be
> inconvenient, if you
> >need a continuous reading as your input voltage goes through
> zero. The
> >circuit can measure both positive and negative voltages, but the
> >integration capacitor has to be initialized differently for the two
> >polarities, which complicates the program if you need to
> measure both.
> >
> >It is probably better to use an analog to digital converter
> unless this
> >curious circuit can serve within its limitations.
> >
> >
> > -- Tracy
> >
> >
> >To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@yahoogroups.com
> >from the same email address that you subscribed. Text in
> the Subject
> >and
> >Body of the message will be ignored.
> >
> >
> >Your use of Yahoo! Groups is subject to
> >http://docs.yahoo.com/info/terms/
>
>
>
>
>
>
> _________________________________________________________________
> MSN Photos is the easiest way to share and print your photos:
> http://photos.msn.com/support/worldwide.aspx
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the
> Subject and Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
I shall attempt to use the info and plan for success.
John Bell
>From: "Al Williams" <alw@a...>
>Reply-To: basicstamps@yahoogroups.com
>To: <basicstamps@yahoogroups.com>
>Subject: RE: [noparse][[/noparse]basicstamps] reading a small voltage
>Date: Wed, 12 Jun 2002 09:21:23 -0500
>
>Hi John,
>
>Read: http://www.wd5gnr.com/opampneg.htm
>
>Note that you can't really use some of the techniques there because you
>have a negative input voltage, but it might give you some insight into
>how this stuff works and there are a few ideas for negative voltage
>there. If you have a - supply, you want to hook it up. Keep in mind, the
>op amp supply does not need to be regulated and it should be a few volts
>above (and below) the maximum (and minimum) voltages you want to output.
>So don't put 5V on the V+ rail and expect to get 5V out unless you have
>a special kind of op amp.
>
>CAZ stands for communtating auto zero. This is a technique used to
>correct offset voltage problems when making differential measurements.
>The amplifier uses a chopping technique to alternately charge a
>capacitor to some input value, and then switch it so that it is
>measured. The Art of Electronics has a good write up on this.
>
>Al Williams
>AWC
>* Easy RS-232 Prototyping
>http://www.al-williams.com/awce/rs1.htm
>
>
>
>
>
> >
Original Message
> > From: JOHN BELL [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=fn_x-Mh2JWvZPJJP8mZFagThvVcbvOieL59XUY_ZpoVq7XpfSgDxxpG4a0sQiv1GWRgVJnOt1ihy74AwKOc]johnabell8@h...[/url
> > Sent: Wednesday, June 12, 2002 9:01 AM
> > To: basicstamps@yahoogroups.com
> > Subject: Re: [noparse][[/noparse]basicstamps] reading a small voltage
> >
> >
> >
> >
> > Thanks to All; esp. the mid nite respondents(Dwayne and Tracy).
> >
> > I am attempting to use a suggested op-amp but being a non
> > hardware/software
> > person (but a Stamp hobbbyist) am having additional trouble
> > with negative
> > voltage: in the book "Electronics for Entrepreneurs" on p.
> > 220 it shows an
> > op-amp with "negative voltage" going into the "negative
> > supply"(-Vs); What
> > is a "negative supply voltage"? Would I hook the -5v of a
> > 5v power supply
> > to this; and if so where would the +5v be attatched? Would this be
> > "Ground"(the schematic on the page shows ground going to the
> > non-inverting
> > input-prob. pin 3 on the 741)? I am using a 741 op-amp so
> > the neg. voltage
> > goes to pin 4.
> >
> > To further explain the input voltage: the input voltage can
> > vary from .6mv
> > to -30mv. I wish the .6mv to be boosted to a high enough
> > voltage so that
> > the stamp will recognize it as "high" on the pin i.e. prob.
> > boost to 3v or
> > higher. That way if the voltage goes neg. the stamp will
> > recognize as a
> > "low" and my prog. can tell the diff. between .6mv and a -mv.
> >
> > In addition What does "CAZ" stand for?
> >
> > Thanks again in advance!
> >
> >
> >
> >
> >
> > >From: Tracy Allen <tracy@e...>
> > >Reply-To: basicstamps@yahoogroups.com
> > >To: basicstamps@yahoogroups.com
> > >Subject: Re: [noparse][[/noparse]basicstamps] reading a small voltage
> > >Date: Wed, 12 Jun 2002 00:57:14 -0700
> > >
> > > >I am trying to get the Stamp to read a very small voltage(.6 mv to
> > > >neg.
> > > >voltage) and I believe Tracy Allen's(www.emesystems.com)
> > fine explanation
> > >on
> > > >"Measuring small voltages with RCtime" (p. 13-15) will work. The
> > > >problem
> > >I
> > > >am having is with the values for R1 and C1 on p. 12. Is anyone
> > > >familiar
> > >with
> > > >this site and the 2 values?
> > > >
> > > >Are there any other suggestions for getting a voltage to the Stamp
> > > >pin
> > >that
> > > >it can read? Perhaps an op-amplifier circuit?
> > > >
> > > >Thanks in advance!
> > > >
> > >
> > >0.6 mV is small, yes. In the first circuit (non-inverting)
> > >
> > > R1*C1 *1.3/Vin =~ t
> > > R1*C1*1.3/0.0006 = t ~ 2160 microseconds.
> > >
> > >So if R1=100 ohms and C1=0.01 uf , then t=~2160
> > microseconds, or a BS2
> > >count of half that. Approximately. It is late and I have
> > not checked
> > >the calcs! As Wayne noted, you would have to use an accurate op-amp
> > >(CAZ) to get the best results. Input offset voltage of the
> > op-amp will
> > >add to your signal.
> > >
> > >I am not sure what you mean by ".6 mv to neg. voltage".
> > >
> > >The RCtime value is inversely proportional to the input
> > voltage and it
> > >blows up as the voltage decreases. That could be
> > inconvenient, if you
> > >need a continuous reading as your input voltage goes through
> > zero. The
> > >circuit can measure both positive and negative voltages, but the
> > >integration capacitor has to be initialized differently for the two
> > >polarities, which complicates the program if you need to
> > measure both.
> > >
> > >It is probably better to use an analog to digital converter
> > unless this
> > >curious circuit can serve within its limitations.
> > >
> > >
> > > -- Tracy
> > >
> > >
> > >To UNSUBSCRIBE, just send mail to:
> > > basicstamps-unsubscribe@yahoogroups.com
> > >from the same email address that you subscribed. Text in
> > the Subject
> > >and
> > >Body of the message will be ignored.
> > >
> > >
> > >Your use of Yahoo! Groups is subject to
> > >http://docs.yahoo.com/info/terms/
> >
> >
> >
> >
> >
> >
> > _________________________________________________________________
> > MSN Photos is the easiest way to share and print your photos:
> > http://photos.msn.com/support/worldwide.aspx
> >
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@yahoogroups.com
> > from the same email address that you subscribed. Text in the
> > Subject and Body of the message will be ignored.
> >
> >
> > Your use of Yahoo! Groups is subject to
>http://docs.yahoo.com/info/terms/
>
>
>To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
>from the same email address that you subscribed. Text in the Subject and
>Body of the message will be ignored.
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
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>
>I am attempting to use a suggested op-amp but being a non hardware/software
>person (but a Stamp hobbbyist) am having additional trouble with negative
>voltage: in the book "Electronics for Entrepreneurs" on p. 220 it shows an
>op-amp with "negative voltage" going into the "negative supply"(-Vs); What
>is a "negative supply voltage"? Would I hook the -5v of a 5v power supply
>to this; and if so where would the +5v be attatched? Would this be
>"Ground"(the schematic on the page shows ground going to the non-inverting
>input-prob. pin 3 on the 741)? I am using a 741 op-amp so the neg. voltage
>goes to pin 4.
There are literally hundreds of types of op amps. The 741 is an
older type, and it is not suitable for this application. It used to
be that most op-amp circuits were powered by a +/- 15 or +/- 12 volt
power supply. That is like taking two 12 volt batteries and
connecting them in series. The junction between the batteries is
attached to ground, and the +12 volts in relation to that was the
positive supply for the op amp and the -12 volts was the negative
supply. Then, the inputs and outputs of the op-amp could swing both
+ and - in relation to ground. The input signal would be fed into
the circuit in relation to ground. The op-amp would not operate
correctly if you tried to force the input or output too near either
+12 or -12 volts. There are still a lot of good reasons to have
both a + and a - power supply.
However, op-amp technology has advanced hugely since the 741 came
out. Now there are op-amps that can operate off of a single 5 volt
power supply, and on some of them the input or the output or both can
swing close to the power supply voltages. In some cases, the input
can even swing beyond the power supply voltage by a small amount.
These are called "single supply" op amps. They are convenient when
working with things like the Stamp, which has a handy 5 volt power
supply.
Another characteristic of the every op-amp is its input offset
voltage. Older op-amps like the '741 have offset voltages on the
order of 10 millivolts. This is like an internal signal that is
added in series with your real signal. It is possible to zero that
out with some adjustments in the circuit, which takes additional
parts and hassle. The worst thing about it is, though, that that
offset changes with temperature and power supply voltage and other
factors. Think about your 0.6 millivolt signal. That is small in
relation to the internal offset of a '741 op-amp. Some standard
single supply op-amps like the LT1078 (one of my favorites) have
offsets on the order of 70 microvolts (0.07 millivolt). That is much
better. And CAZ (commutating auto zero) op amps like the LTC1051 use
"switched capacitor" techniques to achieve offsets well under 10
microvolts. And it operates from a single 5 volt supply. That is
why I suggested that for best accuracy. And that is I guess why you
are confused about the hookup.
>
>To further explain the input voltage: the input voltage can vary from .6mv
>to -30mv. I wish the .6mv to be boosted to a high enough voltage so that
>the stamp will recognize it as "high" on the pin i.e. prob. boost to 3v or
>higher. That way if the voltage goes neg. the stamp will recognize as a
>"low" and my prog. can tell the diff. between .6mv and a -mv.
It sounds like you only need to know whether or not the voltage is +
or -, yes or no. In that case you need a simple comparator, not the
RCtime.
+5 Vdd
1k |\|
signal+>--/\/\-|+\
-30mv | >
P0
to ;---|-/
+0.6mv | |/|
| |
->--o
o--Vss
This needs a single supply op-amp. The output is HIGH when the input
is >0, and LOW when the input is <0. It is okay that the input
signal goes negative by 30 millivolts. The single supply op-amp
input can operate correctly even below its Vss power supply down to
about -0.2 volt. In the data sheet they state, "common mode input
range extends below ground".
>
>In addition What does "CAZ" stand for?
See above. They are called for when very high accuracy is required,
without adjustments. For example, in the above circuit, you could
use an LM358 to get a switching point +/- 0.5 millivolt (not too
good!), or an LT1078 to get within 0.07 millivolt (better), or an
LTC1051 to get within 0.010 millivolt. Or you could use an LT1077
with some additional parts for offset adjustment and put your
switching point exactly where you want it in the range.
good luck,
-- Tracy