ADC question regarding resolution and Vref
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I am considering adding 3 ADC's to measure a small voltage (about
7-20mV). I need something with a high resoution so that small changes
will reflect well. I was thinking of a 24 bit ADC from TI (as an
example). The two questions I have are this:
1. How do I determine the full scale output of a ADC? I'd like to
determine if I really need a 24 bit ADC or something with less
resolution.
2. What is the difference between an internal Vref signal and an
external Vref signal?
Any info anyone could provide would be great, thanks!
7-20mV). I need something with a high resoution so that small changes
will reflect well. I was thinking of a 24 bit ADC from TI (as an
example). The two questions I have are this:
1. How do I determine the full scale output of a ADC? I'd like to
determine if I really need a 24 bit ADC or something with less
resolution.
2. What is the difference between an internal Vref signal and an
external Vref signal?
Any info anyone could provide would be great, thanks!
Comments
sheet for the TI ADS1240 24 bit AD converter, I see some formulas.
After plugging in some of the listed values, I see how they arrive at
the full scale voltage/# of bits.
However, the inital question is still posed regarding the Vref -
Looking at page 20 of the spec sheet, the formulas are seen. I see
two things in the table that confuse me -
there are listings for the "full scale range" and the "differential
input voltage." Since I want to measure from about 7mV to 20mV, would
I set my full scale voltage to 20mV? I see that on the right side
there are two sets of equations, one for RANGE=0 and one for RANGE=1.
What's the difference?
--- In basicstamps@y..., "eccentric_mofo" <eccentric_mofo@y...> wrote:
> I am considering adding 3 ADC's to measure a small voltage (about
> 7-20mV). I need something with a high resoution so that small
changes
> will reflect well. I was thinking of a 24 bit ADC from TI (as an
> example). The two questions I have are this:
>
> 1. How do I determine the full scale output of a ADC? I'd like to
> determine if I really need a 24 bit ADC or something with less
> resolution.
>
> 2. What is the difference between an internal Vref signal and an
> external Vref signal?
>
> Any info anyone could provide would be great, thanks!
eccentric_mofo@y... writes:
> there are listings for the "full scale range" and the "differential
> input voltage." Since I want to measure from about 7mV to 20mV, would
> I set my full scale voltage to 20mV? I see that on the right side
> there are two sets of equations, one for RANGE=0 and one for RANGE=1.
> What's the difference?
>
>
You can't measure a voltage higher than the reference voltage. If you set
Vref to 5 volts - your supply voltage - then you can read a max voltage of 5
volts. You will have to use a pot, preferably a good 10-turn pot, to pick
off 5 volts when the input voltage to the pot is 20 volts. You then have to
process the ADC readings so that when the ADC says 5 volts it really means 20
volts. I have done a bit of programming with the TLC2543, which is a 12-bit
ADC and found ther resolution to be more than adequate. If you need more
help, or help with the code, get in touch.
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
eccentric refers to 20 mV. Still, a good approach would be to use a
low-noise differential instrumentation amp to first amplify the signal
into the 0-5V range, and then use a 12-bit ADC.
Dennis
Original Message
From: Newzed@a... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=szzm2nDhFSOgUV3S-Sx4Z4LF15b_Iy8jK7FxLOvqZS8gtaCGnMj1iY_47f4-sXoP8nrpSK-6TlQ]Newzed@a...[/url
Sent: Tuesday, September 10, 2002 3:11 PM
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] Re: ADC question regarding resolution and
Vref
In a message dated 09/10/2002 18:01:22 Eastern Daylight Time,
eccentric_mofo@y... writes:
> there are listings for the "full scale range" and the "differential
> input voltage." Since I want to measure from about 7mV to 20mV, would
> I set my full scale voltage to 20mV? I see that on the right side
> there are two sets of equations, one for RANGE=0 and one for RANGE=1.
> What's the difference?
>
>
You can't measure a voltage higher than the reference voltage. If you
set
Vref to 5 volts - your supply voltage - then you can read a max voltage
of 5
volts. You will have to use a pot, preferably a good 10-turn pot, to
pick
off 5 volts when the input voltage to the pot is 20 volts. You then
have to
process the ADC readings so that when the ADC says 5 volts it really
means 20
volts. I have done a bit of programming with the TLC2543, which is a
12-bit
ADC and found ther resolution to be more than adequate. If you need
more
help, or help with the code, get in touch.
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
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it a little easier to measure. For instance, you could run the
signal through an op amp set up to a gain of 250 so that your 20mV
signal will give you a 5 volt output, and the 7mV signal will give
you 1.75 volts. You shouldn't need an ADC with such a high
resolution if you're measuring voltages in that range. I'd imagine
12 bits would be fine. With a 12 bit ADC measuring voltages up to 5
volts, you'll have resolution down to 1.22mV. However, if you have
alot of noise on the signal even before it's sent to the amplifier,
that could be a problem.
Good luck!
--- In basicstamps@y..., "eccentric_mofo" <eccentric_mofo@y...> wrote:
> I am considering adding 3 ADC's to measure a small voltage (about
> 7-20mV). I need something with a high resoution so that small
changes
> will reflect well. I was thinking of a 24 bit ADC from TI (as an
> example). The two questions I have are this:
>
> 1. How do I determine the full scale output of a ADC? I'd like to
> determine if I really need a 24 bit ADC or something with less
> resolution.
>
> 2. What is the difference between an internal Vref signal and an
> external Vref signal?
>
> Any info anyone could provide would be great, thanks!
will be powered off of a 9v battery. What I am trying to measure is
the voltage coming off of an oxygen cell (which is typically between
10 to 25mV max).
If I use an instrumentation amp it may be more difficult due to the
fact that I don't have a -5 regulated source (I was going to use the
5v regulated output of the stamp to power the IC's). I was actually
thinking of using a 14-bit ADC from TI (the TLC3544, which is a 4
channel ADC which is what I need). The reason I wanted to go with a
14-bit ADC is because the sensor has to be able to measure small
changes in voltage.
If I use an OP-Amp circuit, I might have to add lots more external
circuitry (such as caps, resistors, etc) - which may add more
parasitic noise into the circuit. How is the noise with an ADC?
I've never used one before, but am learning quickly about them!
Thanks again for all of your help!
--- In basicstamps@y..., "Dennis P. O'Leary" <doleary@e...> wrote:
> Seems to be a difference of 1000 here. Sid, you refer to 20 volts,
but
> eccentric refers to 20 mV. Still, a good approach would be to use a
> low-noise differential instrumentation amp to first amplify the
signal
> into the 0-5V range, and then use a 12-bit ADC.
>
> Dennis
>
>
Original Message
> From: Newzed@a... [noparse][[/noparse]mailto:Newzed@a...]
> Sent: Tuesday, September 10, 2002 3:11 PM
> To: basicstamps@y...
> Subject: Re: [noparse][[/noparse]basicstamps] Re: ADC question regarding resolution and
> Vref
>
>
> In a message dated 09/10/2002 18:01:22 Eastern Daylight Time,
> eccentric_mofo@y... writes:
>
>
> > there are listings for the "full scale range" and
the "differential
> > input voltage." Since I want to measure from about 7mV to 20mV,
would
>
> > I set my full scale voltage to 20mV? I see that on the right
side
> > there are two sets of equations, one for RANGE=0 and one for
RANGE=1.
> > What's the difference?
> >
> >
>
> You can't measure a voltage higher than the reference voltage. If
you
> set
> Vref to 5 volts - your supply voltage - then you can read a max
voltage
> of 5
> volts. You will have to use a pot, preferably a good 10-turn pot,
to
> pick
> off 5 volts when the input voltage to the pot is 20 volts. You then
> have to
> process the ADC readings so that when the ADC says 5 volts it really
> means 20
> volts. I have done a bit of programming with the TLC2543, which is
a
> 12-bit
> ADC and found ther resolution to be more than adequate. If you
need
> more
> help, or help with the code, get in touch.
>
> Sid
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@y...
> 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/
http://www.emesys.com/usea_dat.htm
which is meant for small signals from things like thermopile sensors
and oxygen sensors. It operates from a single +5 volts supply, no -5v
required. The op-amp inside is a CAZ (commutating auto-zero), which
has in input offset voltage of less than 10 microvolts, and the
amplifier gain is set by 0.1% film resistors and a trimmer in two
stages of gain.
One high resolution converter should also work fine. I know Chuck
Gracey has posted Stamp code for the Cirrus CS552x (look back in the
archives on 11/17/01, or http://www.cirrus.com). The CS5523 is 16
bit, 4 channel, and the CS5524 is 24 bit, 4 channel, and they
includes a programmable gain amplifier that can bring bring the full
scale range down to a 25 millivolt span.
I recall seeing some LTC24xx code posted too, maybe by Steve Roberts.
All of these require careful reading of the data sheet--you already
found that out! Also take careful attention to layout, not to mix up
the analog and digital grounds etc. Concerning external versus
internal reference. Usually an external reference will be more
accurate. You usually will want to trim the reference to some exact
value, say 4.096 volts. Internal references cannot usually be
trimmed, and their initial accuracy is seldom better than +/- 1%.
-- best regards
Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
mailto:tracy@e...
>That may work, but I am thinking about ease of application. This
>will be powered off of a 9v battery. What I am trying to measure is
>the voltage coming off of an oxygen cell (which is typically between
>10 to 25mV max).
>
>If I use an instrumentation amp it may be more difficult due to the
>fact that I don't have a -5 regulated source (I was going to use the
>5v regulated output of the stamp to power the IC's). I was actually
>thinking of using a 14-bit ADC from TI (the TLC3544, which is a 4
>channel ADC which is what I need). The reason I wanted to go with a
>14-bit ADC is because the sensor has to be able to measure small
>changes in voltage.
>
>If I use an OP-Amp circuit, I might have to add lots more external
>circuitry (such as caps, resistors, etc) - which may add more
>parasitic noise into the circuit. How is the noise with an ADC?
>I've never used one before, but am learning quickly about them!
>Thanks again for all of your help!
>
>
>--- In basicstamps@y..., "Dennis P. O'Leary" <doleary@e...> wrote:
>> Seems to be a difference of 1000 here. Sid, you refer to 20 volts,
>but
>> eccentric refers to 20 mV. Still, a good approach would be to use a
>> low-noise differential instrumentation amp to first amplify the
>signal
>> into the 0-5V range, and then use a 12-bit ADC.
>>
>> Dennis
>>
>>
Original Message
> > From: Newzed@a... [noparse][[/noparse]mailto:Newzed@a...]
> > Sent: Tuesday, September 10, 2002 3:11 PM
> > To: basicstamps@y...
> > Subject: Re: [noparse][[/noparse]basicstamps] Re: ADC question regarding resolution and
> > Vref
>>
>>
>> In a message dated 09/10/2002 18:01:22 Eastern Daylight Time,
>> eccentric_mofo@y... writes:
>>
>>
>> > there are listings for the "full scale range" and
>the "differential
>> > input voltage." Since I want to measure from about 7mV to 20mV,
>would
>>
>> > I set my full scale voltage to 20mV? I see that on the right
>side
>> > there are two sets of equations, one for RANGE=0 and one for
>RANGE=1.
> > > What's the difference?
> > >