Hi, List.
[excerpt from a post follows]
> If the group members occasionally volunteered this
type of
> information, maybe we might get actual TI engineers checking in -- if
> they are not already -- and make this group THE destination for
> anyone using this chip family.
[end of excerpt]
I had been using an external voltage reference for the ADC of 1.024V.
And I was getting system accuracy of less than 1% error for my
application; this is acceptable.
What I did was send the 1.5V reference voltage out of the Vref+ pin,
then scale it to 1.024V through an op amp, filter it and run it back into the
Veref+ pin. The reason was I was using legacy circuitry from the
previous version of the product.
But I was getting more noise on the ADC output than I liked, so
I started fooling around. I connected to the internal reference and
got significantly better noise performance, so I decided to re-design
things to use the internal 1.5V or 2.5V reference, instead.
Ok. Here's something I just discovered. On p. 48 of the datasheet
SLAS272C MSP430x13x, MSP430x14x MIXED SIGNAL MICROCONTROLLER
I just discovered that when using an external reference, the voltage
applied should be a minimum of 1.4V to assure max accuracy of
the ADC. (footnote 35)
I completely missed this.
Because of this, I _must_ redesign the circuitry to use the internal reference.
Probably, the 2.5V reference.
I also have a question: The internal reference is spec'd as follows:
1.5V: can be as low as 1.44V and as high as 1.56V.
2.5V: can be as low as 2.4V and as high as 2.6V.
(see p. 48)
Q. Am I understanding this correctly? This means that a
reference intended for use with a 12 bit ADC has an error
of +/- 4%? The resolution of the DAC will be 1 in 4096
or .024%, right? So if I want to preserve the accuracy of my
ADC, I need a reference with at least this accuracy?
Thinking about it, though, I guess this isn't really a problem.
I can adjust for this with my external analog trim circuitry, but
then the issue is really how stable is the reference. Is this fully
a function of temperature? Then, I guess I'm really concerned
with the drift spec: +/-100ppm/degree C. Is this correct? Are there
any other accuracy issues I should be aware of as far as the
internal reference goes?
(I've been careful to follow good PCB layout practices, as best I know
how, and I've paid attention to TI's app info concerning layout and
bypassing of the ADC and the digital/analog power supplies. I've
also got a big ground plane.)
Mark Skeels
Engineer
Competition Electronics
www.CompetitionElectronics.com
meskeels@mesk...
Soli Deo Gloria!
internal/external ADC voltage reference info/questions
Started by ●October 23, 2002
Reply by ●October 23, 20022002-10-23
Hi Mark, If you want a high accuracy ADC, then it depends on whether the result is ratiometric or absolute. It sounds like you are int he absolute case as the ratiometric version is easier. I would use one of the information memory locations to store scaling error values for the conversion (cheaper cost of manufacture than a trimpot) and you will also need to adjust for temperature variation of the on-board reference if you want a high accuracy low drift result. Some parts have an on-board diode reference which you can use to track the current core temperature. There are application notes covering this. I have also used a thermistor for this but that consumes another port pin. A piecewise linear fit is good enough though you may need to have reference points every 10 degrees or so over the temperature range the product will operate under. 1% is not overly difficult to achieve but does require care. You are right in assuming the temperature fit needs to not further degrade the ADC accuracy to keep 1% possible. Ray
Reply by ●October 24, 20022002-10-24
Hi Mark, Very interesting. Thanks for sharing your discovery. Clyde -----Original Message----- From: Mark Skeels [mailto:meskeels@mesk...] Sent: Wednesday, October 23, 2002 9:08 AM To: Msp430@Msp4... Subject: [msp430] internal/external ADC voltage reference info/questions Hi, List. [excerpt from a post follows] > If the group members occasionally volunteered this type of > information, maybe we might get actual TI engineers checking in -- if > they are not already -- and make this group THE destination for > anyone using this chip family. [end of excerpt] I had been using an external voltage reference for the ADC of 1.024V. And I was getting system accuracy of less than 1% error for my application; this is acceptable. What I did was send the 1.5V reference voltage out of the Vref+ pin, then scale it to 1.024V through an op amp, filter it and run it back into the Veref+ pin. The reason was I was using legacy circuitry from the previous version of the product. But I was getting more noise on the ADC output than I liked, so I started fooling around. I connected to the internal reference and got significantly better noise performance, so I decided to re-design things to use the internal 1.5V or 2.5V reference, instead. Ok. Here's something I just discovered. On p. 48 of the datasheet SLAS272C MSP430x13x, MSP430x14x MIXED SIGNAL MICROCONTROLLER I just discovered that when using an external reference, the voltage applied should be a minimum of 1.4V to assure max accuracy of the ADC. (footnote 35) I completely missed this. Because of this, I _must_ redesign the circuitry to use the internal reference. Probably, the 2.5V reference. I also have a question: The internal reference is spec'd as follows: 1.5V: can be as low as 1.44V and as high as 1.56V. 2.5V: can be as low as 2.4V and as high as 2.6V. (see p. 48) Q. Am I understanding this correctly? This means that a reference intended for use with a 12 bit ADC has an error of +/- 4%? The resolution of the DAC will be 1 in 4096 or .024%, right? So if I want to preserve the accuracy of my ADC, I need a reference with at least this accuracy? Thinking about it, though, I guess this isn't really a problem. I can adjust for this with my external analog trim circuitry, but then the issue is really how stable is the reference. Is this fully a function of temperature? Then, I guess I'm really concerned with the drift spec: +/-100ppm/degree C. Is this correct? Are there any other accuracy issues I should be aware of as far as the internal reference goes? (I've been careful to follow good PCB layout practices, as best I know how, and I've paid attention to TI's app info concerning layout and bypassing of the ADC and the digital/analog power supplies. I've also got a big ground plane.) Mark Skeels Engineer Competition Electronics www.CompetitionElectronics.com meskeels@mesk... Soli Deo Gloria! . ">http://docs.yahoo.com/info/terms/
Reply by ●October 24, 20022002-10-24
----- Original Message ----- From: "Ray Keefe" <ray@ray@...> To: <msp430@msp4...> Sent: Wednesday, October 23, 2002 4:34 PM Subject: RE: [msp430] internal/external ADC voltage reference info/questions > Hi Mark, > > If you want a high accuracy ADC, then it depends on whether the result is > ratiometric or absolute. It sounds like you are int he absolute case as the > ratiometric version is easier. ...yes, I am using absolute. > > I would use one of the information memory locations to store scaling error > values for the conversion (cheaper cost of manufacture than a trimpot) and I had envisioned this for the next iteration. During manufacture, a method will have to be worked out to determine this value. I was thinking supplying known input values and then having the '430 calculate and store the offset and gain info in FLASH. Is this what you mean? > you will also need to adjust for temperature variation of the on-board > reference if you want a high accuracy low drift result. Some parts have an > on-board diode reference which you can use to track the current core > temperature. There are application notes covering this. I have also used a > thermistor for this but that consumes another port pin. I've got the '149; it has this diode. I'll look for the app note, thanks. > A piecewise linear > fit is good enough though you may need to have reference points every 10 > degrees or so over the temperature range the product will operate under. > By this, you mean a series of short line segments approximating the curve? Then you're saying I may need a different series every 10 degrees. Yes? Doesn't this vary with each unit, or could I expect to use the same cal info for all of them? > 1% is not overly difficult to achieve but does require care. You are right > in assuming the temperature fit needs to not further degrade the ADC > accuracy to keep 1% possible. I don't quite understand this statement. Wouldn't the very object be to improve the accuracy by means of the temperature compensation? Then, of course one would assume the whole reason for doing it in the first place would be to improve accuracy. If it degrades the accuracy, then why do it? And thanks for taking the time to respond! I appreciate it. > > Ray > > > > . > > > > ">http://docs.yahoo.com/info/terms/ > >
Reply by ●October 24, 20022002-10-24
----- Original Message -----
From: "Eisenbeis, Clyde [FRCO/MTN]" <clyde.eisenbeis@clyd...>
To: <msp430@msp4...>
Sent: Thursday, October 24, 2002 6:23 AM
Subject: RE: [msp430] internal/external ADC voltage reference info/questions
> Hi Mark,
>
> Very interesting. Thanks for sharing your discovery.
>
> Clyde
You're welcome. :-)
Reply by ●October 24, 20022002-10-24
Hi Mark, >> I would use one of the information memory locations to store scaling error >> values for the conversion (cheaper cost of manufacture than a trimpot) and >I had envisioned this for the next iteration. During manufacture, a method will >have to be worked out to determine this value. I was thinking supplying known input >values and then having the '430 calculate and store the offset and gain info in FLASH. >Is this what you mean? This sort of correction is what I was thinking of. >> A piecewise linear >> fit is good enough though you may need to have reference points every 10 >> degrees or so over the temperature range the product will operate under. >> >By this, you mean a series of short line segments approximating the curve? >Then you're saying I may need a different series every 10 degrees. Yes? >Doesn't this vary with each unit, or could I expect to use the same cal info >for all of them? The shape of the curve should be the same but the offset and gain may vary from batch to batch. You only need one series. The 10 degrees reference was to the interval between points on the curve. >> 1% is not overly difficult to achieve but does require care. You are right >> in assuming the temperature fit needs to not further degrade the ADC >> accuracy to keep 1% possible. >I don't quite understand this statement. Wouldn't the very object be to >improve the accuracy by means of the temperature compensation? >Then, of course one would assume the whole reason for doing it in the first >place would be to improve accuracy. If it degrades the accuracy, then >why do it? My statement was referring to your comment on the ADC LSB being 0.25%, all things being perfect. This gives you another 0.75% in your error budget before you can no longer meet your 1% accuracy specification. SO all forms of external and internal errors must fit within this range when summed together. Ways to improve basic accuracy are things like temperature compensation to reduce drift, manufacturing setups to remove unit to unit variation and careful design. The way to reduce noise and statistical variation is filtering. I usually do this is software (it is cheaper) but anti-aliasing filtering must be done outside the chip. I have had a lot of analog design experience and guaranteed 1% accuracy is harder to achieve in practice that most people appreciate. You can get better than this but the level of effort goes up enormously once you want 0.1%. Look at the price of a 6 digit multimeter. >And thanks for taking the time to respond! I appreciate it. No problem. Ray