DAC ADC considerations
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[font=arial,helvetica]Hi Group,
I am using the LTC1257 and LTC1298, DAC and ADC chips. They are both 12bit
and easy to work with. I notice that the schematics given in the Stamp
applications I have do not cover voltage reference prep, adding additional
filters, caps or whatever to keep it stable. Are there any "rules of thumb"
for this?
Does anyone have any favorite voltage reference they like to use?
Thanks,
Mark[/font]
I am using the LTC1257 and LTC1298, DAC and ADC chips. They are both 12bit
and easy to work with. I notice that the schematics given in the Stamp
applications I have do not cover voltage reference prep, adding additional
filters, caps or whatever to keep it stable. Are there any "rules of thumb"
for this?
Does anyone have any favorite voltage reference they like to use?
Thanks,
Mark[/font]
Comments
We have a project using an LTC1298 to control a battery charger. My
favorite reference is the 5 volt REF02. There are cheaper references
available, but this chip has some nice features:
~0.1% accuracy
Source 10ma, short circuit protected.
Low current drain.
The charger controller runs from 8 to 24 volts input, with the entire
circuit, including an LCD display, powered directly from the REF02 output.
It draws about 4 ma. when operated from a 9 volt battery. The analog input
to the 1298 is bypassed with a 1uf ceramic, (no problem with a low impedance
source), and the 5 volt REF02 output is bypassed with a 220 uf electrolytic
that we had handy; a smaller cap would work.
Ray McArthur
Original Message
From: <electronguy@a...>
To: <basicstamps@egroups.com>
Sent: Wednesday, January 03, 2001 6:55 PM
Subject: [noparse][[/noparse]basicstamps] DAC ADC considerations
> Hi Group,
>
> I am using the LTC1257 and LTC1298, DAC and ADC chips. They are both 12bit
> and easy to work with. I notice that the schematics given in the Stamp
> applications I have do not cover voltage reference prep, adding additional
> filters, caps or whatever to keep it stable. Are there any "rules of
thumb"
> for this?
> Does anyone have any favorite voltage reference they like to use?
>
> Thanks,
>
> Mark
>
>and easy to work with. I notice that the schematics given in the Stamp
>applications I have do not cover voltage reference prep, adding additional
>filters, caps or whatever to keep it stable. Are there any "rules of thumb"
>for this?
>Does anyone have any favorite voltage reference they like to use?
Hi Mark,
The LTC1298, as you know, uses its power supply as its voltage
reference. I like the National LP2951,
http://www.national.com/ds/LP/LP2950.pdf
It is a voltage regulator, stable enough to qualify as a reference.
The '2951 is adjustable and can be set for a 5.120 volt output, which
then gives a nice 1.25 millivolt per bit scale factor. Seetron uses
the LP2950 with the LT1298 on the Data Collection Proto Board. Be
sure to use at least 1uF output capacitor.
One thing we discovered in a discussion some time back on this list
is a slight problem with the LT1298 in relation to the BS2. You
expect to get an output code of $fff when the input is equal to the
(+) reference, however, what you do get is, say, $ffc maximum. (I
forget exactly what code--maybe Steve S. will recall?) It turned out
that the SPI clock in the BS2 is so slow that the charge on the
sampling capacitors droops a little bit before the end of the
conversion. No big deal, but the problem had us scratching our heads
for a while. It be better on a BS2sx, with the faster SPI clock.
The LTC1257 has its own 2.048 volt reference built in, for 4.096 volt
full scale output. If you want 5 or 5.12 volts full scale output,
you will have to provide an external 2.5 or 2.56 volt reference, from
a low impedance source, say a voltage divider and an op-amp. Yes,
filter it with at least 1uF.
Avoid ground loops! Keep digital signals isolated from analog!
-- Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
"Avoid ground loops! Keep digital signals isolated from analog!"
Could you expand on this a bit?
Sharing a common ground for everything is a problem with DAC ADC?
Mark
Original Message
From: Tracy Allen <tracy@e...>
To: basicstamps@egroups.com <basicstamps@egroups.com>
Date: Thursday, January 04, 2001 1:11 AM
Subject: Re: [noparse][[/noparse]basicstamps] DAC ADC considerations
>>I am using the LTC1257 and LTC1298, DAC and ADC chips. They are both 12bit
>>and easy to work with. I notice that the schematics given in the Stamp
>>applications I have do not cover voltage reference prep, adding additional
>>filters, caps or whatever to keep it stable. Are there any "rules of
thumb"
>>for this?
>>Does anyone have any favorite voltage reference they like to use?
>
>Hi Mark,
>
>The LTC1298, as you know, uses its power supply as its voltage
>reference. I like the National LP2951,
> http://www.national.com/ds/LP/LP2950.pdf
>It is a voltage regulator, stable enough to qualify as a reference.
>The '2951 is adjustable and can be set for a 5.120 volt output, which
>then gives a nice 1.25 millivolt per bit scale factor. Seetron uses
>the LP2950 with the LT1298 on the Data Collection Proto Board. Be
>sure to use at least 1uF output capacitor.
>
>One thing we discovered in a discussion some time back on this list
>is a slight problem with the LT1298 in relation to the BS2. You
>expect to get an output code of $fff when the input is equal to the
>(+) reference, however, what you do get is, say, $ffc maximum. (I
>forget exactly what code--maybe Steve S. will recall?) It turned out
>that the SPI clock in the BS2 is so slow that the charge on the
>sampling capacitors droops a little bit before the end of the
>conversion. No big deal, but the problem had us scratching our heads
>for a while. It be better on a BS2sx, with the faster SPI clock.
>
>The LTC1257 has its own 2.048 volt reference built in, for 4.096 volt
>full scale output. If you want 5 or 5.12 volts full scale output,
>you will have to provide an external 2.5 or 2.56 volt reference, from
>a low impedance source, say a voltage divider and an op-amp. Yes,
>filter it with at least 1uF.
>
>Avoid ground loops! Keep digital signals isolated from analog!
>
> -- Tracy Allen
> electronically monitored ecosystems
> http://www.emesystems.com
>
>
>
>
>
>
>
>
>Could you expand on this a bit?
>
>Sharing a common ground for everything is a problem with DAC ADC?
Hi Mark,
Are you working with heavy currents or inductive loads (motors,
lamps, relays etc.), or high-speed digital circuitry? It is best to
run the wiring so that voltage drops due to the heavy currents or the
noise due to the digital circuits or power supply glitches due to
inductive kicks don't end up fuzzing up the signals from the DAC or
ADC. The symptom is readings that change when the loads turn on or
off, or readings that jump around randomly more than one bit. If
there are no nasty loads, then it may be just fine to connect all of
the grounds together.
-- Tracy