LTC1298 Quirky Numbers
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Posts: 46,084
Hi Everyone:
I am working on a project using a stamp and LTC1298 and I am getting some
quirky numbers being returned.
The LTC1298 is measuring a 0-5VDC signal from a transducer via an isolator.
I am sampling a single channel so pins 2 and 3 are set up for single channel
differential mode and pin 3 is not connected to the rest of the 0V circuit
located on the board.
We were having additional troubles with system noise but we solved this
problem by putting in shielded cabling, disabling the PWM output we assigned
to one pin and adding a 1 uF tantalum capacitor to the input of the LTC1298.
What is happening is that the A/D converter is returning what looks like
noise bursts because when we inject a signal other than 5 VDC (from a
calibrator) the numbers coming back are not always consistent. Example: 0VDC
will return a number of 0 most times but we will see spikes occasionally
that will vary from 23 - 50+. At 5 VDC we always see 4095 being returned as
a number. Noise variations are approaching +/-15%. If the noise was 1% or
less this would be a non-issue.
We have scoped the input to the LTC1298 and it is pretty much a clean DC
input. We have not scoped the 5VDC supply on the board but we are running
everything on the board (stamp, darlington driver and A/D converter) with a
heatsinked LM78M05 regulator and over a 24 hour period its temperature only
rose 10 degrees Fahrenheit above the room's ambient noise temperature. We
also have tantalum bypass capacitors on the stamp, A/D converter, input to
the regulator, output from the regulator and on the output of another
independent power supply on the board. The converter, stamp and input
terminals are all within 1-1/2 inches of each other.
I am using the following sub-routine to query the LTC 1298:
convert:
config=%1001
low CS
shiftout DIO, CLK, lsbfirst, [noparse][[/noparse]config\4]
shiftin DIO, CLK, msbpost, [noparse][[/noparse]ad\12]
high CS
return
Am I missing something obvious here? We have tried sampling the A/D
converter 10 times per second instead of every program cycle but this does
not help at all. Any suggestions would be appreciated.
Thanks and regards in advance.
Gerry Shand
res@i...
I am working on a project using a stamp and LTC1298 and I am getting some
quirky numbers being returned.
The LTC1298 is measuring a 0-5VDC signal from a transducer via an isolator.
I am sampling a single channel so pins 2 and 3 are set up for single channel
differential mode and pin 3 is not connected to the rest of the 0V circuit
located on the board.
We were having additional troubles with system noise but we solved this
problem by putting in shielded cabling, disabling the PWM output we assigned
to one pin and adding a 1 uF tantalum capacitor to the input of the LTC1298.
What is happening is that the A/D converter is returning what looks like
noise bursts because when we inject a signal other than 5 VDC (from a
calibrator) the numbers coming back are not always consistent. Example: 0VDC
will return a number of 0 most times but we will see spikes occasionally
that will vary from 23 - 50+. At 5 VDC we always see 4095 being returned as
a number. Noise variations are approaching +/-15%. If the noise was 1% or
less this would be a non-issue.
We have scoped the input to the LTC1298 and it is pretty much a clean DC
input. We have not scoped the 5VDC supply on the board but we are running
everything on the board (stamp, darlington driver and A/D converter) with a
heatsinked LM78M05 regulator and over a 24 hour period its temperature only
rose 10 degrees Fahrenheit above the room's ambient noise temperature. We
also have tantalum bypass capacitors on the stamp, A/D converter, input to
the regulator, output from the regulator and on the output of another
independent power supply on the board. The converter, stamp and input
terminals are all within 1-1/2 inches of each other.
I am using the following sub-routine to query the LTC 1298:
convert:
config=%1001
low CS
shiftout DIO, CLK, lsbfirst, [noparse][[/noparse]config\4]
shiftin DIO, CLK, msbpost, [noparse][[/noparse]ad\12]
high CS
return
Am I missing something obvious here? We have tried sampling the A/D
converter 10 times per second instead of every program cycle but this does
not help at all. Any suggestions would be appreciated.
Thanks and regards in advance.
Gerry Shand
res@i...
Comments
<res@i...> wrote:
> Hi Everyone:
>
> I am working on a project using a stamp and LTC1298 and I am
getting some
> quirky numbers being returned.
>
> The LTC1298 is measuring a 0-5VDC signal from a transducer via an
isolator.
> I am sampling a single channel so pins 2 and 3 are set up for
single channel
> differential mode and pin 3 is not connected to the rest of the 0V
circuit
> located on the board.
>
> We were having additional troubles with system noise but we solved
this
> problem by putting in shielded cabling, disabling the PWM output we
assigned
> to one pin and adding a 1 uF tantalum capacitor to the input of the
LTC1298.
>
> What is happening is that the A/D converter is returning what looks
like
> noise bursts because when we inject a signal other than 5 VDC (from
a
> calibrator) the numbers coming back are not always consistent.
Example: 0VDC
> will return a number of 0 most times but we will see spikes
occasionally
> that will vary from 23 - 50+. At 5 VDC we always see 4095 being
returned as
> a number. Noise variations are approaching +/-15%. If the noise was
1% or
> less this would be a non-issue.
>
> We have scoped the input to the LTC1298 and it is pretty much a
clean DC
> input. We have not scoped the 5VDC supply on the board but we are
running
> everything on the board (stamp, darlington driver and A/D
converter) with a
> heatsinked LM78M05 regulator and over a 24 hour period its
temperature only
> rose 10 degrees Fahrenheit above the room's ambient noise
temperature. We
> also have tantalum bypass capacitors on the stamp, A/D converter,
input to
> the regulator, output from the regulator and on the output of
another
> independent power supply on the board. The converter, stamp and
input
> terminals are all within 1-1/2 inches of each other.
>
> I am using the following sub-routine to query the LTC 1298:
>
> convert:
> config=%1001
> low CS
> shiftout DIO, CLK, lsbfirst, [noparse][[/noparse]config\4]
> shiftin DIO, CLK, msbpost, [noparse][[/noparse]ad\12]
> high CS
> return
>
> Am I missing something obvious here? We have tried sampling the A/D
> converter 10 times per second instead of every program cycle but
this does
> not help at all. Any suggestions would be appreciated.
>
> Thanks and regards in advance.
>
> Gerry Shand
> res@i...
Hi Gerry,
Anytime I hear darlingtons on the board it makes me think power
supply.
do you have a cap on the power supply that can handle the load for
the darlingtons ?
The power supply should be in a star configuration. That is that the
Stamp should be run directly to a cap on the power supply. the ADC
should be run to a separate cap back to the power supply ditto
anything that will generate pulses. your PWM and possibly the
darlington.
If you have the ability, tie one of the inputs to 5V power for a
reference.
put a cap on the inputs. We offer the ChipCircuit-ADC with an on-
board TLC2543 chip and put capacitors on each input to help with the
noise that you may be seeing.
Also, check the traces to see (this is a long shot) if your PWM
signals are next to your ADC inputs. This is rarely the case as the
ADC's typically go to the board edge and the op-amp for the pwm is
often near the stamp with the 0-5 going to the board edge.
Dave
10 channel ADC....
512K plug-in EEPROM.......
http://www.visualmuses.com/chipcircuit/index.html
a motor in the building that is turning on and you are getting noise from
it. Happens here. Also, we get the local AM radio station sometimes<G>.
What is the timing between the bursts of noise? May be an indication.
Original Message
From: Dave Mucha [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=D6lduaPoCQVyfpYQLscy6odBgo5sqeOCwuPuNUyaTkid1q2vK3p7Tc2Vr7bjU3IO7zLHbg0LcYM]davemucha@j...[/url
Sent: Monday, May 24, 2004 12:08 PM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] Re: LTC1298 Quirky Numbers
--- In basicstamps@yahoogroups.com, "Gerry Shand/Corrinne Bergen"
<res@i...> wrote:
> Hi Everyone:
>
> I am working on a project using a stamp and LTC1298 and I am
getting some
> quirky numbers being returned.
>
> The LTC1298 is measuring a 0-5VDC signal from a transducer via an
isolator.
> I am sampling a single channel so pins 2 and 3 are set up for
single channel
> differential mode and pin 3 is not connected to the rest of the 0V
circuit
> located on the board.
>
> We were having additional troubles with system noise but we solved
this
> problem by putting in shielded cabling, disabling the PWM output we
assigned
> to one pin and adding a 1 uF tantalum capacitor to the input of the
LTC1298.
>
> What is happening is that the A/D converter is returning what looks
like
> noise bursts because when we inject a signal other than 5 VDC (from
a
> calibrator) the numbers coming back are not always consistent.
Example: 0VDC
> will return a number of 0 most times but we will see spikes
occasionally
> that will vary from 23 - 50+. At 5 VDC we always see 4095 being
returned as
> a number. Noise variations are approaching +/-15%. If the noise was
1% or
> less this would be a non-issue.
>
> We have scoped the input to the LTC1298 and it is pretty much a
clean DC
> input. We have not scoped the 5VDC supply on the board but we are
running
> everything on the board (stamp, darlington driver and A/D
converter) with a
> heatsinked LM78M05 regulator and over a 24 hour period its
temperature only
> rose 10 degrees Fahrenheit above the room's ambient noise
temperature. We
> also have tantalum bypass capacitors on the stamp, A/D converter,
input to
> the regulator, output from the regulator and on the output of
another
> independent power supply on the board. The converter, stamp and
input
> terminals are all within 1-1/2 inches of each other.
>
> I am using the following sub-routine to query the LTC 1298:
>
> convert:
> config=%1001
> low CS
> shiftout DIO, CLK, lsbfirst, [noparse][[/noparse]config\4]
> shiftin DIO, CLK, msbpost, [noparse][[/noparse]ad\12]
> high CS
> return
>
> Am I missing something obvious here? We have tried sampling the A/D
> converter 10 times per second instead of every program cycle but
this does
> not help at all. Any suggestions would be appreciated.
>
> Thanks and regards in advance.
>
> Gerry Shand
> res@i...
Hi Gerry,
Anytime I hear darlingtons on the board it makes me think power
supply.
do you have a cap on the power supply that can handle the load for
the darlingtons ?
The power supply should be in a star configuration. That is that the
Stamp should be run directly to a cap on the power supply. the ADC
should be run to a separate cap back to the power supply ditto
anything that will generate pulses. your PWM and possibly the
darlington.
If you have the ability, tie one of the inputs to 5V power for a
reference.
put a cap on the inputs. We offer the ChipCircuit-ADC with an on- board
TLC2543 chip and put capacitors on each input to help with the
noise that you may be seeing.
Also, check the traces to see (this is a long shot) if your PWM
signals are next to your ADC inputs. This is rarely the case as the
ADC's typically go to the board edge and the op-amp for the pwm is
often near the stamp with the 0-5 going to the board edge.
Dave
10 channel ADC....
512K plug-in EEPROM....... http://www.visualmuses.com/chipcircuit/index.html
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on the same power line transformer, can cause these noise spikes at 120
times a second. I used to have a noise problem that started every day in the
early evening, just as the sun was going down. My neighbor two houses away
would turn on her lights in the dining room to about 50% dim, which I could
actually see on my o'scope. When she turned them to 100% there was no noise,
and on the weekends she didn't go in the dining room. Talk about some
detective work to find the noise source.
Mike Sokol
mike@f...
www.fitsandstarts.com
" One should not increase, beyond what is necessary,
the number of entities required to explain anything"...
-William of Occam-
Original Message
From: "Grover Richardson" <grover.richardson@g...>
To: <basicstamps@yahoogroups.com>
Sent: Monday, May 24, 2004 5:21 PM
Subject: RE: [noparse][[/noparse]basicstamps] Re: LTC1298 Quirky Numbers
> Look at the intput to the LTC with an oscope. I would suspect that there
is
> a motor in the building that is turning on and you are getting noise from
> it. Happens here. Also, we get the local AM radio station sometimes<G>.
> What is the timing between the bursts of noise? May be an indication.
>