Digest Number 1501
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Posts: 46,084
Hello Everyone,
I have little problems with stability with LM34 or
other like temp sensors.
Attached is a photo of my temp board. Makes for easy
assembly of temp
sensors. The whole circuit consists of a load on the
signal out 100K to ground.
A series 2.2K after the signal to ground load to
isolate the capacitance of the cable
as per the LM34 Datasheet, (did everyone download this
and read it).
The addition of a Tantalum capacitor on +5 to gnd at
the sensor.
At the ADC end I use a low pass filter to take out any
induced noise picked up by the
cable.
Proper installation of the ADC is essential, i.e.
follow the Datasheet recommendations.
Sincerely,
Ron Anderson
Digital Design Solutions, Inc.
Message: 15
Date: Sat, 23 Nov 2002 11:59:20 -0800
From: "Jonathan Peakall" <jpeakall@m...>
Subject: Re: was: Re: Tracy, math question: LM34s
>Tracy, Pat, All,
> >Here's the odd thing. When I input 5V to the LM34, I
get *very* erratic
> >readings off the ADC. Makes no difference what power
supply I use. When I
> >reduce the input to 2V using a voltage divider, I
get very stable
readings
> >with the bounce factor being around 3mV. I tried a
3.3k resistor in
series
> >with the LM34 output as Tracy suggested, but the
above condition
persists.
>
> Do you mean you are using the following circuit, but
with 6 volts
> replaced with 2 volts?
> ;---
> +6 volts
|
> cable 3 kohm |
> signal ---/\/\
| LM34
> common
|
> `---
> It won't work. The readings are stable because
nothing is happening.
> The absolute minimum supply voltage is 4.5 volts (5
volts per the
> data sheet). Did you check for AC voltages using
your VOM? Connect
> the ground black lead of the VOM to the V- reference
pin of your ADC.
Yes, just as shown above. It IS working with a supply
of 2.2V to the LM34,
quite well in fact, has kept the tub temp stable for 48
hrs. The output of
the LM34 is consistent and linear. As I said, if I up
the supply of the LM34
to 5V, I get wildly varying ADC results. To make sure I
have made clear the
symptoms, here it is:
System details:
LM34DZ sensor, using its own regulated power supply (as
stable as the BOE or
better) and leads on a breadboard
LTC1298 ADC, mounted on and fed from the BOE
With 5V input to the LM34, I get a stable output within
2mV or so on the
output lead. The voltage is around 1.120V at 104F,
equaling 9.7mV/degree.
Hook this up to the ADC, and I get wildly varying
results. With 2.2V input
into the LM34, I get stable output within 2mV or so.
The voltage is around
830mV at 104F, equaling 8mV/degree. Hook this up to the
ADC, and I get nice
stable results, +/- 2 or so. The resistor 3k (I am
using a 3.3k) in series
with the LM34 output makes no difference.
Any idea what gives here? Obviously the LM34 doesn't
like running at 2.2V,
it isn't supposed to and the calibration is off (but
linear!). So I want to
run it on 5V, but I'm going to have to solve this
problem.
> Connect the positive red lead to
> 1) the power supply lead of the LM34?
> 2) the signal lead of the LM34?
> 3) the signal output lead of the LM34?
> Do the same thing with DC voltages, at the same
points, with the
> negative lead connected to the V- reference of the
ADC.
OK. Here are the results, this is with the system as it
is, running the LM34
with 2.2V
1) AC: 4mV, DC: 5V or 2.2V, for the PSU and voltage
divider feeding the
LM34, respectively
2) I'm not sure what you mean by the signal lead.
3) AC: 0, DC 830mV at 104 degreesF
Thanks for all the input!
Jonathan
[noparse][[/noparse]Non-text portions of this message have been removed]
I have little problems with stability with LM34 or
other like temp sensors.
Attached is a photo of my temp board. Makes for easy
assembly of temp
sensors. The whole circuit consists of a load on the
signal out 100K to ground.
A series 2.2K after the signal to ground load to
isolate the capacitance of the cable
as per the LM34 Datasheet, (did everyone download this
and read it).
The addition of a Tantalum capacitor on +5 to gnd at
the sensor.
At the ADC end I use a low pass filter to take out any
induced noise picked up by the
cable.
Proper installation of the ADC is essential, i.e.
follow the Datasheet recommendations.
Sincerely,
Ron Anderson
Digital Design Solutions, Inc.
Message: 15
Date: Sat, 23 Nov 2002 11:59:20 -0800
From: "Jonathan Peakall" <jpeakall@m...>
Subject: Re: was: Re: Tracy, math question: LM34s
>Tracy, Pat, All,
> >Here's the odd thing. When I input 5V to the LM34, I
get *very* erratic
> >readings off the ADC. Makes no difference what power
supply I use. When I
> >reduce the input to 2V using a voltage divider, I
get very stable
readings
> >with the bounce factor being around 3mV. I tried a
3.3k resistor in
series
> >with the LM34 output as Tracy suggested, but the
above condition
persists.
>
> Do you mean you are using the following circuit, but
with 6 volts
> replaced with 2 volts?
> ;---
> +6 volts
|
> cable 3 kohm |
> signal ---/\/\
| LM34
> common
|
> `---
> It won't work. The readings are stable because
nothing is happening.
> The absolute minimum supply voltage is 4.5 volts (5
volts per the
> data sheet). Did you check for AC voltages using
your VOM? Connect
> the ground black lead of the VOM to the V- reference
pin of your ADC.
Yes, just as shown above. It IS working with a supply
of 2.2V to the LM34,
quite well in fact, has kept the tub temp stable for 48
hrs. The output of
the LM34 is consistent and linear. As I said, if I up
the supply of the LM34
to 5V, I get wildly varying ADC results. To make sure I
have made clear the
symptoms, here it is:
System details:
LM34DZ sensor, using its own regulated power supply (as
stable as the BOE or
better) and leads on a breadboard
LTC1298 ADC, mounted on and fed from the BOE
With 5V input to the LM34, I get a stable output within
2mV or so on the
output lead. The voltage is around 1.120V at 104F,
equaling 9.7mV/degree.
Hook this up to the ADC, and I get wildly varying
results. With 2.2V input
into the LM34, I get stable output within 2mV or so.
The voltage is around
830mV at 104F, equaling 8mV/degree. Hook this up to the
ADC, and I get nice
stable results, +/- 2 or so. The resistor 3k (I am
using a 3.3k) in series
with the LM34 output makes no difference.
Any idea what gives here? Obviously the LM34 doesn't
like running at 2.2V,
it isn't supposed to and the calibration is off (but
linear!). So I want to
run it on 5V, but I'm going to have to solve this
problem.
> Connect the positive red lead to
> 1) the power supply lead of the LM34?
> 2) the signal lead of the LM34?
> 3) the signal output lead of the LM34?
> Do the same thing with DC voltages, at the same
points, with the
> negative lead connected to the V- reference of the
ADC.
OK. Here are the results, this is with the system as it
is, running the LM34
with 2.2V
1) AC: 4mV, DC: 5V or 2.2V, for the PSU and voltage
divider feeding the
LM34, respectively
2) I'm not sure what you mean by the signal lead.
3) AC: 0, DC 830mV at 104 degreesF
Thanks for all the input!
Jonathan
[noparse][[/noparse]Non-text portions of this message have been removed]