AD592 and 590 both jumpy... Why?
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I have the Board of Education and I'm using the AD592/cap/resistor
that came with the package. I find the temperature reading jumps
around a lot - +-2 degrees. I thought perhaps I'd gotten a bad one
so I bought another (need 6 temperature sensors eventually for one
project). It has the same jumpiness.
I have it wired this way:
+5 to the + pin
- pin to ground through a 0.22uF cap (actual .229)
-pin to stamp through a 100ohm resistor (actual 98.3)
I tried two other .22 caps just in case, same behavior.
I also bought and tried an AD590, same thing again! Have I missed
something or is this normal for AD590/592s?
that came with the package. I find the temperature reading jumps
around a lot - +-2 degrees. I thought perhaps I'd gotten a bad one
so I bought another (need 6 temperature sensors eventually for one
project). It has the same jumpiness.
I have it wired this way:
+5 to the + pin
- pin to ground through a 0.22uF cap (actual .229)
-pin to stamp through a 100ohm resistor (actual 98.3)
I tried two other .22 caps just in case, same behavior.
I also bought and tried an AD590, same thing again! Have I missed
something or is this normal for AD590/592s?
Comments
>that came with the package. I find the temperature reading jumps
>around a lot - +-2 degrees. I thought perhaps I'd gotten a bad one
>so I bought another (need 6 temperature sensors eventually for one
>project). It has the same jumpiness.
>
>I have it wired this way:
>
>+5 to the + pin
>- pin to ground through a 0.22uF cap (actual .229)
>-pin to stamp through a 100ohm resistor (actual 98.3)
>
>I tried two other .22 caps just in case, same behavior.
>
>I also bought and tried an AD590, same thing again! Have I missed
>something or is this normal for AD590/592s?
The AD590 and AD592 are themselves quite stable. The RCtime circuit
is quite subject to noise in the form of "bounce" on the ground line.
The bounce comes from the digital operation of the Basic Stamp
itself, along with other digital and power circuits you may have
attached to your BOE.
Here is the "ground loop":
On the BOE, the ground path you can see and trace on the top of
circuit board starts at the center terminal of the LM2940 voltage
regulator, over to the (-) terminal of the input connector, and just
beyond that it forks, with one branch off to the power led and the
BS2 pin 23, and and the other branch off all the way around the edge
of the circuit board, connecting to the mounting holes at the
corners. In that loop around the edge it connects to the RS232 DB9
pin 5, the Vss terminal on the expansion connector, and finally near
the very end it arrives at the Vss terminal next to the white block
prototyping area.
Any load currents flowing in that ground loop around the edge of the
board or voltages induce in it are essentially in series with the
0.22uf capacitor. As the ground loop voltage fluctuates, you will
see jumping around in your temperature reading.
Any additional wires from the 0.22uf capacitor over to the Vss
terminal can also contribute to this effect. For example, if you
have a wire from the Vss terminal over to an led, and from there over
to the ground side of the 0.22uF capacitor, that will have an
enormous effect as the current turns on and off in led. If you do
have loads like that on the BOE, it is best to have them turned off
when you do the RCtime measurement.
The ideal would be to have the ground wire on the 0.22uf capacitor
connected directly to pin 23 on the Stamp, or as close to it as
possible. You could do that with an OEM approach.
I think you should be able to achieve +/- 1 degree Celsius on the
BOE, and you could do averaging to get rid of some of the noise.
However, RCtime is not the best for high accuracy or long term
stability. What accuracy do you require?
-- best regards
Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
mailto:tracy@e...
the ad592 and shortened some of the jumpers (was messy anyway). Now
the temp is nice and solid!
As an aside, I'd like to mention that you're website is very
informative. I keep referencing it almost every day.
--- In basicstamps@y..., Tracy Allen <tracy@e...> wrote:
> I think you should be able to achieve +/- 1 degree Celsius on the
> BOE, and you could do averaging to get rid of some of the noise.
> However, RCtime is not the best for high accuracy or long term
> stability. What accuracy do you require?
>
> -- best regards
> Tracy Allen
> electronically monitored ecosystems
> http://www.emesystems.com
> mailto:tracy@e...