Hello everybody
I have a problem with ADS1248 circuit.
For measuring the temperature with thermocouple (TC) or RTD I use the below schematic. Note that in this design 4 channel is the same sensor (TC or RTD). At the beginning, when this circuit is powered on, the type of sensor is set by the operator settings.
The adjustments for ADS1248 is according to below tables.
TC |
|
|
|
RTD |
||
Burnout Current |
2µA |
|
|
|
Burnout Current |
0µA |
Vbias |
AIN(-) |
|
|
|
Vbias |
- |
CLKSTAT |
internal |
|
|
|
CLKSTAT |
internal |
VREFCON |
Internal reference is always on |
|
|
|
VREFCON |
Internal reference is always on |
REFSELT |
Onboard reference selected and internally connected to REF0 input pair |
|
|
|
REFSELT |
Onboard reference selected and internally connected to REF0 input pair |
PGA |
8 |
|
|
|
PGA |
4 |
Sample Rate |
10SPS |
|
|
|
Sample Rate |
10SPS |
DRDY MODE |
0 |
|
|
|
DRDY MODE |
0 |
IMAG |
- |
|
|
|
IMAG |
1500µA |
Cold Junction |
LM74 |
|
|
|
Cold Junction |
LM74 |
Also, I use a subroutine in the microcontroller to average the 4 last results for each channel. In every conversion, first I turn on the “BURNOUT CURRENT” then if I detect the sensor turn the burnout current off and measure the voltage on this channel again and if I doesn’t detect the sensor go to the next channel. Software algorithms are according to below flowchart.
For accurate calibration I use a “PSIP 714” temperature calibrator.
When I use RTD sensor, everything is good and I have no problem and the result doesn’t have any fluctuation. But when I use TC, the amount of fluctuation is large, in a noisy environment the variation is about ±5⁰C.
Now I have some question:
-
What do you think about the fluctuation, is the input filter design incorrect?
-
Is it correct to turn on and off Burnout current in any conversion? Does anybody have better ideas?
-
How often I should use “SYNC” command?
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Is it true to use system monitor for offset in every 4 conversion?