Hi ,
I have connected LM35DZ output to ADS1248. Now I want formula to convert ADS1248 o/p to temperature which is sense by LM35DZ.
Can anybody provide Conversion formula from ADS1248 O/p to Temperature?
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Hi joseph,
Thanks for your reply..
Kindly help with below query of ours.
1. The ADC Common-mode input range formula given in ADC datasheet is applicable for the input of ADC (before PGA) (OR) After PGA (at the input of Delta-Sigma modulator inside the ADC)?
2. As we are giving unipolar supply to ADC, For negative input ranges we are activating VBIAS on –ve input terminal of ADC. Is it Ok or Not?
3. LM35DZ sensor is lead sense or body sense?( According to the datasheet it was mentioned that it is lead sense)
4. If we are giving 0.09V as input to ADC in 0 to 10V range whether the ADC will process it or Not? ( Check this according to the Common-mode input range formula?
Regards,
Yash Dave
Hi Joseph,
Requesting you to go through below query.
ADC input in the range of 0 to 10v is to be applied to ADS1248.
AVDD, DVDD supply is 5V only. AVSS is connected to GND. Hence according to
(AVSS + 0.1V+ (Vin)(Gain)/2) ≤VCMI ≤ (AVDD - 0.1V - (Vin)(Gain)/2)
Input signal has to be attenuated.
Hence I have used attenuator network of 1M & 100K for this .
Reference used for this Configuration is Internal reference (2.048v).
Internal PGA gain is 2.
Sampling rate =2SPS
For 10V input at Vin+ & Vin-, Expected reading are as below:
Input voltage |
Attenuator factor |
Output of attenuator |
PGA gain |
Input at ADC Modulator |
Count Expected (in Decimal) |
Count Expected (in HEX) |
8.9934 (9V Battery) |
0.0909 |
0.909 |
2.00000 |
1.6363 |
6699529 |
663A09 |
10 |
0.0909 |
0.909 |
2.00000 |
1.8181 |
7443921 |
7195D1 |
But Problem is I am applying 10V diifernetial input at Vin+ & vin-, ADC input is applied through Calibrator. Output measured at attenuator is too low. Hence count measured is Low as mentioned in below table.
Input voltage |
Attenuator factor |
Output of attenuator |
PGA gain |
Input at ADC Modulator |
Count observed (in Decimal) |
Count Expected (in HEX) |
8.9934 (9V Battery) |
0.0909 |
0.5814 |
2.00000 |
? |
4510837 |
44D475 |
10 |
0.0909 |
0.1938 |
2.00000 |
? |
967826 |
EC492 |
what cause for voltage observed at AN0 & AN1 is less and also ADC count is less?
Regards,
Yash
Yash,
This is the exact same question as this following post. Are you two working together?
I'll be a bit more detailed in my answer for this just in case if comes up another time. In this case, the input is outside common-mode input range of the PGA. I'm not sure about all of the setup you're describing, but I'll show a diagram to explain what I mean.
First, you describe using a voltage divider for the 0V to 10V input. Is this the way this is set up?
If this is the setup, then the input is outside the common-mode input range. Note the AINN input. If AINN is connected to ground and AVSS, then this input is outside the input range.
(AVSS+0.1V+(Vin)(Gain)/2) ≤ VCMI ≤ (AVDD-0.1V-(Vin)(Gain)/2)
Assuming AVSS=0V, Vin=0.909V and Gain=2, you get the following if you replace the values in the above equation.
1.009 ≤ VCMI ≤ 3.991
With the input at 0 to 0.909, the common-mode voltage is 0.909V/2 = 0.4545V so the common-mode input voltage is not inside the input range.
The common-mode input range is limited by the input and output range of the PGA. Let's look at a diagram of the PGA. It is shown below.
If you look at the PGA, the amplifiers will still be limited by the supplies going from AVSS to AVDD. Again, lets assume that the inputs are AINN=0V and AINP=0.909V and the PGA is set to a gain of 2.
This diagram shows two things that will be wrong with these input voltages. First, the AINN is too low. Even in a gain of 1, the input must be above 0.1V for the input range of A2. Second, the common-mode is too low because the output of A2 cannot go below the negative rail. The output A2 will be limited to maybe 100mV above AVSS.
So using the voltage divider is not going to work when the negative input is tied to ground (or really AVSS for this example).
There is still a small discrepancy for the second case where the input is driven by the DC calibrator. I'm not sure why this gives you a different result, but I suspect it is the same problem. It depends on the exact setup, but the it could be the calibrator has a different DC offset point, and that it still violates the common-mode range.
The common-mode input range is also addressed in a different post listed below. There is also a presentation that gives a couple of examples of how this is limited based on the input PGA limitations. You can find it as an attachment to one of the posts.
I'd read through this post, and the presentation. If you have any more questions, I would also include a diagram of your setup. It's important to note exactly what voltages are seen by AINN and AINP separately.
Joseph Wu