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ADS 1248 voltage input reducing

Amit Patil57
Prodigy 150 points
Other Parts Discussed in Thread: ADS1248, ADS1220

Requesting you to go through below query & reply on urgent basis:

 

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 as shown in below schematic:

 

 

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 (MARTEL MODEL 3001). 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?

  • 0
    TI__Guru* 92655 points
    Amit,


    It depends on how you have your input connected to the ADC, but it is likely that your input is still outside the ADC common-mode input range.

    As you mention, the input must be between:

    (AVSS + 0.1V+ (Vin)(Gain)/2) ≤ VCMI ≤ (AVDD - 0.1V - (Vin)(Gain)/2)

    However, this is for both inputs and the limitation is for both AINN and AINP. In reading through your post, it sounds like you have the AINN connected to the ground node, which is 0V and is below AVSS+0.1V.

    To be able to measure a 0V to 10V signal, you will need to both attenuate the signal and level shift it to get it with the common-mode input range.

    If you need to measure signals to the AVSS rail, the ADS1220 is able to do so by bypassing the input PGA.

    Let me know if this is the problem. If you have any other questions, please post back.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 150 points
    Joseph,
    For debugging above issue I tried.
    I'm giving input directly to ADC1248, up to range 0 to 4.5 (AVdd is 5v that's why I stopped at 4.5v). Then also ADC input measured is reduced. I have given differential input. Differential input impedance is not specified in data sheet. What is differential input impedance of ADS 1248?
  • 0 in reply to Amit Patil57
    TI__Guru* 92655 points
    Amit,


    We don't refer to the input impedance in the Electrical Characteristics table. It is shown in the datasheet in Table 7, which discusses the analog input current. Note the third column that states the equivalent input impedance.

    The input current (and equivalent input impedance) varies with the data rate. In reality, it is the sampling of charge at the modulator rate. The charge being periodically away looks like an equivalent impedance.

    Note that if the input violates the common-mode input range of the device, the input current may rise above the levels mentioned in the table. By over-ranging the PGA, you may pull extra current.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 92655 points
    Amit,


    Are you working with someone else with this question? The questions I have gotten are similar to those found in this post.

    e2e.ti.com/.../1771312

    In this post, I've just posted a more detailed answer to your first question.


    Joseph Wu