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ADS131M06: ADS131M06 Analog Front-End (Current)

Part Number: ADS131M06
Other Parts Discussed in Thread: ALLIGATOR

Hello

See the figures bellow:

Figure 1: My circuit project - analog Front-End (Current).

Figure 2: Texas project - - analog Front-End (Current).

Figure 3: ADS131M06 6-Channel.

In Figure 1, in figure 1 is the circuit I implemented for current measurement, the output of this circuit is connected to an ADS131M06 6-Channel (Figure 3). 

The components in figure 1 are the same as in figure 2 (texas circuit).

However, I'm having some problems.
Figure 4 represents the signal on capacitor C24 of Figure 1. Figure 5 represents the signal on capacitor C25 of Figure 1.
It looks like everything is ok. But the signal on capacitor C30 (Figure 6) is not correct. What could be the possible problem?
 
Figure 4: channel 1: input current; channel 2: voltage on C24.


Figure 5: channel 1: input current; channel 2: voltage on C25.


Figure 6: channel 1: input current; channel 2: voltage on C30.

Thank you.
 
 



  • Hello,

    First, thank you for all of the good information.  It makes it easier to debug the situation.  Would you comment on the ADC measurement accuracy of the differential C30measurement?  I ask because I think this could be a scope GND'ing issue as I don't think a differential active probe is being used.  The GND (alligator clip) on the oscilloscope GND is connected to the Earth GND potential through the scope's eventual connection back to the line voltage input.  

    The voltage on the negative side of C30 is not connected to Earth GND, but by connecting the scope GND there you're effectively shorting that node to earth GND, which may have some potential differences to the AGND signal being used.  Shorting out L7 may help with the measurement, but since it looks like more of a measurement issue w/ the scope than an actual system issue here's a few recommendations:

    1.) Repeat the measurement with a true differential scope probe

    2.) Use two scope channels for both sides of the C30 capacitor and then use the MATH functions in the scope to subtract the two. 

  • Hello Collin,

    First, thank you for your feedback.

    I disconnected the oscilloscope ground from the electrical grid ground. Could this be the problem?

    Thank you.

  • I'm pretty sure it's a grounding problem, although it's hard to determine exactly what the challenge may be.  I've tried to use "Cheater" plugs (don't tell my lab manager Slight smile )in the past to get around this unsuccessfully.  A fully AC isolated Rheostat that powered the oscilloscope w/ a floating GND pin was the best option in the past, but understand these aren't always the easiest to come by.

    Easiest recommendation is going to be to use two channels and then use the Math to subtract the two.  Or possibly use a multi-meter with an AC amplitude measurement as both terminals of multi-meters are usually truly floating from Earth. 

  • Hello Collin,

    See this figure bellow

    Channel 1: current input;

    Channel 2: voltage on the C24 capacitor;

    Channel 3: voltage on the C25 capacitor;

    The math operation is C24 voltage minus C25 voltage. 

    See that the result of the math operation is equal to the input current.

    This is an excellent result. Right? Smiley

     

  • Looks great!  Glad it worked out.