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PGA308: Fault detect function questions

Prodigy 40 points

Replies: 5

Views: 156

Part Number: PGA308

Hello,

I have a question about the PGA308 fault detect function.

we want to detect the sensor fault like the  'Excitation pin open circuit' , 'VIN1 open circuit'  'VIN1 and Excitation short' ...

after setting the CFG1(0111010001011111), we found the output signal is 4.74V(is equal the over scale limite voltage i set ) when VIN1 open circuit,

and the output siganl is 0.28V (is equal the under scale limte voltage i set)when VIN2 open circuit, the output is not correct .

it should output 0V. and we confused why the output is 4.74V and 0.28V.

for my current circuit ,the VIN1 and VIN2 is floating when VIN is open circuit, does it is the reason of the issue?

Do we must need connect the VIN to GND via big value resistor? then can set the VIN to GND when it is open circuit.

otherwise the PGA can't detect the VIN open circuit?

thank you in advance!

  • Hi Roy,

    If fault detection of floating inputs needs to be detected, it is necessary to enable pull-up current sources (IPU1 and IPU2) to each input (VIN1 and VIN2) as shown in Figure 3-12 of the PGA308 User Guide because of the low input bias currents of the PGA308. These 30nA pull-up current sources are designed to be large enough to overcome ESD cell leakages and force the external fault comparators into a fault-detected state for open PGA308 inputs.  The FLT IPU bit (CFG1 Register [D14]) enables IPU1 and IPU2 with a logic '0', and disables them with a logic '1'.

    On your current CFG1 register settings, it appears D14 = '1' where the Pull-Up currents are disabled.  Can you please set D14='0' to enable the current sources?

    If possible, please post an schematic showing your supply, sensor and VREF connections, along with the corresponding bypass capacitors.

    Thank you and Regards,

    Luis

  • In reply to Luis Chioye:

    Hi Luis,

    It works after set D14 from  1 to 0.

    Thank you very much for your kindly reply!

    Roy

  • In reply to Luis Chioye:

    Hello Luis,

    Now I can set the output of the fault to 0V or Vs , but I also see the datasheet description that the output of the fault can be set to 100mV.

    is it possible to set the output of the fault to 100mV? I see only two options for choose, Vs or Ground.

    Could please help check this issue? Thank you!

    Roy

  • In reply to Roy Jia:

    HI Roy,

    The PGA308 does not offer the option to program the output to a particular voltage during a fault condition. The PGA308 output can be  programmed to be either driven high (close to supply VS or VCLAMP), or driven low (close to GND) on a fault condition.

    Vout will reach a voltage between (+Vs) < Vout  < (Vs-100mV) when programmed to go high.  Alternatively, Vout will attempt to reach a voltage between (GND) < Vout < (GND+100mV) when programmed to go low.  In other words, the PGA308 output stage is attempting to reach the positive rail supply (VS) or negative rail supply GND, however the PGA308 output voltage swing is limited by the the Output Voltage Range specification below, depending on the load condition that is present at the output. 

    Thank you and Best Regards,

    Luis Chioye

  • In reply to Luis Chioye:

    Hello Luis,

    I think I understand the fault function of the PGA308 after you detailed explanations,

    Thank you very much for you kindly reply!

    Best regards

    Roy

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