PGA280 Input Diagnostics

Is there anyone that might have some input on this feature of the PGA280?  I would really appreciate any assistance that can be provided.  Thanks!

Hi Justin,

One of my colleagues, Luis Chioye answered this question for another customer, here is his explanation:

The "wire break" function allows diagnosis of wire break or input disconnect.  This function is available to the INP1 channel and the INN1 channel.  During this test, the user must select inputs INP1 and INN1 on the device. 

During this test; depending on the sensor or circuit connected to the inputs of the device; the user may close switch F1 aor F2 or both to source or sink current.  The user could also close G1 or G2 to refer one of the inputs (positive or negative) to VSON which is typically connected to GND.  The mux allows for flexibility and the user can choose the appropiate configuration depending on the sensor connected at the inputs of the device.

Switch D12 is typically used to discharge any external capacitor that may have charged during the "wire break" test.

The 100uA current source will flow through the inputs and the path provided by the switches and produce an input voltage. This voltage will be proportional to the  impedance of the sensor, and the series parasitic resistance of the switches involved in the current path..

A possible example, if the user configures the mux as the circuit shown below closing G2, F1, A1, A2 on the diagram below, assuming VSON is connected to GND, the PGA280 will produce an output voltage:

output Voltage = ((600Ohm +Sensor Impedance +600Ohm ) *100uA)*GAIN 

Where the 600Ohms is the internal series impedance as shown on fig 44.

One thing to keep in mind is to set the PGA gain low enough so the PGA does not saturate.  For example; if the sensor path impedance is 100 ohms; the total impedance through the current path is 1300Ohm; this will produce an input voltage of 0.13V with the 100uA current; which at the higher gain settings will saturate the PGA and appear as an "open"

For example, at a gain of 128; this 0.13V input voltage will saturate the PGA since the device is limited to a suppy VSOP-VSON = 5.5V max

The series and switch on impedance is shown on figure 31 and figure 32 on page 14 as a function of temperature and common-mode voltage.

I hope this helps and I apologize that we didn't address this thread when it was originally posted in February.

John,

Thank you very much for finding this and passing it along to me.  It makes sense and I will try it out as soon as I can.  Do you have similar explanations on how to perform the shorted input and single-ended/differential input tests?

Thanks again,

Justin

Edit: It looks like the same test above might be used to detect shorted input as well.  In this case, there would be no sensor impedance, so the equation would be as such: 
output Voltage = ((600Ohm + 600Ohm ) *100uA)*GAIN or 120mV*GAIN.  Is this correct, or am I not looking at this the right way?  

John,

From the diagram it appears that the current source only attaches to the input A lines.  Is there any way to perform the wire break test for a sensor connected to input B?

Thanks,

Justin