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Differential Voltage Measurement by PT 104

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Differential Voltage Measurement by PT 104

Postby Kate Q » Thu May 14, 2009 9:57 pm

Hello,

I have a PT 104 for voltage measurement. The voltage reading failed from early this week, the data shown by pico recorder is not the voltage across the Pin 2 and pin 3, which I measured using a multimeter.

I connected wires V- and V+ to the to pin 2 and 3 separately, and left Pin 1 and 4 unconnected. The vltage was from -0.3 to -2.2 V, and I scaled it to 0.3 0 2.2 V. At the beginning it gave smaller numbers than what I expected (i.e, when it should be 0.57, the reading was about 0.3), but still reasonable trend curves when the voltage changed. But from this week it seems not working at all. The voltage always slowly dropped from 2.8 V to 0 V after a connection, no matter what the real voltage is.

I wonder where the problem is. I checked the PT 104 by a PT 100 and the 4 channels all worked well.

Thanks a lot!
Kate Q
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Postby PeterF » Fri May 15, 2009 9:55 am

Hi,
You say you have connected pins 2 & 3 but left pins 1 & 4 unconnected. That is your problem. You must have pin 4 connected to the same ground as your voltage source to give a reference to the voltage input pins. The measured voltage is the difference between the voltages at pins 2 & 3 but to measure these two voltages you need a ground reference. Without a ground reference the common mode voltage is undefined and can drift out of the maximum common mode voltage range for the PT-104 causing the effect you see.
Regards,
PeterF.
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Postby Kate Q » Fri May 15, 2009 2:47 pm

[quote="PeterF"]Hi,
You say you have connected pins 2 & 3 but left pins 1 & 4 unconnected. That is your problem. You must have pin 4 connected to the same ground as your voltage source to give a reference to the voltage input pins. The measured voltage is the difference between the voltages at pins 2 & 3 but to measure these two voltages you need a ground reference. Without a ground reference the common mode voltage is undefined and can drift out of the maximum common mode voltage range for the PT-104 causing the effect you see.
Regards,
PeterF.[/
quote]

Thank you. The voltage is across a resistor and it doesn't have a ground reference. What shall I do in this case?
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Postby PeterF » Mon May 18, 2009 9:53 am

Hi,
What is the circuit you have the resistor in? If either end of the resistor is connected to ground then connect the PT-104 ground to that as well. If neither end of the resistor is at ground potential then you must ensure that you do not exceed +/-2.5V at either end of the resistor with respect to ground. (Join the grounds of the PT-104 and the external circuit).
Note on Differential Voltage Mode:
The maximum input voltage range of the PT-104 is 2.5 V. Any voltage in excess of +/- 30 V on any input pin may cause permanent damage to the unit.
In Differential Voltage Mode, the input connector should be treated as a differential input with reference to ground. Both inputs (V+ and V-) must be zero volts or above (it does not matter which input has the higher voltage) and must remain within the input range. A ground reference connection is also required for correct operation. The ground connection of each MiniDIN socket consists of a 100 W resistor to mains earth/ground via the serial cable outer braiding and the PC chassis.
Regards,
PeterF.
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Postby Kate Q » Thu May 21, 2009 9:01 pm

Hi Peter,

Neither end of my resistor has a ground potential. So I grounded Pin 4 but the problem still exists. We tested all the external circuit and it's all good. The differential voltage between pin 2 and 3 is reasonable, just this device cannot read it correctly.
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Postby PeterF » Fri May 22, 2009 7:40 am

Hi Kate,
Using a digital voltmeter, measure the voltage between one end of the resistor and pin 4. Then measure the voltage between the other end and pin 4. Both these voltages must be less than +/-2.5V for the PT-104 to stay in it's common mode voltage range. The two voltages must differ by the amount you would expect to see across the resistor. If both are 0V, then there is no connection between the ground potentials of the external circuit and the PT-104, which is necessary. Please check the PT-104 input again using the PT-100 probe. If that does not work, the input has been damaged and you will need to contact us at support@picotech.com for a returns number for repair.
Regards,
PeterF.
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