connecting high impedance sensor to U3 analog input | LabJack
 

connecting high impedance sensor to U3 analog input

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Richard Goldberg
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connecting high impedance sensor to U3 analog input

I am connecting a high impedance sensor (force sensitive resistor) to the U3 AIN0 pin. The resistance of this sensor varies from 0-1MΩ. According to the datasheet, the max source impedance in this case should be 1k, so I would be exceeding that. What happens if I connect the sensor (in series with a 50k resistor in a voltage divider configuration) directly to the AIN0 pin? Could this damage the Labjack, or will it just result in an error?

I'm assuming that any error will vary according to the source reisstance, but how can I determine the expected error? I know that I could put a voltage follower in between the sensor and the Labjack, but I would prefer to avoid the need to power an op amp if the error would be small.

LabJack Support
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Is the variable resistance

Is the variable resistance what you are supposed to measure?  Does the sensor vary resistance from 0 to 1M and that is what you are supposed to measure?  Or does it generate a voltage somehow, and the change of resistance just happens to happen at the same time?

The basic way to handle a resistive sensor is with the voltage divider circuit shown in "Simple Resistive Sensors" here:

https://labjack.com/support/app-notes/typical-single-ended-AIN

As you have noted, the input impedance of the analog input will affect your measurement.  Rather than trying to model this effect, I suggest you do a calibration of your system, and might as well skip resistance and just develop a relationship of U3 voltage versus force.  Put on some known force values and note the voltages so you can develop a relationship ... not sure if it will be linear or not.

Another complication is that the voltage divider signal is proportional to the excitation voltage, so if you are using VS and it drops 5% the signal will also drop 5%.  VS is not a particularly stable voltage source so you would have to monitor the actual value of VS at all times.  Using a DAC output should be better.

Rather than using AIN0, you could use a low voltage analog input (e.g. FIO4) and enable LongSettling, as this is specified for source impedance up to 200k.  If you use DAC0 for exciation you can set it to a value that works well with the 0-2.4 volt input range.

 

The easiest solution would be the LJTick-Resistance:

https://labjack.com/support/datasheets/accessories/ljtick-resistance

 

Richard Goldberg
r.goldberg's picture
Thanks for your reply. I am

Thanks for your reply. I am using the Sparkfun resistive force sensor that you reference in the first link above. I just tried it using the DAC output and low voltage analog input. Compared to using VS output and a high voltage analog input, I got similar results using a variety of applied forces, so I think it's ok either way in my case. But I didn't realize that the DAC output is better at driving a load. That's good to know.  

 

LabJack Support
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VS has more drive capability,

VS has more drive capability, but the DAC outputs have lower noise and better stability.  There is some applicable discussion in section 1 of post 13 on the following forum topic where we are selecting an excitation source for a bridge circuit:

https://forums.labjack.com/index.php?s=278a8178d4a4f367486e8b4917920c4d&...

 

LabJack Support
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That sparkfun sensor does not

That sparkfun sensor does not generate any power itself, but rather is simply a variable resistor.  You can't damage anything with that sensor.

Exceeding the max source impedance spec causes error, not damage.  Damage is caused by exceeding voltage limits.