Getting a offset in module 9411

[D_1]

Hei everybody,

I am using a cRIO with module 9411, also using a sensor which uses PWM. The output of sensor is connected to the NI module 9411. (My input type is singel ended and I am sinking the sensor ouput) I have supplys one for the sensor 24V and one for the module 9411 5V. I know the module 9411 can be supplied with 24 V, but I am not using it, because I will connect other devices which supports around 5V. So, I created a simple voltage divider in order to lower the signal to 5V. I have a comon ground (connected the both grounds from 5 abd 24 volts supply).

Here is the datasheet from 9411: http://www.ni.com/pdf/manuals/373506e.pdf

Here is the thing I do not understand:

When I connect the circuit to the module I get an offset of 0.9V. This means that module is always HIGH, because the range of the low input is from 0 to 0.8V and because of that I don't see any pulses on Labview. So, I thought I need to additonaly sink the curerent to get rid of offset. Added  pull-down resistors with diffent resistance (from the 10 to 10k Ohms), but the offset was still there. 

However, If do not connect the circuit to the  module then signal is fine, no offset. I even made a circuit in painter 🙂 ( at the moment not uisng my PC).  

So, what am I missing and what kind of circuit I need? thanks in advantage

[Zody]

Hi D_1,

Are you sure the value is 0.9? How did you measure it?

As from specification, when you connect the signal as Single Ended, if it is 0-0.8 - it is Low Logic and if it is 2 - 24 V - it is High Logic.

I believe that in the whole equation of the voltage divider, you forgot the input impedance of the module (according to the specification is 8.4k Ohms), which basically is connected parallel to the 10k Ohms from your circuit. 

But the result is still valid, it means that when you connect 24V to the circuit, you will have 2.4 V as input to module (which is High logic).

Please give us more data about how did you measure the 0.9 V

Best regards,

IR

[D_1]

Hei Zody, sorry for my late response. 

I am preaty sure that the offset is 0.9 V, because I measured it couple of times with osciloscope. Okey I didn't know that input impedance is 8.4k ohms, probely overlooked it, bu good to know for the furture.

In the end we solved it by chacing the input type form single ended to diferencial and it VI code worked properly after that. 

Thanks for the help.