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trouble with NI 9474 digital output

Hello,

 

I have some trouble with a NI 9474 module.

I'm generating with LabView FPGA a rectangular signal at 40 kHz frequency (30% duty cycle). This signal is sent to one or several digital outputs of the NI 9474, which are connected to miniature loudspeakers (impedance 8 Ohm).

 

circuit.JPG

 

I observe several strange things :

 

- the voltage measured between a digital output (e.g. DO 2) and ground (COM) drops to negative values when transition from 1 to 0 occurs (this does not happens when I connect the output to a resistor (~ 50 Ohm) instead of a loudspeaker (Fig 2)

- the supply voltage falls down during the 1 state ; the more digital outputs are activated, the less voltage is supplied during the 1 state (Fig 3)

- even when not activated, a digital output shows voltage variations when other outputs are activated (Fig 4)

 

Could someone explain me such a behavior?

 

Thank you,

 

Fred

 

 

Fig. 1 : lower signal = supply voltage (2V) ; upper signal = DO 2

all DO deactivated

0.JPG

 

 

Fig. 2 : lower signal = supply voltage (2V) ; upper signal = DO 2

DO 2 activated, all other DO deactivated

1.jpeg

 

 

Fig. 3 : lower signal = supply voltage (2V) ; upper signal = DO 2

DO 2 activated, all other DO activated

2.jpeg

 

 

Fig. 4 : lower signal = supply voltage (2V) ; upper signal = DO 2

DO 2 deactivated, all other DO activated

3.jpeg

 

 

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Message 1 of 3
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I think you need a power amp between your outputs and our loudspeakers to match the impedance.

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Message 2 of 3
(4,770 Views)

Coil in speaker is an inductive load like a relay coil.  When you de-energize the coil the field collapses and generates the negative pulse.  You will need to clamp this pulse or you will damage your DO's.  A good overall solution would be to use a ULN2003 driver chip that has the clamping diodes included.

Message 3 of 3
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