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).
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?
Fig. 1 : lower signal = supply voltage (2V) ; upper signal = DO 2
all DO deactivated
Fig. 2 : lower signal = supply voltage (2V) ; upper signal = DO 2
DO 2 activated, all other DO deactivated
Fig. 3 : lower signal = supply voltage (2V) ; upper signal = DO 2
DO 2 activated, all other DO activated
Fig. 4 : lower signal = supply voltage (2V) ; upper signal = DO 2
DO 2 deactivated, all other DO activated
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.