Dither

Each channel of the NI-9470 has a dither engine that allows you to configure amplitude and frequency per channel. The dither engine generates a triangle wave with a zero DC average superimposed on the output setpoint depending on the channel output mode.

Because you can configure the NI-9470 features in a wide range of flexibility, you may encounter cases where the actual dither output on PWM and corresponding average current do not align with the configuration you desire. The following diagram illustrates two general non-ideal cases that may occur independently or simultaneously. 

The ideal case for dither is a triangle waveform with the desired peak amplitude matching the configured amplitude and superimposed on the output setpoint value. In both output modes, if the dither amplitude is too large, or the output setpoint value is too close to bounds, then it results in the PWM output railing at 0% or 100% duty cycle. If the overall ideal dither waveform is truncated, then a non-zero DC offset will be present on the output setpoint and corresponding average current measurement.

In Average Current mode, the PID response will impact dither because PID typically acts like a low-pass filter that zeroes the DC error between the average current output setpoint and readback. The superimposed dither waveform may experience reduced amplitude or will be filtered out if the dither frequency is too high, or the PID response is too slow.

In Duty Cycle mode, be aware that implementing an external average current PID control loop in LabVIEW FPGA will generally nullify the NI-9470 dither that is applied on the duty cycle output setpoint because the PID will normally regulate average current to a setpoint and not allow current variations caused by dither on the PWM duty cycle.