NI ELVISmx Bode Analyzer Phase Incorrect at Lower Frequencies

Issue Details

I have a high-pass filter circuit for which I am using the Bode Analyzer with a myDAQ or NI ELVIS to create Bode plots. When I run the Bode Analyzer, my phase plot is incorrect over a range of lower frequencies. Why is this happening and how do I obtain the correct Bode Plot?

Solution

To correct this behavior, lower the source impedance of the circuit being measured, as described in Eliminate Ghosting on Adjacent Input Channels by Decreasing Source Impedance.

For example, a simple RC high-pass filter with a cutoff frequency of approximately 1600 Hz might consist of a 10 kΩ resistor and a 0.01 µF capacitor, as used to generate the image at top as the figure shown below. The resulting impedance would be the 10 kΩ resistor value, much higher than the low 1 Ω impedance of the myDAQ or NI ELVIS Analog Output . If a 100 Ω resistor and a 1 µF capacitor are substituted to create an equivalent filter with an impedance of 100 Ω. With this modification the correct Bode Plot will be produced, as shown in the image directly below.

Additional Information

This behavior, mainly visible in the phase graph, is due to the fact that both the myDAQ and NI ELVIS use multiplexed Analog Inputs, with the internal Analog to Digital Converter (ADC) being rapidly switched between two sources.

When you use the Bode Analyzer with a high-pass filter, one of the Analog Input channels is connected directly to the Analog Output channel of the device and the other Analog Input is connected to the capacitor, which impedance will be large at low frequencies and low a high frequencies.

When rapidly switching between a high impedance and a low impedance input, the result is distortion of the measured signal, with a greater differences between the signal amplitudes resulting in greater distortion. Since impedance and phase shift will decrease as frequency increases, this behavior is only seen at low frequencies.
 

This distortion does not seem to affect low-pass filters because at very low frequencies, the phase of a low-pass filter will be very close to zero, so this behavior will be masked.

Using a source impedance greater than 1 kΩ is not recommended when using any device with multiplexed Analog Inputs.