This morning, we continued with the investigation about the noise in the output of the DACs in the EEroom, 8 channels DAC, see logbook entry 32713.
We tried at first to measure it with a LAPP ADC, see figure 1 where for the DAC output a temporary signal has been used (INJ_IMC_QF_I_H_TEMP).
We were not able to see the auto-oscillation described in 32713, and it can be due to the low pass filter present in the ADC signals together with the (very) high frequency of this oscillation (~100kHz).
If instead we measure the signal with an oscilloscope or a spectrum analyzer we can see clearly the oscillation.
See figure 2 and 3 where a 200 Hz sinusoidal signal with 0.3 and 0.5 V of amplitude generated by the DAC is shown.
In order to understand if it is due to the fact that the oscilloscope is single ended while the DAC output is differential we also measure it with a spectrum analyzer with differential input.
As it is visible in figure 4 the auto-oscillation (~100kHz, ~2Vpp) is still present even with a differential input.
This behavior is present in many channels, so it seems is not a failure of a single DAC channel.
This DAC can not be then used to drive the Pstab low frequency loop, it is not safe for the laser amplifier electronics, and may be it would be better to do not use even for the Galvo controls.