J. Kissel, P. Thomas, D. Sigg

Patrick grabbed ten seconds of data from the following Beckhoff ADCs with three different input signals: +/- terminals shorted, +/- terminals connected to a 9 [V] battery, and +/- terminals fed with a 1 [Vrms] amplitude, 10 [Hz] sine wave. 
EL 3004, billed as a 12-bit ADC, "presented as 16-bits" datasheet.
EL 3104, billed as a 16-bit ADC datasheet.
EL 3602, billed as a 24-bit ADC datasheet.

The messages:
- Datasheets are deceiving and misleading. So are the default settings. So is what it means to be a 12-bit or 24-bit ADC, since there's no guarantee that all bits are significant.
- The calibration for these ADCs, regardless of what the ADCs are billed as, is 20 / 2^16 [Vpp / ct] for EL3004 and EL3104, and is 20 / 2^32 [Vpp / ct]
- The noise performance of the EL 3104 16-bit ADC makes the most sense, and is a consistent 30 [uV/rtHz] regardless of input.
- The noise performance of the EL 3004 12-bit ADC is confusing, but makes sense if you consider it to be a 12 bit internal ADC chip running at 50 [Hz] whose [least significant / "last"] bit is not random, sampled by the internal processor at 1 [ms] padded to produce 16 bits. If we treat the injected sine wave's noise performance as the expected noise, 200 [uV/rtHz].
- The noise performance of the EL 3602 24-bit ADC (two channels of which were measured) is non-sense. Further investigation is required.
- The noise performance of the EL 3104 looks like it would be just about good enough to handle the pressure sensor signals as they stand in the HEPI Pump Servo (recall LHO aLOG 16500).
- We should consider looking into some Beckhoff PID modules as well, such that the computation and control is all done inside the Beckhoff system, and EPICs is merely used as a monitor.

We're (as in Patrick) going to get a longer data on Monday set so we can better assess the low-frequency performance, and we'll focus on why the time series of the EL 3602 seems to be pre-averaging the data, causing a huge increase in noise when an AC signal is driven through it.

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Data, plots, and analysis script live in
/ligo/svncommon/SeiSVN/seismic/Common/Documents/T1500159_BeckhoffADCNoise/