J. Kissel, H. Radkins,

In further pursuit of HEPI Pump Servo Noise -- found to be coherent with the IFO at low-frequencies (see G1500099, and LHO aLOGs 16386, 16342, and 16239) -- Hugh and I briefly terminated the pressure sensor inputs to the HEPI pump servo (see times in LHO aLOG 16419), in attempt to measure the ADC noise of the PC/104, Athena II, 16-bit ADC (see Diamond System's product website for data sheet and user manual). 

Sadly, there appears to be no difference between the channels when the sensors are plugged in and when the sensors are terminated.

What's confusing about this, is that we know the pressure preamp (D1101839, which adds a gain of 151 [V_diff/V_diff] to each supply and return sensor) and the input instrumentation amplifier inside the servo (pg 2 of D0901559, adding a gain of 2 [V_se / V_diff] for a total of 300 [V_se / V_diff]) are functional and calibrated correctly in the EPICs database. If that weren't true, the DC signal on each sensor's EPICs readback would not match the dial gauges on the system.

Further, as mentioned on pg 13 of G1500099, if the pressure sensor's calibration is 0.3 [mV/PSI] (0 to 100 [mV] over a 0 to 300 [PSI] range), typical supply pressure is 70-80 [PSI], then the pre-amplified, instrumentation-amplified, supply pressure signal reaching the Pump Servo ADC should be roughly 6.3-7.2 [V] single-ended, using plenty of the range of the 0-10 [V] ADC range. On the other hand, the return pressure, typically ~5 [PSI], would arrive at the ADC as only 0.45 [V]. Small, but still well above the 10 / 2^16 = 1.5e-4 [V/ct] resolution.

So, the DC signals add up and agree with dial gauges, the signals should fit well within the ADC's range well above it's bit resolution, and responds appropriately to little steps in the control pressure. So why don't we get a change in ASD between terminated and connected?

We'll continue to investigate tomorrow morning. 

I attach three things:
(1) 2015-02-02_H1HPI_PumpServo_ADCNoiseCharacterization.pdf 
The pudding. Amplitude spectral densities of the Supply Pressure (H1:HPI-PUMP_L0_BSC2SUP_PRESS), Return Pressure (H1:HPI-PUMP_L0_BSC2RET_PRESS), and the calculated Differential Pressure (H1:HPI-PUMP_L0_DIFF_PRESSURE) for three different times. RED is with the pump servo closed (the current, nominal running state), BLUE is the with the pump servo open, but the sensors still connected, and MAGENTA is with the pump servo open and the sensor inputs terminated. As you can see -- no change between BLUE and MAGENTA.

(2) 2015-02-02_HPI_PumpServo_Pics_A.pdf 
A collection of pictures of the HEPI Pump system, as built at LHO.

(3) 2015-02-02_HPI_PumpServo_Pics_B.pdf 
More pictures of the HEPI Pump system, including pictures of the terminating plug used and how / where it was connected (starting on pg 9).

Following the pin out shown in D0901559, where (on pg 1, upper left corner) connectors J1A and J1B are shown to bring in the field Supply and Return pressure signals, I crafted two, dummy, DB9, pressure sensor terminators by connecting the + and - legs (pins 4 and 6) of the differential input with a 100 [ohm] resistive load. All other pins (i.e. the +10 [V] supply voltage to the pressure sensor, and the +/- 15 [V] supply voltage to the pressure sensor preamp) were left open. Again, see pgs 9 and 10 of 2015-02-02_HPI_PumpServo_Pics_B.pdf.