SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
Figure 4-9 - Back plane connections to the photometer LIA cards. The spectrometer LIA boards are conceptually
indentical.
LIA Amplifiers: The Lock-in Amplifiers are used to read out the AC signals from the detectors and convert
them into DC. Each bolometer has a dedicated amplifier. The amplifiers are grouped together in groups of
32 on each LIA board. Figure 4-9 shows the interconnection between the LIA boards on the motherboard.
The analogue output from each of the amplifiers is placed on a bus connected to the DAQ + I/F board.
Each individual amplifier has;
(i) a differential input pre-amplifer circuit that uses Analog Devices OP-400 operational amplifier;
(ii) an Intersil, DG303A digital switch. This takes the pre-amplified AC signal from the bolometer and
full wave rectifies it using the signal from the bias generators. The switching is signal is phase
commandable to compensate for any signal phase change that occurs between the detector and the
digital switch in order to maximize the output from the switch;
(iii) an active post low pass filtering circuit that removes the bias frequency and bias harmonic
components from the rectified signal. The low pass filter circuit uses two Analog Devices OP-400
operational amplifiers.
These three components are illustrated in Figure 3-26. There are a total of 12 LIA boards; with nine being
used for the photometer and three used for the spectrometer.
The DAQ + I/F boards: There are a total of
In order to limit the mechanical properties of this unit it is now divided into two boxes which are
respectively the DCU and the FCU including the MCU, the SCU and PSU.
The SCU function is manifold: it is in charge of interfacing FPU sub-systems such as the
cooler heaters, the calibrators and the thermometry sensors plus housekeeping parameters
with the DPU through digital interfaces. This function covers the sensors biasing, the signal
amplification and the digitisation of the Analog parameters. The SCU is also in charge of
providing all the sub-systems with secondary power supply lines from the S/C 28 V interface
by means of the PSU (a set of DC/DC converters). This PSU is associated to a supply current
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