In this section we introduce the elements related to the ISOPHOT signal flow. For the understanding of the instrument output related to an astronomical observation, systems on both the cold and warm side of the cryostat are of importance, see also Fig. 3.1:

**Figure 3.1:** Main systems for data collection
$\begin{figure} \begin{center} \begin{tabular} {c} {\epsfxsize=8cm \epsfbox{sig_flow.eps}} \\ \end{tabular}\end{center} \end{figure}$

The function of the Cold readout Electronics (CRE) unit is to pre-amplify and sample the photo-currents generated in the detector. To reach the maximum efficiency, the CRE is integrated on the chip containing the detector. In Fig 3.2, a diagram is presented of the cold electronics chain.

**Figure 3.2:** Schematic diagram showing the detector - CRE amplifier chain, the capacitance C_F is also denoted by C_int in this document.
$\begin{figure} \begin{center} \begin{tabular} {c} {\epsfxsize=10cm \epsfbox{cre_fig.eps}} \\ \end{tabular}\end{center}\end{figure}$

The Detector/CRE chain in Fig 3.2 represents an integrating amplifier with capacitive feedback. Each detector pixel is connected to one input channel of a CRE. In case the detector is receiving IR photons from an astronomical or internal source, the voltage V at the output of the CRE will increase as a function of time. The voltage increase is proportional to the current through the detector which is in turn proportional to the number of photons falling on the detector. In the remainder of this manual this voltage increase or slope (in V/s) will be referred to as photo-current or signal. For the observer the signal is a fundamental concept: once signals from calibration standards have been measured, it is possible to relate the signal to a power from the source in the filterband.

After the amplifier stage there is a sample & hold (S&H) stage. By applying a sample pulse the voltage will be clamped and while the front-end continues to integrate, the clamped signal is read out through a multiplexer (MUX). In that way the readout does not disturb the integration.

Since the output voltage should stay within a limited range, the voltage is reset by short circuiting the capacitors using both the ground as well as reset switch. A reset pulse is applied in addition to a sample pulse after a number of desired voltages have been sampled. The readout associated with this reset is called the destructive readout, the other sampled voltages are called non-destructive readouts The time between two reset pulses (in seconds) is called the fundamental integration time or reset interval (RI). The duration of a reset interval is denoted by t_RI. All readouts collected during one reset interval are part of one integration ramp or ramp. In Fig 3.3, the readout data stream is shown schematically.

The voltage of the first readout after reset is not zero but starts at a arbitrary voltage level. This level is called the reset level since it has more or less the same value during a measurement. Variations are caused by instabilities in the amplifier electronics.

**Figure 3.3:** Schematic diagram showing CRE output for a ramp of 7 non-destructive readouts (NDR=3)
$\begin{figure} \begin{center} \begin{tabular} {c} {\epsfxsize=10cm \epsfbox{cre_output.eps}} \\ \end{tabular}\end{center}\end{figure}$

3.2.3 Detector Interface Electronics (DIE)

The detector interface electronics (DIE) acquires the output signals from the CREs and processes these for the telemetry transmission. It also serves as a multiplexer thus reducing the number of output lines. There are two DIEs in the EEU, each interfacing with the following groups of detectors:

The DIE subtracts a commandable offset from the CRE output voltage and amplifies the difference with a commandable gain factor. Note that for PHT the gain is a constant unlike other instruments for which the gain is used to adjust the dynamical range.

The DIE also converts the analogue signal from the CRE to a digital signal. This DA conversion produces a 12 bits value per readout which means that each readout has a digitalisation resolution of $2^{12}=\,$ 4096. The fact that there are several readouts per ramp increases the resolution in the signal.

3.2.4 readout timing control

To cover a dynamic range equivalent to the flux density range of astronomical sources (a few mJy to hundreds of Jy) within the available commanding bits in the telemetry, the design of the readout parameters has been based on an exponential scale with base 2. Only a power of 2 readouts per reset interval is commanded, and the duration of a reset interval (in seconds) has also be a power of 2. Since there are 4 bits available for commanding, the reset interval can have 16 values ranging from 1/256, 1/128, 1/64, . . . 128 s. Per ramp there are 2ⁿ readouts. The last one, the destructive readout, is generally not reliable since it is disturbed by the reset. The number of non-destructive readouts is provided by the parameter NDR, it is equal to 2^NDR-1.

The parameter DR has been defined to set the chopper plateau time. DR is defined such that 2^DR is equal to the amount of destructive readouts (or number of ramps) per chopper plateau.

The chopper dwell time or duration of a chopper plateau t_chop can be derived from:

The commanding of PHT imposes at least 4 ramps per chopper plateau, thus DR ${\geq}2$ for chopped observations. The maximum plateau time is 128 s. The first ramp of a chopper plateau is affected by the chopper transition and therefore unreliable.

For high readout rates the amount of data to be transferred can be too high. In order to avoid telemetry overflow, the parameter data reduction (DAT_RED) has been introduced. The value of DAT_RED is an integer indicating that only the first ramp of a sequence of DAT_RED ramps should be transmitted. For example, DAT_RED = 4 means that only the first of every 4 ramps is collected. DAT_RED = 1 indicates no reduction. Observations of bright sources will have a DAT_RED parameter larger than 1. The value of DAT_RED depends not only on NDR and RI, but also on the number of pixels per detector array.

3.2 Data Collection with ISOPHOT

3.2.1 Signal Flow in ISOPHOT

3.2.2 Cold readout Electronics (CRE)

3.2.3 Detector Interface Electronics (DIE)

3.2.4 readout timing control