This module has four identical stages, each of which is associated
with its own front panel potentiometer. Jumper links on the board allow each
stage to be configured so that the potentiometer provides either a variable
gain or a variable DC offset adjustment. Each stage can receive its input from
any of the 16 output signal lines on the system backplane (A1-A8 and B1-B8) and
it can place its output on any other of these lines. This arrangement takes
advantage of the fact that in any one system it is most unlikely that all 16
signal lines would already be in use. The input and output signal selection is
done by two jumpers fitted to the long rows of pins immediately below each
amplifier stage on the board. One jumper is placed to link a pin in the central
column to the adjacent left-hand pin to select the input line, and the other
jumper is place to link another pin in the central column to its adjacent
right-hand pin to select the output line. From the bottom upwards, the pin
order is for data lines 1B, 1A, 2B, 2A, and so on; this arrangement is labelled
on the board. There is also a facility to link the input of the second stage to
the output of the first, by connecting a jumper between the pins labelled
LKl-2, and a similar facility is provided for the third and fourth stages by
the LK3-4 pins. In this mode, the jumper selecting the input signal line for
stage 2 (or 4) should be removed, and the jumper selecting the output signal
line for stage 1 (or 3) can optionally be removed. The purpose of this facility
is to allow one stage to provide variable gain and another stage to provide
variable offset on the same channel if required, without using backplane signal
lines to interconnect the two stages.
Although the variable gain option clearly has its uses, it is not essential for most applications. In contrast, the variable offset facility is an essential requirement for ratiometric fluorescence measurements, to allow subtraction of a signal equivalent to the autofluorescence background at that wavelength. The module is therefore normally supplied with all four stages configured for variable offset, using signal lines A1-A4 as inputs and B1-B4 as the corresponding outputs. If required, a second module can be configured to perform the same functions for filter positions 5-8. The choice of gain or offset configuration is made by the pair of jumpers at the top left of each amplifier stage. As labelled on the board, connecting these jumpers between each central pin and the pin just above it provides the variable gain mode, whereas connecting each central pin to the one just below it provides the variable offset mode (in which the module is normally supplied).
Immediately below this pair of jumpers on each amplifier stage is another jumper that sets either the maximum gain or the maximum value of the DC reference voltage in the offset mode. The maximum gain, as labelled on the board, is either x1 with the jumper connected between the central pin and the upper pin, or x10 with the jumper connected between the central pin and the lower pin; the corresponding DC reference voltages in the offset mode are 1V and 10V.
To the right of this jumper is another double jumper link, with alternative jumper positions analogous to those that set the gain or the offset mode of operation. This is labelled "normal" and "invert" for the upper and lower positions respectively, and it should usually be left in the normal position as supplied. If this pair of jumpers is moved to the invert position, then in offset mode the DC offset voltage will be added to rather than subtracted from the signal voltage, whereas in gain mode the signal voltage itself will be inverted. This facility is included to make the module a fully general-purpose one. It should also be noted that other configurations of each stage, in which pairs of filter signals can be added together or subtracted from each other, are possible. Further information on these possibilities is available on request, but please note that we would normally recommend use of our combiner module for such applications.
