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When several items of equipment are connected together, problems can sometimes be experienced as a result of interference being coupled between them through the signal and/or mains earth lines. This problem can be particularly troublesome when one item can generate high voltages and/or currents, as xenon arc lamps do during switch on.
To minimise such effects, we can offer the following suggestions. First, we suggest that all equipment which has any signal connection to the spectrophotometer (this includes the computer if a computer interface is used) is connected to the same mains wall socket, via a multiway extension lead. The light source should be connected to a different mains wall socket. With this configuration, any mains-borne interference generated by the light source (or other equipment) will be more or less isolated from the sensitive equipment.
The spectrophotometer system box includes a mains filter. This filter differs from most other mains filters in that it also filters the earth line. Our experience is that the earth-line filtering dramatically reduces the susceptibility to mains-borne interference. However, the benefits are lost if the signal and mains earth lines are interconnected in any of the other equipment which has signal connections to the spectrophotometer, as this short-circuits the earth filter.
To guard against this problem, we suggest that all other such equipment is used in conjunction with a similar filter. We can advise on sources of supply or obtain suitable filters for you if required. Suppliers include Farnell Electronic Components (Leeds, UK, tel 0532 636311), part number 148-217. One filter should suffice if all these other items of equipment are powered via it. Note that this type of filter can also prevent interference from being injected INTO the mains, so putting another filter in the mains connection for the light source may also be helpful (see separate notes concerning the generation of interference during switch-on of the light source).
Finally, we should also point out the general problem of mains earth loops, which can occur when several items of equipment are interconnected. The spectrophotometer signal earths are connected to the mains earth line, but if required, they can be separated. To do this, remove the top cover of the system box (you will need to remove all the screws along the front and back as well as the two on each side) in order to gain access to the main system board, which is on the left-hand side of the enclosure. To break the connection between the system and mains earth, remove the small black jumper link which is just in front of the metal rail to which various power semiconductors are bolted. We recommend that you try this only if you experience significant mains pickup or mains-borne interference problems even after trying the other measures outlined above.