Section C. Design Input 3/11/97 Page 16
SCOPE AND LEVEL OF DETAIL. Design input requirements must be
comprehensive. This may be quite difficult for manufacturers who are implementing a
system of design controls for the first time. Fortunately, the process gets easier with
practice. It may be helpful to realize that design input requirements fall into three
categories. Virtually every product will have requirements of all three types.
• Functional requirements specify what the device does, focusing on the operational
capabilities of the device and processing of inputs and the resultant outputs.
• Performance requirements specify how much or how well the device must
perform, addressing issues such as speed, strength, response times, accuracy, limits
of operation, etc. This includes a quantitative characterization of the use
environment, including, for example, temperature, humidity, shock, vibration, and
electromagnetic compatibility. Requirements concerning device reliability and
safety also fit into this category.
• Interface requirements specify characteristics of the device which are critical to
compatibility with external systems; specifically, those characteristics which are
mandated by external systems and outside the control of the developers. One
interface which is important in every case is the user and/or patient interface.
What is the scope of the design input requirements development process and how much
detail must be provided? The scope is dependent upon the complexity of a device and the
risk associated with its use. For most medical devices, numerous requirements
encompassing functions, performance, safety, and regulatory concerns are implied by the
application. These implied requirements should be explicitly stated, in engineering terms,
in the design input requirements.
Determining the appropriate level of detail requires experience. However, some general
guidance is possible. The marketing literature contains product specifications, but these
are superficial. The operator and service manuals may contain more detailed
specifications and performance limits, but these also fall short of being comprehensive.
Some insight as to what is necessary is provided by examining the requirements for a very
common external interface. For the power requirements for AC-powered equipment, it is
not sufficient to simply say that a unit shall be AC-powered. It is better to say that the
unit shall be operable from AC power in North America, Europe, and Japan, but that is
still insufficient detail to implement or validate the design. If one considers the situation
just in North America, where the line voltage is typically 120 volts, many systems are
specified to operate over the range of 108 to 132 volts. However, to account for the
possibility of brownout, critical devices may be specified to operate from 95 to 132 volts
or even wider ranges. Based on the intended use of the device, the manufacturer must
choose appropriate performance limits.
There are many cases when it is impractical to establish every functional and performance
characteristic at the design input stage. But in most cases, the form of the requirement can
be determined, and the requirement can be stated with a to-be-determined (TBD)
numerical value or a range of possible values. This makes it possible for reviewers to