Evaluation (Validation and Verification) - Medical Device Design: Innovation from Concept to Market - page 222

o/p

o/p

do

FSD

i/p

i/p

Figure 9.9
Reproducibility: (a) good reproducibility; (b) poor reproducibility

CASE STUDY 9.4
A 12-bit converter is used to measure a voltage of 0-1 V. The range of the ADC is 0-10 V.
Determine the resolution and suggest an improvement.

The number of divisions of an ADC is given by 2 n , where n is the number of bits. Hence a 12-bit
converter has 4096 divisions. Therefore the resolution of the ADC is

∆ R /4 96

0 0 000 . 24V

Inserting an amplifier of gain 10 between the signal and the ADC can improve the resolution.
This now uses the full range of the ADC and changes the resolution to

R R/k 0000 000000 24/1 24V

∆∆

k

. .

Please note that modern televisions and cameras use the term high definition and relate this to
high resolution. This is in fact wrong. TV sets with better definition almost certainly have low
values of resolution!

I am always amazed how people have accepted digital information to be more accurate than
analog. This is not the case; it is certainly easier to deal with but it is by no means more accurate.

9.3.3.6 Linearity
The last calibration term for us to consider is linearity . This is simply defined as the greatest
deviation away from linearity as a proportion of FSD ( Figure 9.10 ):

dl
FSD

L 100% (9.4)

9.3.3.7 Summary of Calibration
It cannot be stressed enough that calibration is important whatever your device. It is, by far,
the best way to confirm that your device is performing as it should. You may not need all of
the terms we have met and, equally, these are not all of the terms associated with calibration.

Consider the risk analysis associated with two dialysis machines where one has been
calibrated and the other has not; we know exactly how much flow is generated for a particular