Biomedical Engineering Reference
In-Depth Information
Fig. 2.21. Block diagram of AFM electronics functions. Top: electronics as implemented with
analogue electronics; Bottom: as implemented with a high-speed DSP chip.
the maximum potential is 100 volts, then the scan range is 100 microns. The maximum
achievable resolution is set by the noise floor of the driving voltage. A noise floor of 1
millivolt would give an
X-Y
resolution of 1 nanometre in this case.
It is important that the bit noise associated with the
X-Y
scan generators be less than the
analogue noise floor of the electronic controller. For example, if the scan range is 100
m
and the analogue noise floor is less than 1 nm, then the number of bits required is at least
100,000, which is greater than 2 [16] bits. This is significant because most DACs store
such data as 16 bit numbers (i.e. they can have no more than 65,536 possible values). Thus,
if this is the case, some resolution would be lost when the data was digitized. To overcome
this, one option is to use a scale and offset DAC and amplifier if the scanning DAC does
not give enough bit resolution. This overcomes the resolution problem because although
