PRODUCT SPECIFICATION
TMC1203
Specification Notes
Bandwidth
Bandwidth specification of an A/D converter is somewhat
different from the normal frequency-response specification
used in amplifiers and filters. An understanding of the differ-
ences will help in selecting converters properly for particular
applications.
A/D conversion comprises two distinct processes: sampling
and quantizing. Sampling is
grabbing
a snapshot of the input
signal and holding it steady for quantizing. The quantizing
process is approximating the analog input to its nearest
numerical value within the conversion range. While
sampling is a high-frequency process, quantizing operates on
a dc signal, held steady by the track/hold circuit. Therefore,
the sampling process relates to the dynamic characteristics of
an A/D converter.
Sampling involves an aperture time, the time needed for the
track/hold circuit to capture the input signal and settle on a
dc value to hold. It is analogous to the shutter speed of a
camera: the shorter the A/D aperture (or faster the shutter)
the less the signal (or picture) will be blurred, and the less
uncertainty there will be in the quantized value. This is not to
be confused with the camera lens opening (aperture), which
is entirely different.
For example, a 10 MHz sinewave with a 1V peak amplitude
(2Vp-p) has a maximum slew rate of 2pfA at zero crossing,
or 62.8V/ms. With an 8-bit A/D converter, q (the quantiza-
tion step size) = 2V/255 = 7.8mV. The input signal will slew
one LSB in 124ps. To limit the error (and noise) contribution
due to aperture effects to 1/2LSB, the aperture must be
shorter than 62ps.
This is the primary reason that the signal to noise ratio drops
off as full scale frequency increases. Notice that the slew rate
is directly proportional to signal amplitude, A. A/Ds will
handle lower-amplitude signals of higher bandwidth, but
other distortion effects will be worsened.
All this is of particular interest in applications such as digi-
tizing analog VGA RGB signals, or the output of a CCD
imaging chip. These data are effectively pre-sampled: there
is a period of rapid slewing from one pixel value to another,
followed by a relatively stable dc level before the signal
slews to the next pixel value. The goal is, of course, to sam-
ple on these stable pixel values, not on the slewing between
pixels. During the aperture time, the A/D sees essentially a
dc signal, and bandwidth considerations are less important.
As long as the input circuit can slew and settle to the new
value in the prescribed period, an accurate conversion will be
made.
The TMC1203 is capable of slewing a full 2V and settling
between samples taken as little as 25ns apart, making it ideal
for digitizing analog VGA and CCD outputs.
tSTD
VINX
Sample N
Sample N+2
Sample N+3
Sample N+1
tPWL
CLKX
tDO
tHO
Hi-Z
DX7-0
Data N-3
Data N-2
Data N-1
Data N
tPWH
1/fS
tDIS
tENA
OEX
65-3720-02
Figure 1. Timing
5
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