
PRODUCT SPECIFICATION
TMC1203
5
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 2
p
fA 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.
Figure 1. Timing
VINX
Sample N
Sample N+1
Data N-3
Data N-2
Data N-1
Data N
Hi-Z
Sample N+2
Sample N+3
tSTD
tPWL
tPWH
tDIS
tENA
tDO
tHO
1/fS
CLKX
65-3720-02
DX7-0
OEX