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1203KLC40

Triple Video A/D Converter 8-Bit, 50Msps

Cadeka Microcircuits LLC. 

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.

VINX

CLKX

DX7-0

OEX

tSTD

Sample N

tPWL

Sample N+1

tPWH

1/fS

Sample N+2

tHO

Data N-3

tDO

Data N-2

tDIS

Hi-Z

Data N-1

tENA

Figure 1. Timing

Sample N+3

Data N

65-3720-02

5

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