Rainbow-electronics ADC1251 Manual de usuario descargar pdf (Pagina 16)

4.0 Dynamic Performance (Continued)

Effective number of bits can also be useful in describing the

A/D’s noise performance. An ideal A/D converter will have

some amount of quantization noise, determined by its reso-

lution, which will yield an optimum S/N ratio given by the

following equation:

S/N

(6.02

1.8) dB

where n is the A/D’s resolution in bits.

The effective bits of a real A/D converter, therefore, can be

found by:

n(effective)

S/N(dB)

1.8

6.02

As an example, an ADC12451 with a

5V, 10 kHz sine

wave input signal will typically have a S/N of 78 dB, which is

equivalent to 12.6 effective bits.

Two sample/hold specifications, aperture time and aperture

jitter, are included in the Dynamic Characteristics table

since the ADC12451 has the ability to track and hold the

analog input voltage. Aperture time is the delay for the A/D

to respond to the hold command. In the case of the

ADC12451, the hold command is internally generated.

When the Auto-Zero function is not being used, the hold

command occurs at the end of the acquisition window, or

seven clock periods after the rising edge of the WR

. The

delay between the internally generated hold command and

the time that the ADC12451 actually holds the input signal is

the aperture time. For the ADC12451, this time is typically

100 ns. Aperture jitter is the change in the aperture time

from sample to sample. Aperture jitter is useful in determin-

ing the maximum slew rate of the input signal for a given

accuracy. For example, an ADC12451 with 100 ps of aper-

ture jitter operating with a 5V reference can have an effec-

tive gain variation of about 1 LSB with an input signal whose

slew rate is 12 V/ms.

5.0 Typical Applications

Power Supply Bypassing

TL/H/11025–24

Protecting the Analog Inputs

TL/H/11025–25

Note: External protection diodes should be able to withstand the op amp current limit.

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