Rainbow Electronics ADC1251 - Manual (Page 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 e (6 02 c n a 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) e
S N(dB)b1 8
6 02
As an example an ADC12451 with a g 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
16