Datasheet

2
f1
2
Fn
2
f2
10
x
A
A++A
logTHD = 20
/
ADC081C021, ADC081C027
SNAS447C –FEBRUARY 2008–REVISED MARCH 2013 www.ti.com
Specification Definitions
ACQUISITION TIME is the time required for the ADC to acquire the input voltage. During this time, the hold
capacitor is charged by the input voltage.
APERTURE DELAY is the time between the start of a conversion and the time when the input signal is internally
acquired or held for conversion.
CONVERSION TIME is the time required, after the input voltage is acquired, for the ADC to convert the input
voltage to a digital word.
DIFFERENTIAL NON-LINEARITY (DNL) is the measure of the maximum deviation from the ideal step size of 1
LSB.
EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE BITS) is another method of specifying Signal-to-Noise
and Distortion or SINAD. ENOB is defined as (SINAD - 1.76) / 6.02 and says that the converter is equivalent to a
perfect ADC of this (ENOB) number of bits.
FULL POWER BANDWIDTH is a measure of the frequency at which the reconstructed output fundamental
drops 3 dB below its low frequency value for a full scale input.
GAIN ERROR is the deviation of the last code transition (111...110) to (111...111) from the ideal (V
REF
- 1.5
LSB), after adjusting for offset error.
INTEGRAL NON-LINEARITY (INL) is a measure of the deviation of each individual code from a line drawn from
negative full scale (½ LSB below the first code transition) through positive full scale (½ LSB above the last code
transition). The deviation of any given code from this straight line is measured from the center of that code value.
INTERMODULATION DISTORTION (IMD) is the creation of additional spectral components as a result of two
sinusoidal frequencies being applied to an individual ADC input at the same time. It is defined as the ratio of the
power in both the second and third order intermodulation products to the power in one of the original frequencies.
Second order products are f
a
± f
b
, where f
a
and f
b
are the two sine wave input frequencies. Third order products
are (2f
a
± f
b
) and (f
a
± 2f
b
). IMD is usually expressed in dB.
MISSING CODES are those output codes that will never appear at the ADC output. The ADC081C021 is
ensured not to have any missing codes.
OFFSET ERROR is the deviation of the first code transition (000...000) to (000...001) from the ideal (i.e. GND +
0.5 LSB).
SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in dB, of the rms value of the input signal to the rms
value of the sum of all other spectral components below one-half the sampling frequency, not including
harmonics or d.c.
SIGNAL TO NOISE PLUS DISTORTION (S/N+D or SINAD) Is the ratio, expressed in dB, of the rms value of the
input signal to the rms value of all of the other spectral components below half the clock frequency, including
harmonics but excluding d.c.
SPURIOUS FREE DYNAMIC RANGE (SFDR) is the difference, expressed in dB, between the desired signal
amplitude to the amplitude of the peak spurious spectral component, where a spurious spectral component is
any signal present in the output spectrum that is not present at the input and may or may not be a harmonic.
TOTAL HARMONIC DISTORTION (THD) is the ratio, expressed in dBc, of the rms total of the first n harmonic
components at the output to the rms level of the input signal frequency as seen at the output. THD is calculated
as
(1)
where A
f1
is the RMS power of the input frequency at the output and A
f2
through A
fn
are the RMS power in the
first n harmonic frequencies.
THROUGHPUT TIME is the minimum time required between the start of two successive conversions. It is the
acquisition time plus the conversion time.
LEAST SIGNIFICANT BIT (LSB) is the bit that has the smallest value or weight of all bits in a word. This value is
LSB = V
A
/ 2
n
(2)
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