Z2

Z1

Z0

DB11

DB3

DB2

DB1

DB0

Track

3 leading zero bits

12 data bits

CS

SCLK

SDATA

1

2

3

4

5

12

13

14

15

16

TRI-STATE

Hold

17

18

19

20

ADC121S021

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SNAS305H – JULY 2005 – REVISED MARCH 2013

Figure 3. ADC121S021 Serial Timing Diagram

Specification Definitions

ACQUISITION TIME is the time required to acquire the input voltage. That is, it is time required for the hold

capacitor to charge up to the input voltage. Acquisition time is measured backwards from the falling edge of CS

when the signal is sampled and the part moves from track to hold. The start of the time interval that contains

user must ensure that the time between the 13th rising edge of SCLK and the falling edge of the next CS is not

APERTURE DELAY is the time after the falling edge of CS to when the input signal is acquired or held for

conversion.

APERTURE JITTER (APERTURE UNCERTAINTY) is the variation in aperture delay from sample to sample.

Aperture jitter manifests itself as noise in the output.

CONVERSION TIME is the time required, after the input voltage is acquired, for the ADC to convert the input

voltage to a digital word. This is from the falling edge of CS when the input signal is sampled to the 16th falling

edge of SCLK when the SDATA output goes into TRI-STATE.

DIFFERENTIAL NON-LINEARITY (DNL) is the measure of the maximum deviation from the ideal step size of 1

LSB.

DUTY CYCLE is the ratio of the time that a repetitive digital waveform is high to the total time of one period. The

specification here refers to the SCLK.

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.

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 the ADC input at the same time. It is defined as the ratio of the power in

the second and third order intermodulation products to the sum of the power in both of the original frequencies.

IMD is usually expressed in dB.

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