REV. 0

–6–

AD9821

DEFINITIONS OF SPECIFICATIONS

Differential Nonlinearity (DNL)

An ideal ADC exhibits code transitions that are exactly 1 LSB

apart. DNL is the deviation from this ideal value. Thus, every

code must have a finite width. No missing codes guaranteed to

12-bit resolution indicates that all 4096 codes, respectively,

must be present over all operating conditions.

Peak Nonlinearity

Peak nonlinearity, a full signal chain specification, refers to the

peak deviation of the output of the AD9821 from a true straight

line. The point used as “zero scale” occurs 1/2 LSB before the

first code transition. “Positive full scale” is defined as a Level 1,

1/2 LSB beyond the last code transition. The deviation is mea-

sured from the middle of each particular output code to the true

straight line. The error is then expressed as a percentage of the

2 V ADC full-scale signal. The input signal is always appropriately

gained up to fill the ADC’s full-scale range.

Total Output Noise

The rms output noise is measured using histogram techniques.

The standard deviation of the ADC output codes is calculated

in LSB and represents the rms noise level of the total signal

chain at the specified gain setting. The output noise can be

converted to an equivalent voltage using the relationship 1 LSB =

(ADC Full Scale/2

For the AD9821, 1 LSB is 500

µV.

Power Supply Rejection (PSR)

The PSR is measured with a step change applied to the supply

pins. This represents a very high frequency disturbance on the

AD9821’s power supply. The PSR specification is calculated

from the change in the data outputs for a given step change in

the supply voltage.

Internal Delay for SHA

The internal delay (also called aperture delay) is the time delay

that occurs from when the sampling edge is applied to the AD9821

until the actual sample of the input signal is held. The DATACLK

samples the input signal during the transition from low to high,

so the internal delay is measured from each clock’s rising edge

to the instant the actual internal sample is taken.

EQUIVALENT INPUT CIRCUITS

330

DVDD

DVSS

Figure 1. Digital Inputs— DATACLK, CLPOB, PBLK, SCK, SL

DVDD

DVSS

DRVSS

DRVDD

THREE-

STATE

DATA

DOUT

Figure 2. Data Outputs—D0–D11

AVDD

ACVSS

ACVSS

60

Figure 3. VIN+ and VIN– (Pins 30 and 31)

330

DVDD

DVSS

DVDD

DVSS

DVSS

DATA IN

RNW

DATA OUT

Figure 4. SDATA (Pin 47)