ADC12451 datasheet(15/18 Pages) NSC

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30 Analog Considerations (Continued)

35 INPUT BYPASS CAPACITORS

An external capacitor can be used to filter out any noise due

to inductive pickup by a long input lead and will not degrade

the accuracy of the conversion result

36 INPUT SOURCE RESISTANCE

The analog input can be modeled as shown in

Figure 6

External RS will lengthen the time period necessary for the

voltage on CREF to settle to within

LSB of the analog

input voltage With tA e 35 ms RS s 1kX will allow a 5V

analog input voltage to settle properly

37 POWER SUPPLIES

Noise spikes on the VCC and V

supply lines can cause

conversion errors as the comparator will respond to this

noise The AD is especially sensitive during the Auto-Zero

or -Cal procedures to any power supply spikes Low induc-

tance tantalum capacitors of 10 mF or greater paralleled

with 01 mF ceramic capacitors are recommended for supply

bypassing Separate bypass capacitors should be placed

close to the DVCC AVCC and V

pins If an unregulated

voltage source is available in the system

a separate

LM340LAZ-50 voltage regulator for the A-to-D’s VCC (and

other analog circuitry) will greatly reduce digital noise on the

supply line

38 THE CALIBRATION CYCLE

On power up the ADC12451 goes through an Auto-Cal cy-

cle which cannot be interrupted Since the power supply

reference and clock will not be stable at power up this first

calibration cycle will not result in an accurate calibration of

the AD A new calibration cycle needs to be started after

the power supplies reference and clock have been given

enough time to stabilize During the calibration cycle cor-

rection values are determined for the offset voltage of the

sampled data comparator and any linearity and gain errors

These values are stored in internal RAM and used during an

analog-to-digital conversion to bring the overall full-scale

offset and linearity errors down to the specified limits Full-

scale error typically changes g02 LSB over temperature

and linearity error changes even less therefore it should be

necessary to go through the calibration cycle only once af-

ter power up if Auto-Zero is used to correct the zero error

change Since Auto-Zero cannot be activated with S H con-

version method it may be necessary to do a calibration cy-

cle more than once

39 THE AUTO-ZERO CYCLE

To correct for any change in the zero (offset) error of the

AD the auto-zero cycle can be used It may be necessary

to do an auto-zero cycle whenever the ambient temperature

changes significantly (See the curve titled ‘‘Zero Error

Change vs Ambient Temperature’’ in the Typical Perform-

ance Characteristics) A change in the ambient temperature

will cause the VOS of the sampled data comparator to

change which may cause the zero error of the AD to be

greater than g1 LSB An auto-zero cycle will typically main-

tain the zero error to g1 LSB or less

40 Dynamic Performance

Many applications require the AD converter to digitize ac

signals but the standard dc integral and differential nonlin-

earity specifications will not accurately predict the AD con-

verter’s performance with ac input signals The important

specifications for ac applications reflect the converter’s abil-

ity to digitize ac signals without significant spectral errors

and without adding noise to the digitized signal Dynamic

characteristics such as signal-to-noise (SN)

signal-to-

noiseadistortion ratio (S(NaD)) effective bits full power

bandwidth aperture time and aperture jitter are quantitative

measures of the AD converter’s capability

An AD converter’s ac performance can be measured using

Fast Fourier Transform (FFT) methods A sinusoidal wave-

form is applied to the AD converter’s input and the trans-

form is then performed on the digitized waveform S(NaD)

and SN are calculated from the resulting FFT data and a

spectral plot may also be obtained Typical values for SN

are shown in the table of Electrical Characteristics and

spectral plots of S(NaD) are included in the typical per-

formance curves

The AD converter’s noise and distortion levels will change

with the frequency of the input signal with more distortion

and noise occurring at higher signal frequencies This can

be seen in the S(NaD) versus frequency curves These

curves will also give an indication of the full power band-

width (the frequency at which the S(NaD) or SN drops

3 dB)

TLH11025 – 23

FIGURE 6 Analog Input Equivalent Circuit

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