For absolute accuracy where the analog input varies be-

tween very specific voltage limits the reference pin can be

biased with a time and temperature stable voltage source

In general the magnitude of the reference voltage will re-

quire an initial adjustment to null out full-scale errors

32 INPUT CURRENT

Because the input network of the ADC12441 is made up of

a switch and a network of capacitors a charging current will

flow into or out of (depending on the input voltage polarity)

pend on the actual input voltage applied

33 NOISE

The leads to the analog input pin should be kept as short as

possible to minimize input noise coupling Both noise and

undesired digital clock coupling to this input can cause er-

rors Input filtering can be used to reduce the effects of

these noise sources

34 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

35 INPUT SOURCE RESISTANCE

The analog input can be modeled as shown in

Figure 3

LSB of the analog

settle properly

36 POWER SUPPLIES

b

supply lines can cause

conversion errors as the comparator will respond to this

noise The AD is especially sensitive during the auto-zero

or auto-cal procedures to any power supply spikes Low in

ductance tantalum capacitors of 10 mF or greater paralleled

with 01 mF ceramic capacitors are recommended for supply

bypassing Separate bypass capacitors whould be placed

b

pins If an unregulated

voltage source is available in the system

a separate

other analog circuitry) will greatly reduce digital noise on the

supply line

37 THE CALIBRATION CYCLE

On power up the ADC12441 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

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

38 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 curved titled ‘‘Zero Error

Change vs Ambient Temperature’’ in the Typical Perform-

ance Characteristics) A change in the ambient temperature

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

TLH11017 – 21

FIGURE 3 Analog Input Equivalent Circuit

12