Rainbow-electronics ADC0848 Bedienungsanleitung PDF herunterladen (Seite 10)

Applications Information (Continued)

‘‘

’’ input and then the ‘‘

’’ inputs is (/2 of a clock period.

The change in the common-mode voltage during this short

time interval can cause conversion errors. For a sinusoidal

common-mode signal this error is:

ERROR(MAX)

peak

(2q f

)

0.5

where f

is the frequency of the common-mode signal,

peak

is its peak voltage value and t

is the conversion time.

For a 60 Hz common-mode signal to generate a (/4 LSB

error (

5 mV) with the converter running at 40 mS, its peak

value would have to be 5.43V. This large a common-mode

signal is much greater than that generally found in a well

designed data acquisition system.

3.2 Input Current

Due to the sampling nature of the analog inputs, short dura-

tion spikes of current enter the ‘‘

’’ input and exit the ‘‘

’’

input at the clock edges during the actual conversion. These

currents decay rapidly and do not cause errors as the inter-

nal comparator is strobed at the end of a clock period. By-

pass capacitors at the inputs will average these currents

and cause an effective DC current to flow through the out-

put resistance of the analog signal source. Bypass capaci-

tors should not be used if the source resistance is greater

than 1 kX.

3.3 Input Source Resistance

The limitation of the input source resistance due to the DC

leakage currents of the input multiplexer is important. A

worst-case leakage current of

1 mA over temperature will

createa1mVinput error witha1kXsource resistance. An

op amp RC active low pass filter can provide both imped-

ance buffering and noise filtering should a high impedance

signal source be required.

4.0 OPTIONAL ADJUSTMENTS

4.1 Zero Error

The zero of the A/D does not require adjustment. If the

minimum analog input voltage value, V

IN(MIN)

, is not ground,

a zero offset can be done. The converter can be made to

output 0000 0000 digital code for this minimum input voltage

by biasing any V

(

) input at this V

IN(MIN)

value. This is

useful for either differential or pseudo-differential modes of

input channel configuration.

The zero error of the A/D converter relates to the location

of the first riser of the transfer function and can be mea-

sured by grounding the V

input and applying a small mag-

nitude positive voltage to the V

input. Zero error is the

difference between actual DC input voltage which is neces-

sary to just cause an output digital code transition from 0000

0000 to 0000 0001 and the ideal (/2 LSB value ((/2 LSB

9.8

mV for V

REF

5.000 V

4.2 Full-Scale

The full-scale adjustment can be made by applying a differ-

ential input voltage which is 1 (/2 LSB down from the desired

analog full-scale voltage range and then adjusting the mag-

nitude of the V

REF

input for a digital output code changing

from 1111 1110 to 1111 1111.

4.3 Adjusting for an Arbitrary Analog Input Voltage

Range

If the analog zero voltage of the A/D is shifted away from

ground (for example, to accommodate an analog input sig-

nal which does not go to ground), this new zero reference

should be properly adjusted first. A V

(

) voltage which

equals this desired zero reference plus (/2 LSB (where the

LSB is calculated for the desired analog span, 1 LSB

analog span/256) is applied to selected ‘‘

’’ input and the

zero reference voltage at the corresponding ‘‘

’’ input

should then be adjusted to just obtain the 00

HEX

to 01

HEX

code transition.

TL/H/5016–16

a) Ratiometric

TL/H/5016–17

b) Absolute with a Reduced Span

FIGURE 2. Referencing Examples

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 19 20

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