Campbell 21X Manuale dell'Operatore Scaricare il pdf (Pagina 137)

SECTION

13.

21X MEASUREMENTS

{D SLOW

MEASUREMENT

Setup

Amplifier Iniegrotion

AfD Conversion

Settling

13.1 FAST

A

SEQUENGE

The 21X malkes

voltage

measurements by

'

integrating tfie

input signal

for a fixed time and

then holding the

integrated

value

for

the

analog

to

digital

(A/0)

conversion.

The

A,/D

conversion

is made with a

14 bit

successive approximation

technique

wflich resolves

the signal voltage

to

approximately

one

part

in 15,000 of the

full

scale range

pn

a differential

measurement

(e.9.,

1/15,000 x 5V

=

333pV).

The

resolution of a

single-ende(

measurement

is

one

part

in 7500.

Integrating the

signal

removes

noise

that

could

create

an

erior

if the signal

were

sources of

nbise

is

60

Hz

from

AC

power

lines.

The slow

i

time

of 16.67 milliseconds

(1.67ms

on

{ne

5V

range)

is equal

to

one 60

Hz

cycle.

the

integration

time, the

AC noise

will

to 0.

With

ditferent software

and a

hardware

,

the slow

integration time

can be

le

to

20 ms

for countries that

have 50

Hz

There are

sdveral

situations

where

the

fast

integration

tifne of

250

microseconds

(25ps

on

the 5V

rang4)

is

preferred.

The tast integration

time

minimides

time

skew between

measuremehts

and

increases

the throughput

rate.

The current

drain on the

21X batteries

is

lower when

fast

integration

time is used.

The

fast integratipn

time should

ALWAYS be used

with the

AC

half

bridge

(lnstruction

5) when

measuring

AC

resistance

or

the

output

of an

LVDT.

An

AC

resistive

sensor

will

polarize

if a

DC

voltage is

applied,

causing

erroneous

readings

anfl sensor

decay.

The induced

voltage

in aft LVDT

decays

with

time as

current

in the

primaly

coil

shifts

from the inductor

to the

series

resistpnce;

a

long

integration time

would

result

in mogt of

the

integration taking

place

after

the

sigPal

had disappeared.

.

450

uS

250uS fost

550uS

Reset

Inteqrotor

16.67mS slow

FIGURE 13.1-1. Timing

of Single-Ended

Measurement

Before making a series

of measurements

prescribed

by an

lnput

Instruction, the

21X

makes a calibration

measurement.

The

calibration

is accomplished

by measuring two

known voltages

which are sent

through the

same amplifier circuit

that

will be used

for

the

measurements. The calibration

for a single-

ended

measurement consists

of

measuring a

voltage

which

is 4/5ths of

full scale

and

then

making a measurement

with the

input

grounded.

A ditferential

measurement

is made

once

with

the

inputs as

connected

and a second

time

with

the

inputs

reversed

(Section

13.2);

calibration for differential

measurements uses

voltages

at

r4l5ths

of full

scale.

13.2

SINGLE.ENDED

AND

DIFFERENTIAL

VOLTAGE

MEASUREMENTS

NOTE: The channel

numbering

on the 21X

panel

refers

to differential

channels.

Either

the high or

low side

of a ditferential channel

can

be

used

for

single

ended

measurements. Each

side

must be

counted

when

numbering

single-ended

channels; e.9., the

high and

low sides of

differential channel

8 are single-ended

channels

15 and 16,

respectivelY.

The timing and sequence

of a single-ended

measurement

is shown

in Figure

13.1-1. A

single-ended

measurement

is made on a single

input which is referenced

to

ground.

A single

integration is

performed

for each

measurement.

A differential measurement

measures

the

difference

in

voltage between

two

inputs. The

measurement sequence

on a differential

measurement

involves two

integrations:

First

with the high input

referenced

to the

low,

then

with the inputs

reversed

(Figure

13.2-1).

The

21X computes

the differentialvoltage

by

instantaneoUsly

sampled

and

held

for the

A/D

conversion.

The

slow

integration

time

provides

a more

noisq-free

reading

than the fast

integration tiine.

One of the

most common

13-1

1 2 ... 132 133 134 135 136 137 138 139 140 141 142 ... 190 191

Commenti su questo manuale

Nessun commento