Campbell 21X Instrukcja Operatora Pobierz pdf (Strona 139)

13.3 THE

EF

ECT

OF

SENSOR

LEAD

N THE

SIGNAL

SETTLING

LENGTH

TIME

Whenever

an

analog

input is switched

into

the

21X

measurdment

circuitry

prior

to

making

a

measuremedt,

a

finite

amount

of

time

is

required

for tfe

signal to stabilize

at its correct

value.

The ralte

at

which

the signal

settles

is

determined

by

the input settling

time constant

which

is a

function of both

the

source

resistance,

ahd

input

capacitance

(explained

below).

The

P1X

allows a

450ps

settling time

before

initiatihg the

measurement.

In

most

applications

lhis

settling time

is adequate,

but

the

additionaf

wire

capacitance

associated

with

long

sensor

can

increase

the settling time

constant

to tlhe

point

that

measurement errors

are three

potential

sources of

settle before

the

measurement

1.

The

must rise to

its correct

value.

2.

A

small

(

smV)

caused

by

switchi

the analog

input

into the

circuitry

must

settle.

nsient,

usually

about

40mVA/,

the

switched,

precision

excitation

in resistive

bridge

must

settle.

The

of this section

is to

bring attention

to

potential

errors

caused

when

the

input

time

constant

gets

too large

procedures

whereby

the

effects

on the

measurement

can be

estimated.

lrl addition,

physicalvalues

are

given

for three

typps

of wire

used

in Campbell

Scientific

sensors,

and error

estimates

for

given

lead

lengths are

provided.

Finally,

techniques

are discuss4d

for minimizing

input settling error

when

long

l{ads are

mandatory.

13.3.1

THE

INFUT SETTLING

TIME CONSTANT

The

rate at which

an

input

voltage

rises to its

full

value or

that

a transient decays

to the correct

input

level

afe both determined

by

the input

settling

time constant.

ln both

cases the

waveform

is an exponential.

Figure

13.3-1

shows

both

p

rising

and

decaying

waveform

settling

to

tfip signal

level,

Vso.

The rising input

voltage

is d{scribed

by Equation

13.3-1 and

the

decaying

in$ut

voltage

by

Equation

13.3-2,

SECTION

13.

21X MEASUREMENTS

FIGURE

13.3-1.

Input

Voltage

Rise and

Transient Decay

v"

=

v"o(t-"-tlRocr

)

rise

[13.3-1]

V"

=

v"o

*(v"o

-

v"o)e-tlRo"t, d"""y

t13.3-21

where V, is the

input

voltage, V"o the

true signal

voltage, V"o the

peak

transient

voltage, t

is time

in seconds,

Ro the source

resistance

in ohms,

and C1

is the total

capacitance

between

the

signal lead and

ground

(or

some

other

fixed

reference

value)

in farads.

The

settling

time

constant

in seconds,

t, and the

capacitance

relationships are

given

in

Equations

13.3-3 through

13.3-5,

U.J

may

occur.

error

which

is

made:

3.

A larger

caused

voltage

where

Cr

is the

fixed 21X input capacitance

in

farads, C", is the

wire capacitance

in farads/foot,

and L is the

wire

length in feet.

Equations 13.3-1 and

13.3-2

can

be used

to

estimate the

input settling error,V.,

directly. For

the

rising

case,

V"

=

V"o-V", whereas

for the

decaying transient,

Vs

=

Vso*V". Substituting

these relationships

for

V,

in Equations

13.3-1

and 13.3-2,

respectively,

yields

expressions

in

V",

the

input

settling

error:

t

=

RoCr

CT

=

Cf+CwL

Cf

=

3.3

nfd

Ve

=

Vso€-t/Rocr,

rise

Ve

=

V'eo e-t/RocT, decay

[13.3-3]

[13.3-4]

[13.3-5]

[13.3-6]

[13.3-7]

and to

of

lead

Where V'"o

=

V"o-V.o, the difference

between

the

peak

transient

voltage and the

true

signal

voltage.

13-3

1 2 ... 134 135 136 137 138 139 140 141 142 143 144 ... 190 191

Komentarze do niniejszej Instrukcji

Brak uwag