Campbell 21X Manuale dell'Operatore Scaricare il pdf (Pagina 140)

SECTION 13. 21X

MEASUREMENTS

NOTE:

Since

the

peak

transient,

V"o,

causes significant

error

only if

it is

several

times larger

than the signal, V"o,

error

calculations

made

in this section

approximate

V'"o

by

V"o;

i.e., V'"o

=

V"o.

lf

the

input

settling

time constant,

t, is known, a

quick

estimation

of the settling error

as a

percentage

of the maximum

error

(V.o

for rising,

V'"o

for

decaying)

is

obtained by knowing

how

many time

constants

(Vt)

are

contained

in the

450ps

21X

input

settling

interval

(t).

The

familiar

exponential

decay

relationship

is

given

in

Table 13.3-1 for

reference.

TABLE

13.3-1.

Exponential

Decay, Percent

of

Maximum

Error

vs.

Time

in

Units of t

Time

o/o

Time

o/o

Constants

Max.

Error Constants

Max. Error

0 100.0

5

0.7

1

36.8 7

0.'l

3

5.0 10

0.004

Before

proceeding

with

examples of the

effect

of long

lead

lengths

on the

measurement,

a

discussion on obtaining

the

source resistance,

Ro,

and

lead

capacitance, C*L, is necessary.

DETERMINING

SOURCE

RESISTANCE

The source resistance

used to estimate the

settling time constant

is the

resistance

the 21X

input

"sees"

looking

out at

the sensor.

For our

purposes

the source

resistance can be

defined

as the

resistance

from

the

21X input

through

all

external

paths

back

to the

21X. Figure

13.3-2

shows a typical resistive

sensor,

(e.9.,

a

thermistor) configured

as

a half

bridge. Figure

13.3-3 shows Figure

13.3-2 redrawn in

terms

of

the resistive

paths

determining the

source

resistance Ro, is

given

by

the

parallel

resistance

of Rs and Rf,

as

shown

in Equation

13.3-8.

F|GURE

13.3-2. Typical

Resistive Half

Bridge

13-4

.21X

HI

OR LO

INPUT

FIGURE 13.3-3.

Source

Resistance Model

for Half

Bridge

Connected to the

21X

Ro

=

Fl.Rr/(R'+Rr)

[13.3-8]

lf Rl is much smaller,

equalto or

much

greater

than R.,

the

source resistance can

be

approximated by Equations 13.3-9 through

1 3.3-1 1, respectively.

Ro

=

Rf, RF<R,

Ro

=

R/2,

Rr=R,

Ro

=

R., Rt>>B,

[13.3-e]

[13.3-10]

[13.3-1

1]

The source

resistance

for

several

Campbell

Scientific sensors are

given

in column 3 of

Table 13.3-5.

DETERMINING LEAD CAPACITANCE

Wire manufacturers typically

provide

two

capacitance

specifications:

1) the

capacitance

between

the

two leads with

the

shield floating,

and 2)

the

capacitance

between

the two leads

with

the shield

tied to one lead. Since the input

lead

and the

shield are

tied to

ground (often

through

a

bridge resistor, R) in single-ended

measurements

such as

Figure 13.3-2,

the

second

specification

is

used

in determining lead

capacitance. Figure 13.3-4 is

a

representation

of this capacitance, Cn, usually

specified

as

pfd/ft.

C*

is actually the

sum of

capacitance

between

the

two conductors

and

the

capacitance

between

the top conductor and the

shield.

Capacitance

for

3

Belden lead wires

used in

Campbell Scientific sensors

is shown in

column 6 of

Table 13.3-2.

1 2 ... 135 136 137 138 139 140 141 142 143 144 145 ... 190 191

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