Campbell Scientific CR10 CR10 Measurement and Control - Page 169

In

order

to

make

a

differential

measurement,

the

inputs must be within

the

CR10

common

mode

range

of

t2.5

V.

The

common mode

range

is

the

voltage

range, relative

to

CR10 ground, within

which

both inputs of

a differential

measurement

must lie in order for

the differential

measurement

to

be

made.

For

example,

if

the

high side of

a

differential

input

is

at

2

V and

the

low

side

is

at

1

V

relative to

CR10 ground,

there

is no problem; a

measurement made on the

+2.5V

range would

indicate a signal

of

1

V.

However,

if

the

high input

is at

2.8 V and

the

low input

is

at 2 V,

the

measurement

cannot

be made because

the

high

input is

outside

of

the

common mode

range.

The

CR10

will

indicate

the

overrange with

the

maximum

negative number

(Section 3.5.)

Problems

with

exceeding

common mode

range

may be encountered when the

CRl0

is used

to

read

the

output of external

signalconditioning

circuitry

if

a

good ground connection

does

not

exist between the

externalcircuitry

and

the

CR10.

When

operating where

AC

power

is

available, it is not always safe

to

assume that

a

good ground connection exists through the AC

wiring.

lf a CR10 is used

to

measure the output

from

a laboratory

instrument (both plugged into

AC power

and

referencing ground to outlet

ground), it is best

to

run a ground

wire

between

the CR10 and

the

externalcircuitry.

Even with

this

ground connection,

the

ground potential of

the

two

instruments may not be at exactly the

same level,

which

is

why

a differential

measurement

is

desired

(Section 7.2).

lf a

differential

measurement is used on a sensor

that

is not referenced

to

CR10 ground

through

a

separale connection

(e.9.,

a

net radiometer), a

jumper

wire

should

be

connected

between

the

low

side

of

the

ditferential input and analog ground to

hold

the sensor

in

common

mode range.

A

differential

measurement has

better

noise

rejection

than

a

single-ended

measurement.

Integrating

the

signal in both directions also

reduces input offset

voltage

due to thermal

effects

in

the

amplifier section of the CR10.

Input offset voltage on

a

single-ended

measurement is less

than 5

microvolts;

the

input offset

voltage

on a differential

measurement

is less

than

1

microvolt.

A

single-ended

measurement

is

quite

satisfactory

in

cases

where

noise is not a

SECTION

13.

CRlO

MEASUREMENTS

problem and

care

is

taken

to

avoid ground

potential

problems.

Channels are available

for

twice as

many

single-ended measurements.

A

single-ended

measurement

takes about

half

the

time

of a

differential

measurement,

which

is

valuable

in

cases

where

rapid

sampling

is

a

requirement.

NOTE:

Sustained

voltages

in excess of

+16 VDC

applied

to

the analog

inputs will

damage

the

CR10

input

circuitry.

13.3

THE

EFFECT OF

SENSOR LEAD

LENGTH

ON

THE SIGNAL

SETTLING

TIME

Whenever an

analog

input is

switched

into

the

CR10 measurement

circuitry

prior

to

making a

measurement,

a

finite

amount

of

time

is

required for

the

signal to stabilize

at

its

correct

value.

The

rate at which

the

signal settles

is

determined by

the

input settling time

constant

which

is a

function

of both

the

source

resistance, and input

capacitance

(explained

below).

The

CR10 allows a

450

ps settling

time

before

initiating

the measurement.

In

most

applications

this

settling

time

is

adequate,

but

the

additional wire

capacitance

associated with

long sensor leads

can

increase the settling time

constant to the point

that

measurement errors

may

occur.

There

are

three

potential sources of

error

which

must

settle

before

the

measurement

is made:

1.

The signal must

rise

to

its

correct value.

2.

A

smalltransient

(-5

mV) caused by

switching the

analog

input into the

measurement

circuitry

must settle.

3.

A

larger

transient,

usually about 40

mVAy',

caused

by

the

switched, precision excitation

voltage

used in resistive bridge

measurements

must settle.

The

purpose of this

section

is

to

bring attention

to

potential measurement errors caused when

the

input settling time constant gets

too

large

and

to

discuss

procedures whereby the effects

of lead length on

the

measurement

can

be

estimated.

ln

addition, physicalvalues

are given

for three types of

wire

used in CSI sensors, and

error estimates

for

given lead lengths are

provided.

Finally, techniques are discussed

for

minimizing input settling error

when

long leads

are mandatory.

13-3

Campbell Scientific CR10 CR10 Measurement and Control - Page 169

Sensor Lead, Length, The Signal

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