®

ISC300

7

CKI

CKI

CKI

CKI

CKI

CKI

Optional Clock Driver

MEASUREMENT CHANNEL

CALIBRATION

The ISC300 is designed to allow easy system calibration

and 50 to be measured.

System calibration would typically proceed as follows:

Lab Calibration

• Set ISC300 gain.

• Set input to 0V reference, measure Offset.

• Connect external precision V reference, measure Gain.

• Remove external V reference and set input to 10V or

0.1V reference.

Offset and Gain are now calibrated to an external preci-

sion reference—record the numbers.

Field Calibration

• Set ISC300 gain.

• Set input to 0V reference, measure Offset.

• Set input to 10V or 0.1V, measure Gain.

• Recalibrate system.

SYNCHRONIZATION

As the internal modulation frequencies of several ISC300s

can be marginally different, ‘beat’ frequencies ranging from

a few Hz to a few kHz can exist in multi ISC300 applica-

tions. The internal clock (see Figure 5) starts when power is

applied and runs at typically 50kHz. The ISC300 design

accommodates ‘internal synchronous’ noise which is caused

by minute timing differences, but synchronous beat fre-

quency noise will not be strongly attenuated, especially at

low frequencies if it is introduced via the power, signal or

ground paths. To overcome this problem, the design allows

the synchronization of each oscillator in the system to one

frequency. This is done by connecting the CKI (clock in)

39k

220pF

CKI

f = 50kHz

Simplified Schematic

Ω

FIGURE 5. CKI Input.

pins of each ISC300 in the system together (see Figure 6).

The ISC300 can also be synchronized by an external clock

driver.

INPUT

GAIN

SELECT

SELECT

SELECT

G

AND GAIN

RST

CLK

Com 1

0

0

0.5

0

No Change

1

0

+0.1V

0

1

50

1

No Change

1

1

+10V

1

0

Latch

1

^

Signal

1

1

RESET

0

X

FIGURE 7. Mode Selection Jumpers.

FIGURE 6. Synchronizing Multi-ISC300 Applications.

NOISE

Output noise is generated by the residual components of the

25kHz carrier that have not been removed from the signal.

This noise may be reduced by adding an output low pass

filter (see Figure 15 for an example of a 2 pole filter with

amplification, giving a

±10V output). The filter time con-

stants should be set below the carrier frequency. The output

of the ISC300 is a switched capacitor and requires a high

impedance load to prevent degradation of linearity. Loads of

less than 1M

Ω will cause an increase in noise at the carrier

frequency and will appear as ripple in the output waveform.

APPLICATIONS

This section describes the design criteria of various applica-

tions of the ISC300.

2, 3 AND 4 WIRE RESISTANCE MEASUREMENTS

Two wire resistance measurements are prone to errors due to

lead resistances. The voltage error can be significant since

the voltmeter measures on the lines supplying the RTD

+V

ISO

Sense 2

–In

+In

Filter and

MUX

Com 1

–In

+In

–V

ISO

I

REF2

–V

ISO

I

REF1

–V

ISO

Sense 1

Com 1

Com

3

1

2

J

1

1

J

2

2

3

Voltage

RTD

J

1

1-2

2-3

J

2

1-2

2-3

Mode

Jumper

PGA

20M

Ω

20M

Ω