10

®

3650/52

APPLICATIONS

Figure 8 shows a system where isolation amplifiers (3650)

are used to measure the armature current and the armature

voltage of a motor.

The armature current of the motor is converted to a voltage

gain) and isolated by the 3650.

The armature voltage is sensed by the voltage divider (ad-

justable) shown and then amplified and isolated by the 3650.

The 3650 provides the advantage of accurate current mea-

surement in the presence of high common-mode voltage.

Both 3650s provide the advantage of isolating the motor

ground from the control system ground. Isolated power is

provided by an isolated DC/DC converter (BB Model 722 or

equivalent).

The 3652 is ideally suited for patient monitoring applica-

tions as shown in Figure 9. The fact that it is a true balanced

input instrumentation amplifier with very high differential

and common-mode impedance means that it can greatly

reduce the common-mode noise pick up due to imbalance in

lead impedances that often appear in patient monitoring

situations. The 3kV and 6kV shown in Figure 9 are the 10ms

and differential input voltages with respect to input com-

mon. The rating of the isolation barrier is 2000Vpk continu-

FIGURE 8. Isolated Armature Current and Voltage Sensor.

P+

V–

+V

O

3650HG

11

10

12

20

4.99k

Ω

+V

CC

23

G = 1V/V

To

Voltage

Sense

C

–V

O

722

1.3k

Ω

E

V+

1M

Ω

4.99k

Ω

500

Ω

9.76k

Ω

14

13

17

26

–V

CC

O/P Com

+V

–V

I/O Com

3650JG

11

10

20

4.75k

Ω

23

4.99k

Ω

14

12

13

+V

–V

V

To

Current

Sense

17 O/P Com

100V

S

26

+V

CC

–V

CC

+15VDC

–15VDC

O/P Com

G = 100V/V

500

Ω

V

A

(500V)

V

S

(100mV)

R

S

V

OUT

+

–

+

–

V

CM

CMRR

IMRR

COMMON-MODE AND

ISOLATION-MODE REJECTION

The expression for the output error due to common-mode

and isolation mode voltage is:

+

GUARDING AND PROTECTION

To preserve the excellent inherent isolation characteristics of

these amplifiers, the following recommended practice should

be noted.

1. Use shielded twisted pair of cable at the input as with

any instrumentation amplifier.

2. Care should be taken to minimize external capacitance.

A symmetrical layout of external components to achieve

balanced capacitance from the input terminals to output

common will preserve high IMR.

3.

External components and conductor patterns should be

at a distance equal to or greater than the distance

between the input and output terminals to prevent HV

breakdown.

4.

Though not an absolute requirement, the use of lamin-

ated or conformally coated printed circuit boards is

recommended.

V

ISO