8

®

3650/52

+

C

R

G1

R

G2

R

IN

V

ISO

V

2

11

10

12

17

(1)

V

1

V

OUT

–

23

+

–

C

NOTE: (1) The offset adjustment circutry and power supply connections

have been omitted for simplicity. Refer to Figure 5 for details.

10

6

Ω

R

V

V

ISO

IMRR

FIGURE 6a. 3650 with Differential Current Sources.

FIGURE 6b. 3650 with Differential Voltage Sources.

ERROR ANALYSIS

A model of the 3650 suitable for DC error analysis of offset

voltage, voltage drift versus temperature, bias current, etc.,

is shown in Figure 7.

ered to be ideal. Separate external generators are used to

from the gain equation. The feedback configuration, optics

simple circuit analysis gives the following expression for the

+

C

R

IN

V

ISO

I

2

11

10

12

17

(!)

I

1

V

OUT

–

23

+

–

C

NOTE: (1) The offset adjustment circutry and power supply connections

have been omitted for simplicity. Refer to Figure 5 for details. (2) IMRR

here is in pA/V, typically 5pA/V at 60Hz and 1pA/V at DC.

V

(2)

FIGURE 5. Power and Offset Adjust Connections.

NOTE: (1) Optional Offset Adjust.

+V

CC

–V

CC

R

IN

+15VDC

50k

15

16

3M

12

C

Bal

14

13

+V

O

C

–V

O

722

1.3k

Ω

32

29

17

23

C

20

26

10k

–15VDC

Output

Output

Common

P+

V+

E

V–

Bal

Model 722 DC/DC

converter or equivalent

+V

–V

+

–

INPUT CONFIGURATIONS

Some possible input configurations for the 3650 and 3652

are shown in Figures 6a, 6b, 6c. Differential input sources

are used in these examples. For situations with nondifferential

inputs, the appropriate source term should be set to zero in

the gain equations and replaced with a short in the diagrams.

Figure 6a shows the 3650 connected as a transconductance

amplifier with input current sources. Voltage sources are

shown in Figure 6b. In this case the voltages are converted

perform as gain setting resistors in the voltage transfer

function. When a single voltage source is used, it is recom-

mended (but not essential) that the gain setting resistor

remain split into two equal halves in order to minimize

errors due to bias currents and common-mode rejection (see

Typical Performance Curves).

Figure 6c illustrates the connections for the 3652 when the

tion provides an isolation amplifier with high input imped-

ance (both common-mode and differential, and good com-

mon-mode and isolation-mode rejection. It is a true isolated

instrumentation amplifier which has many benefits for noise

rejection when source impedance imbalances are present.

In the 3652, the voltage gain of the buffer amplifiers is

slightly less than unity, but the gain of the output stage has

been raised to compensate for this so that the overall transfer

function from the

±I or ±I

summing can be done at the

±I or ±I

shows the +I and –I inputs used. If more input voltage

used. This will increase the input noise due to the contribu-

tion from the 1.6M

Ω resistors, but will provide additional

differential and common-mode protection (10ms rating of

3kV).