7

®

3650/52

function of the accuracy of the matching and is further

enhanced by the use of negative feedback in the input stage.

Advanced laser trimming techniques are used to further

compensate for residual matching errors.

A model of the 3650 suitable for simple circuit analysis is

shown in Figure 3. The output is a current dependent voltage

the 3650 is a transconductance amplifier with a gain of one

volt per microamp. When voltage sources are used, the input

current is derived by using gain setting resistors in series

with the voltage source (see Installation and Operating

ance. The common-mode and isolation impedances are very

high and are assumed to be infinite for this model.

FIGURE 3. Simple Model of 3650.

A simplified model of the 3652 is shown in Figure 4. The

isolation and output stages are identical to the 3650. Addi-

tional input circuitry consisting of FET buffer amplifiers and

input protection resistors have been added to give higher

Ω),

lower bias currents (50pA) and overvoltage protection. The

differential and 3000V common-mode (see Definitions for a

discussion of common-mode and isolation-mode voltages.)

The addition of the buffer amplifiers also creates a voltage-

INSTALLATION AND

OPERATING INSTRUCTIONS

POWER SUPPLY CONNECTIONS

The power supply connections for the 3650 and 3652 are

shown in Figure 5. When a DC/DC converter is used for

isolated power, it is placed in parallel with the isolation

barrier of the amplifier. This can lower the isolation imped-

ance and degrade the isolation-mode rejection of the overall

circuit. Therefore, a high quality, low leakage DC/DC con-

verter such as the Burr-Brown Model 722 should be used.

OFFSET VOLTAGE ADJUSTMENTS

The offset nulling circuits are identical for the 3650 and

3652 and are shown in Figure 5. The offset adjust circuitry

is optional and the units will meet the stated specifications

with the BAL terminals unconnected. Provisions are avail-

able to null both the input and output stage offsets. If the

amplifier is operated at a fixed gain, normally only one

adjustment will be used: the output stage (10k

Ω adjustment)

for low gains and the input stage (50k

Ω adjustment) for high

gains, (

>10).

Use the following procedure if it is desired to null both input

and output components. (For example, if the gain of the

amplifier is to be switched). The input stage offset is first

nulled (50k

Ω adjustment) with the appropriate input signal

pins connected to input common and the amplifier set at its

maximum gain. The gain is then set to its minimum value

and the output offset is nulled (10k

Ω adjustment).

FIGURE 4. Simple Model of 3652.

(Output)

+

R

IN

+

–

23

10

11

I

IN

(Input)

C

12

14

13

+V

–V

R

OUT

V

D

–

17

+V

CC

–V

CC

26

20

V

1V

µA

C

R

G1

R

G2

11

10

C

(Output)

23

C (Input)

A

1

A

2

R

O

2

R

O

2

8

9

–I

R

+I

R

+I

–I

I

IN

Same as 3650 in Figure 3.

1.6M

Ω

1.6M

Ω

17

6

4

3

1

12