Applications

Circuit

implementations

using

the

THAT

1420/1430 are relatively straightforward. A quiet,

solid ground reference, stiff voltage supplies, and ad-

equate supply bypassing are all that is required to

achieve excellent performance out of both ICs. Both

devices are stable into any capacitive load, and the

maximum capacitance is limited only by slew rate

and frequency response considerations.

For the purposes of the frequency response calcu-

lation, the line driver’s 50W sense resistors can be

lumped into a single 100W resistor. The correct ca-

ble capacitance to use is the sum of the

inter-conductor capacitance and the two conduc-

tor-to-shield capacitances. Unfortunately, some man-

ufacturers

only

specify

the

inter-conductor

capacitance and the capacitance of one conductor to

the other while connected to the shield, and some ex-

traction may be required.

As an example, one manufacturer supplies a

shielded, twisted pair with 30pF/ft of inter-conductor

capacitance and 25pF/ft of conductor to shield capac-

itance. The corner frequency of the THAT 1420/1430

driving 500 ft of this cable will be

fkHz

C

pF

ft

pF

ft

pF

ft

=»

´´

´

+

+

1

2

100

500 30

25

25

40

pW

()

One must also consider the slew rate limitations

posed by excessive cable and other capacitances. We

know that

dt

=

and that

dV

dt

Peak

Vf

=´

´

2p

Rane Corporation has published a document titled

ments for a balanced line driver, including a)

stablility into reactive loads, b) output voltage swing

of at least ±11 volts peak (+20dBu), and c) reliabil-

ity. This document also suggests a reasonable rule

by which to calculate the output current require-

ments at 20kHz. The author concludes that the ac-

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com

Rev. 4/24/01

Page 5

Preliminary Information

In

4

Gnd

3

5

2

Out-

1

Out+

8

7

6

U1

THAT1420

In

VCC

VEE

1

In-

2

In+

3

4

7

Out

6

U2

THAT1243 or equiv.

VEE

VCC

Out

C4

100n

C5

100n

C6

100n

C7

100n

Vcc

Sens+

Vee

Sens-

Vcc

Vee

Ref

Figure 4. Basic THAT 1420 applications circuit

4. Copyright ã 1991 Rane Corporation