tual worst case peak level for various types of music

and speech will be flat out to 5kHz, and roll off at

6dB/octave above this frequency. Thus the peak lev-

els at 20kHz will be 12dB below those at 5kHz.

Using these, we can calculate the required slew

rate and current drive. Since both outputs can swing

dV

dt

V

s

VkHz

=´

´

=

25 5

20

0 69

p

m

..

As a consequence,

ift

mA

pF

ft

pF

ft

pF

ft

V

s

=´

+

+

´

»

500

30

25

25

0 69

28

()

.

m

(well within the 1420/1403’s capabilities). Figure 4

shows the most basic connection between the THAT

1420 and a typical line receiver (like the THAT

1243). The only external components that are abso-

lutely required are the local 100nF bypass capaci-

tors, and these could, in fact, be shared with another

nearby component. There are no common mode out-

put offset reduction capacitors, and the line driver’s

outputs are connected directly to their respective

sense inputs. The outputs are also DC coupled to the

line receiver. If large common mode voltages are ex-

pected, the designer may choose to incorporate large,

non-polarized capacitors to isolate the THAT 1420’s

outputs.

Figure 5 shows the basic THAT 1430 applications

circuit. This circuit includes external components

for common mode offset reduction. This IC is spe-

cially designed to allow common mode offset reduc-

tion with only a small resistor and capacitor, and is

ideal for new designs where space is at a premium.

Other considerations that apply to the THAT 1420

apply to the THAT 1430.

Figure 6 shows a THAT 1420 with common mode

offset reduction, RFI protection and surge protection,

but these last two additions could be added to the

THAT 1430 as well. One should also note that the

THAT 1420 is pin-for-pin compatible with industry

standard line drivers.

These line drivers can easily drive cables hun-

dreds of feet in length without becoming unstable,

but attaching such a long cable can act as an antenna

(even for AM stations) which can pick up RFI and di-

rect it into the circuit. C3 and C8 are 100pF capaci-

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

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

Page 6

THAT1420/1430 Balanced Line Driver

Preliminary Information

In

4

Gnd

3

5

2

Out-

1

Out+

8

7

6

U1

THAT1430

In

Vcc

Vee

C1

100n

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

Cap1

Vee

Cap2

Vcc

Vee

Ref

R1

1M0

Figure 5. Basic THAT 1430 application circuit with output common mode offset reduction