tors whose purpose is to redirect this RF energy to

the chassis before it can circulate and effectively form

a single loop transformer that magnetically couples

RF into the remainder of the circuit. Ferrite beads

are also included to ensure that RFI current is di-

rected to the chassis and not through the relatively

low impedance (at RF frequencies) output of the

THAT 1420/1430. The devices will have no effect on

the gain error of these line drivers at audio frequen-

cies.

While both of these chips have diode protection to

the rails, this protection might not be adequate for

some conditions seen in the field. The most obvious

problem that one might foresee would be having the

line driver’s output plugged directly into a micro-

phone preamplifier input that has +48V phantom

power applied. This situation can result in surge

currents of several amps, which can cause open cir-

cuits in the metal traces or failure of the protection

diodes on the IC.

This circuit uses a discrete diode bridge com-

posed of SB160’s to clamp potentially damaging

surges to the IC’s supply rails.

Closing thoughts

The integrated balanced line driver is one of those

highly useful, cost-effective functional blocks that can

provide significant improvement over discrete de-

signs. The THAT 1420 and 1430 go a step or two

further by improving over existing components. Both

incorporate OutSmartsä technology to tame the ab-

errant single-ended clipping behavior of conventional

cross-coupled output stages. The THAT 1430's de-

sign gives reasonably low output offset voltage with

only a resistor and a single film or ceramic capacitor,

though it is not pin-compatible with existing IC out-

put stages.

For more information on these or other THAT

Corporation integrated circuits, please contact us di-

rectly, or through one of our international distribu-

tors.

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 7

Preliminary Information

In

4

Gnd

3

Out-

1

Out+

8

U1

THAT1420

In

VCC

VEE

C1

10u

C2

10u

C4

100n

C5

100n

D3

SB160

D4

SB160

D5

SB160

D6

SB160

C3

100p

C8

100p

L1

Ferrite Bead

L2

Ferrite Bead

Out Hi

Out Lo

5

6

7

2

Vee

Sens-

Sens+

Vcc

Figure 6. 1420 with output common mode offset protection, RFI protection, and surge protection