THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
THAT 1420, 1430
Description
The THAT 1420 and 1430 are a new genera-
tion of audio differential line drivers with im-
proved
performance
over
conventional
cross-coupled monolithic designs. Both models
exhibit low noise and distortion, high slew rate,
stability under difficult loads, wide output swing,
and have outputs which are short-circuit pro-
tected.
In addition both models incorporate patented
OutSmarts technology, a dual feedback-loop de-
sign that prevents the excessive ground currents
typical of cross-coupled output stages (CCOS)
when clipping into single-ended loads
1
.
To overcome this problem, the THAT 1420
and 1430 use two individual negative-feedback
loops to separately control the differential output
voltage and common mode output currents, mak-
ing the design inherently more stable and less
sensitive to component tolerances than the CCOS.
Most importantly, the dual-feedback design pre-
vents the loss of common-mode feedback that
plagues the CCOS designs, avoiding the excessive
ground currents and overly-distorted output
waveform that can result when driving sin-
gle-ended loads.
Where minimum output offset voltage with
minimum parts count is desired, the THAT 1430
further improves over existing designs. In con-
ventional CCOS circuits, two relatively high-value
electrolytic capacitors are required to reduce the
offset voltage. By contrast, the THAT 1430 topol-
ogy requires only a resistor and a single film or
ceramic capacitor to achieve the same effect at
lower parts count and price.
T H A T
C o r p o r a t i o n
OutSmarts Balanced Line Drivers
FEATURES
OutSmarts technology tames
clipping behavior into single-ended
loads
Pin-compatible with SSM2142
Balanced, floating output delivers
transformer-like behavior
Stable when driving long cables
and capacitive loads
THAT 1430 delivers low output
offset voltage using single capacitor
APPLICATIONS
Differential Line Driver
Audio Mix Consoles
Distribution Amplifiers
Audio Equalizers
Dynamic Range Processors
Digital Effects Processors
Telecommunications Systems
Instrumentation
Hi-Fi Equipment
Din+
Cin+
Cin-
Din-
Out-
Sens+
Out+
Sens-
Vcc
In+
Vee
Gnd
Dout-
Dout+
10k
10k
50
5k
5k
10k
10k
10k
10k
20k
20k
10p
50
THAT 1420
C
EXT
C
EXT
D
C
A
A
&
Figure 1. THAT 1420 Equivalent Circuit Diagram
1. See Gary Hebert's paper, An Improved Balanced, Floating Output Driver IC, presented at the 108th AES Convention, Feb. 2000
DIP Pin
Number
SO Pin
Number
1420 Pin
Name
1430 Pin
Name
1
3
Out-
Out-
2
4
Sens-
Cap2
3
5
Gnd
Gnd
4
6
In
In
5
11
Vee
Vee
6
12
Vcc
Vcc
7
13
Sens+
Cap1
8
14
Out+
Out+
Table 1. THAT 1420/1430 pin assignments
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 2
THAT1420/1430 Balanced Line Driver
Preliminary Information
Absolute Maximum Ratings (T
A
= 25C)
Positive Supply Voltage (Vcc)
+18 V
Operating Temperature Range (T
OP
)
-40 to +85C
Negative Supply Voltage (Vee)
-18 V
Storage Temperature (T
ST
)
-40 to +150C
Output Short Circuit Duration
Continuous
Junction Temperature (T
J
)
150C
Power Dissipation (P
D
)
TBD mW
Lead Temperature (T
LEAD
)(Soldering 60 sec)
300C
SPECIFICATIONS
2
2. All specifications are subject to change without notice.
3. All measurements taken with V
S
=18, T=25C, unless otherwise noted
Electrical Characteristics
3
Parameter
Symbol
Conditions
Min.
Typ.
Max.
Units
Input Impedance
Z
IN
4
5
kW
Gain
G1
R
L
=600W
Balanced
4.35
4.65
4.95
dB
Single Ended
4.4
4.6
4.8
dB
Gain
G2
R
L
=100kW
Balanced
5.8
6
6.2
dB
Single Ended
5.8
6
6.2
dB
DC Power Supply
Rejection Ratio
PSRR
4V to 18V
80
105
dB
Output Common-Mode
Rejection Ratio
OCMRR
f=1kHz, BBC Method
50
68
dB
Output Signal Balance Ratio
SBR
f=1kHz, BBC Method
28
40
dB
THD+N (Balanced)
THD+N
1
20Hz-20kHz
0.001
%
1kHz
0.0005
%
THD+N (Single Ended)
THD+N
2
V
O
=10 V
RMS
, R
L
=600W, 20Hz-20kHz
0.0018
%
Output Noise
SNR
Bal. Mode, 20 kHz BW
-104
dBV
Headroom
HR
0.1% THD+N
25
dBV
Slew Rate
SR
16
V/mS
Output Common Mode
Voltage Offset
V
OCM
R
L
=600W, w/o Sense capacitors
-300
60
300
mV
THAT1420
V
OCM
R
L
=600W, w/ Sense capacitors
-6
4
6
mV
Output Common Mode
Voltage Offset
V
OCM
R
L
=600W, w/o Sense capacitor
-400
80
400
mV
THAT1430
V
OCM
R
L
=600W, w/ Sense capacitor
-20
10
20
mV
Theory of Operation
OutSmarts technology
The THAT 1420 and 1430 are similar devices,
both employing the OutSmarts topology, a variation
of circuitry originally developed at Audio Toys, Inc.
OutSmarts topology employs two negative-feedback
loops -- one to control the differential signal, and a
separate loop to control the common mode output
levels.
Figures 2 and 3 show the gain core common to
both the THAT 1420 and 1430. The gain core is a
single amplifier that includes two differential input
pairs, C
in+/-
and D
in+/-
, and complementary outputs,
V
out+
and V
out-,
related to each other by two gain ex-
pressions, A
D
(s) and A
C
(s). The first pair of differen-
tial inputs, D
in+/-
, are connected to the differential
feedback network between the outputs and the input
signal. The second differential input pair, C
in+/-
, is
connected to a bridge circuit which generates an er-
ror signal that is used to servo the common-mode be-
havior of the outputs. The loop equations are then,
(
)
D
D
D
A D
D
OUT
OUT
OUT
D
IN
IN
+
-
+
-
-
=
=
-
D
where A
D
is the differential open-loop gain, and
(
)
D
D
D
A C
C
OUT
OUT
OUT
C
IN
IN
+
-
+
-
+
=
=
-
where A
C
is the common-mode open-loop gain.
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 3
Preliminary Information
Electrical Characteristics (cont'd.)
Parameter
Symbol
Conditions
Min.
Typ.
Max.
Units
Differential Output Offset
V
OOD
R
L
=600W
-10
4
10
mV
Differential Output
Voltage Swing,Pos
V
IN
= 18V
V
CC
-2
V
Differential Output
Voltage Swing,Neg
V
IN
= 18V
V
EE
+2
V
Output Impedance
Z
O
40
50
60
W
Quiescent Supply Current
I
S
Unloaded, V
IN
= 0
4
5.2
mA
Short Circuit Output Current
I
SC
60
70
mA
Voltage Supply Range
4
18
V
Din+
Cin+
Cin-
Din-
Out-
Sens+
Out+
Sens-
Vcc
In+
Vee
Gnd
Dout-
Dout+
10k
10k
50
5k
5k
10k
10k
10k
10k
20k
20k
10p
50
THAT 1420
C
EXT
C
EXT
D
C
A
A
&
Figure 2. THAT 1420 Equivalent Circuit Diagram
These equations can be solved much like standard
op-amp loop equations, and for the differential case,
we can see that (using superposition) resistor feed-
back results in
D
D
In
IN
OUT
+
-
+
=
+
(
)
1
3
2
3
and
D
D
IN
OUT
-
+
=
1
3
Substituting and simplifying into the equation that
defines differential operation yields
D
D
D
A
In
OUT
D
D
OUT
=
+
-
+
(
)
3
2
3
Dividing through by A
D
(assuming that A
D
>> 3) and
simplifying yields
( )
D D
In
OUT
=
+
2
as one would expect for a +6dB line driver.
The derivation for the common mode equation is
more complicated
1
in that it is dependent on the at-
tached load, and in any event doesn't yield much
insight into the device's operation.
In op-amp analysis or in the above derivation, the
combination of negative feedback and high open-loop
gain results in the open-loop gain "dropping out" of
the equation, and the differential inputs being forced
to the same potential. If we start with that assump-
tion, we can intuitively discern the operation of the
common-mode feedback loop as follows:
Referring again to Figures 2 and 3, the com-
mon-mode input actually senses the sum of the IC's
output currents by way of two 50 ohm resistors and
the bridge network (the 10pF capacitor simply limits
the maximum frequency at which this action occurs).
The resulting error signal is amplified and then
summed into both outputs, with the net effect being
to force the sum of the currents to be zero, and thus
the common mode output current to zero. Since this
is negative feedback, the common-mode loop can
raise the effective output impedance at audio fre-
quencies without the side effects of circuits that use
positive feedback to implement this function.
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 4
THAT1420/1430 Balanced Line Driver
Preliminary Information
Out-
Cap1
Cap2
Out+
Vcc
In+
Vee
Gnd
10k
10k
50
5k
5k
10k
10k
10k
10k
7k
7k
10p
50
THAT 1430
C
EXT
R
EXT
Din+
Cin+
Cin-
Din-
Dout-
Dout+
D
C
A
A
&
Figure 3. THAT 1430 Equivalent Circuit Diagram
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
f
kHz
C
pF
ft
pF
ft
pF
ft
=
+
+
1
2
100
500 30
25
25
40
p
W
(
)
One must also consider the slew rate limitations
posed by excessive cable and other capacitances. We
know that
i C
dV
dt
=
and that
dV
dt
Peak
V
f
=
2p
Rane Corporation has published a document titled
RaneNote 126
4
, which specifies some of the require-
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 1991 Rane Corporation
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
11V, the V
Peak
is actually 22V (below 5kHz), and at
20kHz, V
Peak
is 5.5V. Therefore,
dV
dt
V
s
V
kHz
=
=
2
5 5
20
0 69
p
m
.
.
As a consequence,
i
ft
mA
pF
ft
pF
ft
pF
ft
V
s
=
+
+
500
30
25
25
0 69
28
(
)
.
m
Thus, driving this 40nF cable requires 28mA
Peak
(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
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 OutSmarts 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
Package Information
The THAT1420/1430 are available in both 8-pin
mini-DIP and 16-pin SOIC packages. The package
dimensions are shown in Figures 7 and 8, while
pinouts are given in Table 1.
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 8
THAT1420/1430 Balanced Line Driver
Preliminary Information
B
A
K
F
H
E
D
G
J
C
ITEM
A
B
C
D
E
F
G
H
J
K
1
MILLIMETERS
9.52 0.10
6.35 0.10
7.49/8.13
0.46
2.54
3.68/4.32
0.25
3.18 0.10
8.13/9.40
3.30 0.10
INCHES
0.375
0.250 0.004
0.295/0.320
0.018
0.100
0.145/0.170
0.010
0.125 0.004
0.320/0.370
0.130 0.004
0.004
Figure 7. -P (DIP) version package outline drawing
D
E
B
C
J
A
F
H
G
1
ITEM
A
B
C
D
E
F
G
H
J
MILLIMETERS
10.11/10.31
7.40/7.60
10.11/10.51
0.36/0.46
1.27
2.44/2.64
0.23/0.32
0.51/1.01
0.10/0.30
INCHES
0.398/O.406
0.291/0.299
0.398/0.414
0.014/0.018
0.050
0.096/0.104
0.009/0.013
0.020/0.040
0.004/0.012
Figure 8. -S (SO) version package outline drawing
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