ETC THAT1420

T H AT Corporation
OutSmartsä Balanced Line Drivers
THAT 1420, 1430
FEATURES
APPLICATIONS
OutSmartsÔ technology tames
clipping behavior into single-ended
loads
·
Differential Line Driver
·
Audio Mix Consoles
·
Pin-compatible with SSM2142
·
Distribution Amplifiers
·
Balanced, floating output delivers
transformer-like behavior
·
Audio Equalizers
·
Dynamic Range Processors
·
Stable when driving long cables
and capacitive loads
·
Digital Effects Processors
·
THAT 1430 delivers low output
offset voltage using single capacitor
·
Telecommunications Systems
·
Instrumentation
·
Hi-Fi Equipment
·
Description
The THAT 1420 and 1430 are a new generation of audio differential line drivers with improved
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 protected.
In addition both models incorporate patented
OutSmartsÔ technology, a dual feedback-loop design that prevents the excessive ground currents
typical of cross-coupled output stages (CCOS)
when clipping into single-ended loads1.
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-
THAT 1420
10k
Vcc
50
OutCEXT
Sens+
5k
In+
CinGnd
10k
Dout-
Din+
10k
Cin+
5k
10p
20k
AD & AC
Din-
Dout+
10k
10k
20k
Sens-
10k
Vee
CEXT
50
Out+
Figure 1. THAT 1420 Equivalent Circuit Diagram
ing the design inherently more stable and less
sensitive to component tolerances than the CCOS.
Most importantly, the dual-feedback design prevents 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 single-ended loads.
Where minimum output offset voltage with
minimum parts count is desired, the THAT 1430
further improves over existing designs. In conventional CCOS circuits, two relatively high-value
electrolytic capacitors are required to reduce the
offset voltage. By contrast, the THAT 1430 topology requires only a resistor and a single film or
ceramic capacitor to achieve the same effect at
lower parts count and price.
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
1. See Gary Hebert’s paper, An Improved Balanced, Floating Output Driver IC, presented at the 108th AES Convention, Feb. 2000
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
SPECIFICATIONS 2
Absolute Maximum Ratings (T A = 25°C)
Positive Supply Voltage (Vcc)
+18 V
Operating Temperature Range (TOP)
-40 to +85°C
Negative Supply Voltage (Vee)
-18 V
Storage Temperature (TST)
-40 to +150°C
Output Short Circuit Duration
Continuous
Junction Temperature (TJ)
150°C
Power Dissipation (PD)
TBD mW
Lead Temperature (TLEAD)(Soldering 60 sec)
300°C
Electrical Characteristics 3
Parameter
Symbol
Min.
Typ.
4
5
Balanced
4.35
4.65
4.95
dB
Single Ended
4.4
4.6
4.8
dB
Balanced
5.8
6
6.2
dB
Single Ended
5.8
6
6.2
dB
PSRR
±4V to ±18V
80
105
dB
OCMRR
f=1kHz, BBC Method
50
68
dB
SBR
f=1kHz, BBC Method
28
40
dB
THD+N (Balanced)
THD+N1
20Hz-20kHz
1kHz
0.001
0.0005
%
%
THD+N (Single Ended)
THD+N2
VO=10 VRMS, RL=600W, 20Hz-20kHz
0.0018
%
SNR
Bal. Mode, 20 kHz BW
-104
dBV
Headroom
HR
0.1% THD+N
25
dBV
Slew Rate
SR
16
V/mS
Input Impedance
ZIN
Gain
G1
Gain
G2
DC Power Supply
Rejection Ratio
Output Common-Mode
Rejection Ratio
Output Signal Balance Ratio
Output Noise
Conditions
Max.
Units
kW
RL=600W
RL=100kW
Output Common Mode
Voltage Offset
VOCM
RL=600W, w/o Sense capacitors
-300
±60
300
mV
THAT1420
VOCM
RL=600W, w/ Sense capacitors
-6
±4
6
mV
Output Common Mode
Voltage Offset
VOCM
RL=600W, w/o Sense capacitor
-400
±80
400
mV
THAT1430
VOCM
RL=600W, w/ Sense capacitor
-20
±10
20
mV
2. All specifications are subject to change without notice.
3. All measurements taken with VS=±18, T=25°C, unless otherwise noted
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
Preliminary Information
Page 3
Electrical Characteristics (cont’d.)
Parameter
Differential Output Offset
Symbol
Conditions
Min.
Typ.
Max.
Units
VOOD
RL=600W
-10
±4
10
mV
Differential Output
Voltage Swing,Pos
VIN = ±18V
VCC-2
V
Differential Output
Voltage Swing,Neg
VIN = ±18V
VEE+2
V
Output Impedance
ZO
Quiescent Supply Current
IS
Short Circuit Output Current
ISC
40
Unloaded, VIN = 0
60
Voltage Supply Range
50
60
W
4
5.2
mA
70
±4
mA
±18
V
Theory of Operation
OutSmartsÔ 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, Cin+/- and Din+/-, and complementary outputs,
Vout+ and Vout-, related to each other by two gain ex-
pressions, AD(s) and AC(s). The first pair of differential inputs, Din+/-, are connected to the differential
feedback network between the outputs and the input
signal. The second differential input pair, Cin+/-, is
connected to a bridge circuit which generates an error signal that is used to servo the common-mode behavior of the outputs. The loop equations are then,
DOUT + - DOUT - = D DOUT = AD ( DIN + - DIN - )
where AD is the differential open-loop gain, and
DOUT + + DOUT - =
å DOUT = AC( CIN + - CIN - )
where AC is the common-mode open-loop gain.
THAT 1420
10k
Vcc
50
OutCEXT
Sens+
5k
In+
10k
Cin+
Cin-
Gnd
10k
Dout-
Din+
5k
10p
20k
AD & AC
Din-
Dout+
10k
10k
20k
Sens-
10k
Vee
CEXT
50
Out+
Figure 2. THAT 1420 Equivalent Circuit Diagram
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
THAT 1430
CEXT
Cap2
Cap1
REXT
50
Vcc
Out-
10k
5k
10k
Dout-
Din+
In+
10k
Cin+
Cin5k
Gnd
10p
7k
AD & AC
Din-
Dout+
10k
10k
Vee
10k
7k
50
Out+
Figure 3. THAT 1430 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 feedback results in
DIN + = (13 DOUT - +
DIN - =
1
2
3 In+ )
and
3 DOUT +
Substituting and simplifying into the equation that
defines differential operation yields
D DOUT =
- DD
AD ( 3OUT
+
2
3
In+ )
Dividing through by AD (assuming that AD >> 3) and
simplifying yields
D DOUT = 2 ( In+ )
as one would expect for a +6dB line driver.
The derivation for the common mode equation is
more complicated1 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 assumption, we can intuitively discern the operation of the
common-mode feedback loop as follows:
Referring again to Figures 2 and 3, the common-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 frequencies 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
Rev. 4/24/01
Preliminary Information
Page 5
VCC
VCC
C4
100n
6
In
C6
100n
7
Vcc
8
In Sens+
Out+
3
OutGnd Sens1
Vee
U1
2 THAT1420
5
2
4
7
In-
Vcc
6 Out
Out
Ref
Vee 1
3
In+
U2
4
THAT1243 or equiv.
C5
100n
VEE
C7
100n
VEE
Figure 4. Basic THAT 1420 applications circuit
Applications
Circuit implementations using the THAT
1420/1430 are relatively straightforward. A quiet,
solid ground reference, stiff voltage supplies, and adequate 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 calculation, the line driver’s 50W sense resistors can be
lumped into a single 100W resistor. The correct cable capacitance to use is the sum of the
inter-conductor capacitance and the two conductor-to-shield capacitances. Unfortunately, some manufacturers
only
specify
the
inter-conductor
capacitance and the capacitance of one conductor to
the other while connected to the shield, and some extraction 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
fC =
1
2´p ´100 W´500 ( 30 pF ft + 25 pF ft + 25 pF ft)
» 40kHz
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
= VPeak ´ 2p ´ f
Rane Corporation has published a document titled
4
RaneNote 126 , which specifies some of the requirements for a balanced line driver, including a)
stablility into reactive loads, b) output voltage swing
of at least ±11 volts peak (+20dBu), and c) reliability. This document also suggests a reasonable rule
by which to calculate the output current requirements at 20kHz. The author concludes that the ac-
4. Copyright ã 1991 Rane Corporation
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
Vcc
C4
100n
C1
R1
100n
1M0
Vcc
C6
100n
6
In
7
Vcc
8
4
In Cap1
Out+
3
OutGnd Cap2
1
Vee
U1
2
5
THAT1430
C5
100n
2
7
InVcc
6 Out
Out
Ref
Vee 1
3
In+
U2
4
THAT1243 or equiv.
Vee
C7
100n
Vee
Figure 5. Basic THAT 1430 application circuit with output common mode offset reduction
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 levels 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 VPeak is actually 22V (below 5kHz), and at
20kHz, VPeak is 5.5V. Therefore,
dV
dt
= 2p ´ 5.5V ´ 20kHz = 0.69 mVs
As a consequence,
i = 500 ft ´ (30
pF
ft
+ 25
pF
ft
+ 25
pF
)
ft
´ 0.69 mVs » 28mA
Thus, driving this 40nF cable requires 28mAPeak
(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 absolutely required are the local 100nF bypass capacitors, and these could, in fact, be shared with another
nearby component. There are no common mode output 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 expected, 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 specially designed to allow common mode offset reduction 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 hundreds 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 direct 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
Rev. 4/24/01
Preliminary Information
Page 7
VCC
C1
C4
100n
10u
6
In
7
Vcc
8
In Sens+
Out+
Out3 Gnd
1
SensU1
Vee
2 THAT1420
5
C2
D3
SB160
D4
SB160
4
10u
C5
100n
D5
SB160
Out Hi
L1
Ferrite Bead
C8
100p
L2
Ferrite Bead
D6
SB160
C3
100p
Out Lo
VEE
Figure 6. 1420 with output common mode offset protection, RFI protection, and surge protection
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 directed 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 frequencies.
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 microphone preamplifier input that has +48V phantom
power applied. This situation can result in surge
currents of several amps, which can cause open circuits in the metal traces or failure of the protection
diodes on the IC.
This circuit uses a discrete diode bridge composed 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 designs. The THAT 1420 and 1430 go a step or two
further by improving over existing components. Both
incorporate OutSmartsä technology to tame the aberrant single-ended clipping behavior of conventional
cross-coupled output stages. The THAT 1430's design 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 output stages.
For more information on these or other THAT
Corporation integrated circuits, please contact us directly, or through one of our international distributors.
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
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.
E
J
C
B
F
1
B
C
G
A
H
1
K
F
D
G
A
H
J
D
ITEM
A
B
C
D
E
F
G
H
J
K
E
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±0.004
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
Figure 7. -P (DIP) version package outline drawing
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
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com