InGenius
High-CMRR
Balanced Input Line Receiver ICs
THAT
1200, 1203, 1206
FEATURES
•
High CMRR: typ. 90 dB at 60 Hz
•
Extremely high common-mode input
impedance
• Maintains balance under real-world
conditions
•
Transformer-like performance in an IC
•
Excellent audio performance
• Wide bandwidth: typ. > 22 MHz
• High slew rate: typ. 12 V/us
• Low distortion: typ. 0.0005 % THD
• Low noise: typ. -106 dBu
•
Several gains: 0 dB, -3 dB, & -6 dB
APPLICATIONS
•
Balanced Audio Line Receivers
•
Instrumentation Amplifiers
•
Differential Amplifiers
•
Transformer Front-End Replacements
•
ADC Front-Ends
Description
The THAT 1200-series InGenius balanced
line receivers overcome a serious limitation of
conventional balanced input stages: poor com-
mon mode rejection in real-world applications.
While conventional input stages measure well in
the lab and perform well on paper, they fail to
live up to their CMRR specs when fed from even
slightly unbalanced source impedances — a
common situation in almost any pro sound envi-
ronment. This is because conventional stages
have low common-mode input impedance, which
interacts with imbalances in source impedance to
unbalance common-mode signals, making them
indistinguishable from desired, balanced signals.
Developed by Bill Whitlock of Jensen Trans-
formers, the patented InGenius input stage uses
clever bootstrapping to raise its common-mode
input impedance into the megohm range without
the noise penalty from the obvious solution of
using high-valued resistors. Like transformers,
InGenius line receivers maintain their high
CMRR over a wide range of source impedance
imbalances — even when fed from single-ended
sources. But unlike transformers, these wide
bandwidth solid state devices offer dc-coupling,
low distortion, and transparent sound in a small
package at reasonable cost.
Pin Name
R6
DIP Pin
1
2
3
4
5
6
7
8
SO Pin
1
2
3
4
5
6
7
8
IN-
R7
OA1
+1
R10
24K
R1
R2
Vcc
Vee
Vout
Ref
In-
In+
Vee
CM In
Vout
REF
OA4
+1
OA3
-
+
R4
R5
24K
R8
IN+
R9
R11
24K
+1
OA2
R3
Vcc
CM Out
CM IN
CM OUT
Table 1. 1200-series pin assignments
Cb
Gain
Part no.
THAT1200
THAT1203
THAT1206
R6 , R9
0
7k
7k
R7 , R8
24 k
17 k
17 k
R1 , R3
6k
6k
7k
R2 , R4
6k
6k
5k
Plastic DIP
1200P
1203P
1206P
Plastic SO
1200S
1203S
1206S
0 dB
-3 dB
-6 dB
Figure 1. THAT 1200-series equivalent circuit diagram
Table 2. Ordering information
Protected under U.S. Patent Numbers, 5,568,561 and 6,160,451. Additional patents pending.
InGenius is a registered trademark of THAT 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
Copyright © 2017, THAT Corporation; All rights reserved; Doc 600033 Rev 01
InGenius High-CMRR
Balanced Input Line Receiver ICs
Page 2 of 12
Document 600033 Rev 01
SPECIFICATIONS
1
Absolute Maximum Ratings (T
A
= 25° C)
Positive Supply Voltage (V
CC
)
Negative Supply Voltage (V
EE
)
Storage Temperature Range (T
ST
)
+20 V
-20 V
-40 to +125 ºC
THAT1200
Input Voltage (V
IN
)
± 25 V
Output Short-Circuit Duration (t
SH
)
Operating Temperature Range (T
OP
)
Junction Temperature (T
J
)
THAT1203
± 31 V
THAT1206
± 31 V
Continuous
0 to +85 ºC
125 ºC
Elec trical Characteristics
2 , 3 , 4
Parameter
Supply Current
Supply Voltage
Input Bias Current
Symbol
I
CC
V
CC
, V
EE
I
B
No signal; Either input
connected to GND
No signal
Conditions
No Signal
Min
—
±3
—
700
Typ
4.7
Max
8.0
±18
1,400
Units
mA
V
nA
Input Offset Current
Input Voltage Range
I
B-OFF
—
±12.5
21.0
24.0
24.0
—
±13.0
21.5
24.5
24.5
48.0
with bootstrap
10.0
3.2
±300
—
—
—
—
nA
V
dBu
dBu
dBu
kΩ
MΩ
MΩ
V
IN-CM
Common mode
V
IN-DIFF
Differential (equal and opposite swing)
THAT 1200
THAT 1203
THAT 1206
Z
IN-DIFF
Z
IN-CM
Differential
Common mode
60 Hz
20 kHz
Input Impedance
Common Mode Rejection Ratio
CMRR
1
Matched source impedances; V
CM
= ±10V
DC
70
60 Hz
70
20 kHz
—
90
90
85
—
—
—
dB
dB
dB
Common Mode Rejection Ratio
5
CMRR
IEC
10Ω unmatched source impedances; V
CM
= ±10V
DC
—
90
60 Hz
—
90
20 kHz
—
85
600Ω unmatched source impedances; V
CM
= ±10V
60 Hz
—
70
20 kHz
—
65
At 60 Hz, with V
CC
= -V
EE
THAT 1200
THAT 1203
THAT 1206
—
—
—
dB
dB
dB
Common Mode Rejection Ratio
CMRR
2
—
—
dB
dB
Power Supply Rejection Ratio
6
PSRR
—
—
—
82
80
80
—
—
—
dB
dB
dB
1
2
3
4
All specifications subject to change without notice.
Unless otherwise noted, T
A
= 25°C, V
CC
= +15V, V
EE
= -15V
See test circuit in Figure 2.
0 dBu = 0.775 Vrms
5
6
Per IEC Standard 60268-3 for testing CMRR of balanced
inputs.
Defined with respect to differential 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
Copyright © 2017, THAT Corporation; All rights reserved.
InGenius High-CMRR
Balanced Input Line Receiver ICs
Page 3 of 12
Document 600033 Rev 01
Elec trical Characteristics ( con 't)
Parameter
Total Harmonic Distortion
Symbol
THD
Conditions
Min.
Typ.
Max
Units
V
IN_DIFF
= 10 dBu; BW = 20 kHz; f = 1 kHz
R
L
= 2 kΩ
—
BW = 20 kHz
THAT1200
THAT1203
THAT1206
No signal
R
L
= 2 kΩ; C
L
= 300 pF
R
L
= 10 kΩ; C
L
= 10 pF
THAT1200
THAT1203
THAT1206
f = 1 kHz, R
L
= 2 kΩ
At max differential input
THAT1200
THAT1203
THAT1206
R
L
= R
Lcm
= 0 Ω
At CM output
0.0005
—
%
Output Noise
e
N(OUT)
—
—
—
—
7
-105
-104
-106
—
12
—
—
—
±10
—
dBu
dBu
dBu
mV
V/µs
Output Offset Voltage
Slew Rate
Small Signal Bandwidth
V
OFF
SR
BW
-3dB
—
—
—
—
22
27
34
0
—
—
—
±0.05
MHz
MHz
MHz
dB
Output Gain Error
Maximum Output Voltage
G
ER(OUT)
V
O
21
21
18
—
—
21.5
21.5
18.5
±25
±10
—
—
—
—
—
dBu
dBu
dBu
mA
mA
Output Short Circuit Current
I
SC
I
CMSC
R
Lmin
R
LCMmin
C
Lmax
C
LCmax
Minimum Resistive Load
At CM output
2
10
—
—
—
—
kΩ
kΩ
Maximum Capacitive Load
At CM output
—
—
—
—
300
50
pF
pF
Cb
+
R5
100R
CM Out
Gnd
In-
220u
R3
600R
Vcc
2
8
In-
C4
C1
56p
In+
R1
200k
R2
200k
7
100n
CMout
Vcc
5
Out
CMin
Ref
6
Vee
3
1
In+
4
U1
R6
100R
R4
2k
C2
300p
Main Out
Gnd
THAT120x
C3
100n
Ext. DC Source
Gnd
Vee
Figure 2. THAT 1200-series test circuit
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478-9200; Fax +1 508 478-0990; Web:
www.thatcorp.com
Copyright © 2017, THAT Corporation; All rights reserved.
InGenius High-CMRR
Balanced Input Line Receiver ICs
Page 4 of 12
Document 600033 Rev 01
Theory o f Operation
The InGenius concept was invented to overcome
limitations of traditional approaches to active input
stage design. Because of the many misconceptions about
the performance of conventional input stages, and to set
the stage for discussion of InGenius, we will begin by
discussing conventional approaches.
mode rejection ratio. Any difference between the ratios
R
2
/R
1
and R
4
/R
3
will lead to less than perfect CMRR.
The Impact of Driving Source Impedance
However, in the real world, where sources have non-
zero output impedance, the situation is more complicat-
ed. Figure 4 shows the equivalent circuit of a real-world
differential application.
In this case, the source
connected to the differential receiver has source
impedance of R
s+
in the positive side, and R
s-
in the
negative side. Because these two resistive elements are
in series with each other, they only serve to attenuate the
signal V
diff
relative to the input impedance of the
differential stage. Even if they (R
s+
and R
s-
) are mis-
matched, this attenuation is the only consequence of
non-zero source impedance.
Vin-
R1
R2
-
Vdiff
R3
+
R4
Vout
Rs-
Vin-
R1
R2
Figure 3. Basic differential amplifier
Vdiff
+
2
-
-
+
R3
R4
Vout
Traditional Balanced Input Stages
The typical balanced input stage used in most pro-
fessional audio products is shown in Figure 3. It
amplifies differential signals but rejects common-mode
interference based on the precision of the match in the
ratios R
2
/R
1
and R
4
/R
3
. In this circuit,
=
1+
+
+
+
Vdiff
-
2
Rs+
Figure 4. Basic differential amplifier showing
mismatched source impedances
However, the same cannot be said for common-mode
interference. Common-mode signals appear in phase
between the two input terminals. For in-phase signals,
the source impedances can have significant impact. As
shown in Figure 5, this is because each leg of the source
impedance forms a voltage divider when it interacts with
the input impedance of its respective input of the
differential amplifier.
Because the + and - inputs of the operational ampli-
fier are forced by feedback to maintain the same voltage,
the individual common-mode impedances of each side of
the differential stage are:
=
=
+
; and
.
In modern integrated circuits (such as the THAT
1240 series), these resistor ratios are trimmed (usually
with a laser) to extreme precision, resulting in typical
match of ±0.005%. So, one can assume that
R
2
/R
1
=R
4
/R
3
. In this case, we can simplify this formula
as follows:
=
=
1+
1+
1
+
yielding:
+
CMRR Depends on Resistor Match
When driven from a theoretical, true voltage source,
the precisely matched resistor ratios deliver extremely
high CMRR. With perfectly matched resistor ratios, for
V
in+
=-V
in-
(this corresponds to a pure differential input
signal), then V
out
=2*(V
in+
)*R
2
/R
1
. On the other hand, for
V
in+
=V
in-
(this corresponds to a pure common mode
signal), then V
out
=0. This produces an infinite common
So long as R
1
=R
3
, these impedances, which form a
load for common-mode input signals, are identical.
(This is why, in discrete applications, it is wise to choose
R
1
=R
3
, and why, in all integrated applications, these
resistors are chosen to be the same value.)
The total common-mode input impedance is
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478-9200; Fax +1 508 478-0990; Web:
www.thatcorp.com
Copyright © 2017, THAT Corporation; All rights reserved.
InGenius High-CMRR
Balanced Input Line Receiver ICs
=
.
Page 5 of 12
Document 600033 Rev 01
Source Impedance Mismatches Ruin Good CMRR
Even if R
1
perfectly matches R
3
, any mismatch in the
source impedances R
S+
and R
S-
will cause the voltage
dividers to be unequal between the two input legs. This
means that V
in-
and V
in+
in Figure 5 are no longer equal
to each other. Essentially, imbalances in the two source
impedances convert the common mode signal to a
differential signal, which will not be rejected by the input
stage no matter how high its theoretical CMRR is.
To see how this plays out in practice, consider the
case of a typical unity-gain conventional balanced line
receiver with common-mode input impedance of 10 kΩ.
In such cases, a source impedance imbalance of only
10 Ω can degrade CMRR to no better than 66 dB. A 10 Ω
mismatch could be caused by tolerances in coupling
capacitors or output build-out resistors. The situation
becomes much worse when a conventional balanced line
receiver is driven from an unbalanced source, where it is
common to use at least 100 Ω in series with the output
for protection.
(With a 100 Ω unbalanced output
impedance, and a 10 kΩ common-mode input imped-
ance, even a
perfect
simple input stage can provide no
more than 46 dB CMRR!)
In-
Ri1
+
R1
R2
-
-
+
Out
R4
In+
Ri2
-
+
R3
Figure 6. Instrumentational amplifier
and OA
2
from the balanced stage (OA
3
along with R
1
, R
2
,
R
3
, and R
4
). In this circuit, Z
CM-
= R
i1
, and Z
CM+
= R
i2
.
To retain 90 dB CMRR in the face of a 10 Ω mis-
match in source impedance would require R
i1
and R
i2
to
be > 317 kΩ. Of course, any difference in the values of
R
i1
and R
i2
themselves would further unbalance common
mode signals as well, so these resistors would ideally be
trimmed just like the resistors in the single opamp stage
of Figure 3. Unfortunately for this approach, it is
difficult and expensive to make precision trimmed
resistors with such high values.
Furthermore, since the input bias current for ampli-
fiers OA
1
and OA
2
flows through these resistors, their
input currents must be extremely low if they are not to
cause significant offsets. Practically, this necessitates
using FET input stages for OA
1
and OA
2
. While FETs
may be a viable alternative, it is difficult to achieve with
them the low noise performance of modern bipolar input
stages.
Vin-
R1
R2
-
Zcm
Vcm
Zcm-
+
R3
R4
Vout
Transformer Input Stages
Zcm+
Figure 5. Basic differential amplifier driven
by common-mode input signal
The best solution to this problem is to increase the
line receiver's common-mode input impedance enough
to minimize the unbalancing effect of the voltage divider.
Preferably, this means achieving input impedances on
the order of several megohms. However, in a conven-
tional differential amplifier, this requires high-value
resistances in the circuit. High resistance carries with it
a high noise penalty, making this straightforward
approach impractical for quality audio devices.
From the point of view of common mode input im-
pedance, as well as that of electrical isolation, a
transformer in front of the first active input stage is
really the best possible solution. Transformers are the
only approach of which we are aware that provides true
electrical isolation with reasonable fidelity. Further-
more, their common-mode input impedance is easily
extremely high (tens of Megohms), and almost complete-
ly decoupled from their differential input impedance.
But, transformers have many other limitations. They
do not offer dc coupling, and suffer from saturation at
low frequencies unless they are physically large and
carefully made. Again, unless they are carefully made
(which usually equates to high cost), they introduce
phase shift at high audio-band frequencies. Further-
more, they tend to be big and heavy and pick up external
magnetic fields, sometimes making it difficult to locate
transformer-coupled equipment to avoid interference.
Fortunately, audio equipment usually does not re-
quire true electrical isolation. In most cases, transform-
ers out-perform conventional input stages only because
Instrumentation Amplifiers
Some designers prefer the more elaborate approach
of an instrumentation amplifier, as shown in Figure 6.
In this circuit, it is possible to raise the input impedance
(both common-mode and differential) of the stage
because the load seen by the source is decoupled by OA
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
Copyright © 2017, THAT Corporation; All rights reserved.
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