Pre-trimmed Analog Engine
®
Dynamics Processor IC
THAT
4305
FEATURES
Pre-trimmed Blackmer
®
VCA &
RMS-level detector
Wide supply voltage range:
±4.5V ~ ±16V
Low supply current:
3.5 mA typ. (±15V)
Wide dynamic range:
117 dB (VCA)
60 dB (RMS-level detector)
APPLICATIONS
•
Compressors & Limiters
Gates & Expanders
AGCs
Line-operated dynamics processors
De-Essers
Duckers
Mixers
Level indicators
Companding noise reduction systems
Description
The THAT4305 is a single-chip Analog Engine
®
optimized for low-cost applications. It incorporates a
high-performance
Blackmer
®
voltage-controlled
amplifier (VCA) and log-responding RMS-level
sensor. The VCA and RMS detector are pre-trimmed
at wafer stage to deliver low distortion without
further adjustment.
Available in a small (QSOP) surface-mount
package, the 4305 is aimed at line-operated audio
applications such as compressor/limiters, gates, and
other dynamic processors. The part normally
operates from a split supply voltage up to ±16Vdc,
drawing only 3.5mA at ±15V. This IC also works at
supply voltages as low as ±4.5V, making it useful in
some battery-operated products as well.
The 4305 was developed specifically for use in
low-cost dynamics processors, drawing from THAT's
long history and experience with such designs. Both
VCA control ports and the detector input and output
are available for the designer to connect as s/he sees
fit. As a result, the part is extremely flexible and can
be configured for a wide range of applications includ-
ing single- and multi-band companders, digital
overload protectors, voltage-controlled faders, level
indicators, etc.
What really sets the 4305 apart from other
manufacturers’ offerings is the transparent sound of
its Blackmer VCA, coupled with its accurate true-
RMS level detector. This makes the IC useful in a
wide range of analog audio products.
NC
VCA
IN
NC
VCA
OUT
EC-
EC+
NC
VCC
16
15
14
13
12
11
10
9
IN
VCA
OUT
EC+ EC-
IN
RMS
OUT
CT
1
NC
2
RMS
IN
3
NC
4
CT
5
RMS
OUT
6
GND
7
NC
8
VEE
Pin Name
No Connection
RMS IN
No Connection
C
TIME
RMS OUT
GND
No Connection
Vee
Vcc
No Connection
EC+
EC-
VCA OUT
No Connection
VCA IN
No Connection
Pin Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Table 2. Pin assignments
Package
Order Number
4305Q16-U
Figure 1. THAT 4305 equivalent block diagram
16 pin QSOP
Table 1. Ordering information
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Copyright © 2015, THAT Corporation; Document 600067 Rev 02
Document 600067 Rev 02
Page 2 of 20
THAT4305 Pre-trimmed
Analog Engine
®
Dynamics Prcoessor IC
SPECIFICATIONS
Absolute Maximum Ratings
1
Operating Temperature Range (T
OP
)
Junction Temperature (T
J
)
Power Dissipation (P
D
) at T
A
=85 ºC
-40 to +85 ºC
-40 to +125 ºC
400mW
Supply Voltages (V
CC
, V
EE
)
VCA Control Voltage
Storage Temperature Range (T
ST
)
±18V
±0.6 V
-40 to +125 ºC
Electrical Characteristics
2
Parameter
Power Supply
Positive Supply Voltage
Negative Supply Voltage
Supply Current
I
CC
I
EE
I
CC
I
EE
Voltage Controlled Amplifier (VCA)
Max. I/O Signal Current
VCA Gain Range
Gain at 0V Control
Gain-Control Constant
Gain-Control Tempco
G
0
E
C+
/Gain (dB)
ΔE
C
/ΔT
CHIP
E
C+
= E
C-
= 0V
-60 dB < gain < +60 dB
Ref T
CHIP
=27ºC
R
OUT
= 20 kΩ
0 dB gain
+15 dB gain
+30 dB gain
Output Noise
e
N(OUT)
0 dB gain
22Hz~22kHz, R
IN
=R
OUT
=20 kΩ
Total Harmonic Distortion
RMS Level Detector
Output Voltage at Reference i
IN
e
O(0)
i
IN
= 7.5
μA
RMS
i
IN
= 200 nA RMS
i
IN
= 200
μA
RMS
Scale Factor Match to VCA
-20 dB < VCA gain < +20 dB
1
μa<
i
IN(RMS)
< 100
μA
.95
1
1.05
-
-9
0
±1
±1
+9
±3
±3
mV
dB
dB
THD
V
IN
= -5dBV, 1kHz, E
C+
= E
C-
= 0V
-
-97.5
0.07
-95
0.15
dBV
%
-
-
-
1
3
10
15
30
50
mV
mV
mV
i
IN(VCA)
+ i
OUT(VCA)
-60
-1.0
-
-
0
6.2
+0.33
±1.8
+60
+1.0
-
-
mA
peak
dB
dB
mV/dB
%/ºC
V
CC
V
EE
Referenced to GND
Referenced to GND
No Signal
V
CC
=+15V, V
EE
= -15V
V
CC
=+15V, V
EE
= -15V
V
CC
=+5V, V
EE
= -5V
V
CC
=+5V, V
EE
= -5V
3.5
-3.5
2
-2
5
-5
mA
mA
mA
mA
+4.5
-4.5
-
-
+16
-16
V
V
Symbol
Conditions
Min
Typ
Max
Units
Output Offset Voltage Change
3
Δ
V
OFF(OUT)
Output Error at Input Extremes e
O(RMS)error
1. If the devices are subjected to stress above the Absolute Maximum Ratings, permanent damage may result. Sustained operation at or near the Absolute Maximum
Ratings conditions is not recommended. In particular, like all semiconductor devices, device reliability declines as operating temperature increases.
2. Unless otherwise noted, T
A
=25ºC, V
CC
=+15V, V
EE
= -15V.
3. Reference is to output offset with -60 dB VCA gain.
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Copyright © 2015, THAT Corporation; All rights reserved.
Document 600067 Rev 02
Page 3 of 20
THAT4305 Pre-trimmed
Analog Engine
®
Dynamics Prcoessor IC
Electrical Characteristics (con’t)
2
Parameter
Rectifier Balance
Timing Current
Filtering Time Constant
Output Tempco
Load Resistance
Capacitive Load
I
T
τ
ΔE
O
/ΔT
CHIP
R
L
C
L
Ref T
CHIP
= 27 ºC
-250mV < V
OUTRMS
< +250mV
-
2
150
Symbol
Conditions
±7.5μA DC
IN
-
Min
Typ
±1
7.5
3467 X C
TIME
+0.33
-
Max
±3
-
Units
dB
μA
s
%/ºC
kΩ
pF
C5
100p
VCA In
C2
10u
+15V
R4
6k8
R3
20k
11
Ec+ 9
Vcc
13
VCA In VCA Out
15
Gnd
Vee
6
Ec-
8
12
R2
20k
RMS In C1
C3
10u
22p NPO
VCA Out
U2
R1
5k1
2
U1B
THAT4305
RMS In
RMS Out
5
CT
4
RMS Out
R5
2k
+15V
C4
10u
C
TIME
10u
Control Voltage
-15V U1A
THAT4305
Figure 2. Simplified application circuit
Theory of Operation
The THAT 4305 Dynamics Processor combines
THAT Corporation's proven exponentially controlled
Blackmer® Voltage-Controlled Amplifier (VCA) and
log-responding RMS-Level Detector building blocks
in a small package optimized for low cost designs.
The part is fabricated using a proprietary, fully
complementary, dielectric-isolation process. This
process produces very high-quality bipolar transis-
tors (both NPNs and PNPs) with unusually low collec-
tor- substrate capacitances. The 4305 takes
advantage of these devices to deliver wide bandwidth
and excellent audio performance while consuming
very low current and operating over a wide range of
power supply voltages.
For details of the theory of operation of the VCA
and RMS Detector, we refer the interested reader to
THAT Corporation's data sheets on the 2180-Series
VCAs and the 2252 RMS Level Detector. Theory of the
interconnection of exponentially controlled VCAs and
log-responding level detectors is covered in THAT
Corporation's application note AN101A,
The Mathe-
matics of Log-Based Dynamic Processors.
The VCA - in Brief
The VCA in the 4305 is based on THAT Corpora-
tion's highly successful complementary log-antilog
gain cell topology (the Blackmer® VCA) as used in
THAT 2180-Series IC VCAs. VCA symmetry is
trimmed during wafer probe for minimum
distortion. No external adjustment is allowed.
Input signals are currents in the VCA's VCA
IN
pin
(pin 15). This pin is a virtual ground with a small dc
offset, so in normal operation an input voltage is
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Copyright © 2015, THAT Corporation; All rights reserved.
Document 600067 Rev 02
Page 4 of 20
THAT4305 Pre-trimmed
Analog Engine
®
Dynamics Prcoessor IC
converted to input current via an appropriately sized
resistor (R3 in Figure 2). Because the dc current
associated with dc offsets present at the input pin
plus any dc offset in the preceding stages will be
modulated by gain changes (thereby becoming
audible as thumps), the input pin is normally
ac-coupled. This blocks such offset currents and
reduces dc offset variation with gain.
The VCA output signal, VCA
OUT
(pin 13), is also a
current, inverted with respect to the input current. In
normal operation, the output current is converted to
a voltage via an external op-amp, where the current-
to-voltage conversion ratio is determined by the
feedback resistor connected between the op-amp's
output and its inverting input (R2 in Figure 2). The
resulting signal path through the VCA plus op-amp is
noninverting.
The VCA gain is controlled by the voltage applied
between E
C+
(pin 11) and E
C-
(pin 12). Note that any
unused control port should be connected to ground
(as E
C+
is in Figure 2). The gain (in decibels) is
proportional to (E
C+
-E
C-
). The constant of propor-
tionality is 6.2 mV/dB for the voltage at E
C+
(relative
to E
C-
). Note that neither E
C+
or E
C-
should be driven
more than ±0.6 V away from ground.
The VCA's noise performance varies with gain in
a predictable way, but due to the way internal bias
currents vary with gain, noise at the output is not
strictly the product of a static input noise times the
voltage gain commanded. At large attenuation, the
noise floor is usually limited by the input noise of the
output op-amp and its feedback resistor. At 0 dB
gain, the noise floor of ~ -97.5 dBV is the result of
the VCA’s output noise current, converted to a
voltage by the typical 20k I-V converter resistor (R2
in Figure 2). In the vicinity of 0 dB gain, the noise
increases more slowly than the gain: approximately
5 dB noise increase for every 10 dB gain increase.
Finally, as gain approaches 30 dB, output noise
begins to increase directly with gain.
While the 4305's VCA circuitry is very similar to
that of the THAT 2180 Series VCAs, there are several
important differences, as follows.
1. Supply current for the 4305 VCA depends on
the supply voltage. At ±5 V, approximately 800
μA
is
available for the sum of input and output signal
currents. This increases to about 1.8 mA at ±15 V.
(Compare this to ~1.8 mA for a 2180 Series VCA
when biased as recommended.)
2. The SYM control port (similar to that on the
2180 VCA) is not brought out to an external pin; it is
driven from an internally trimmed current generator.
3. The control-voltage constant is approximately
6.2 mV/dB, due primarily to the higher internal
operating temperature of the 4305 compared to that
of the 2180 Series.
The RMS Detector - in Brief
The 4305's detector computes RMS level by
rectifying input current signals, converting the recti-
fied current to a logarithmic voltage, and applying
that voltage to a log-domain filter. The output signal
is a dc voltage proportional to the decibel-level of the
RMS value of the input signal current. Some ac
component (at twice the input frequency plus higher-
order even harmonics) remains superimposed on the
dc output. The ac signal is attenuated by a log
domain filter, which constitutes a single-pole rolloff
with cutoff determined by an external capacitor (C4
in Figure 2).
The rectifier is balanced to within ±3 dB, so a
small amount of fundamental (and higher odd-order
harmonics) ripple can be present at the detector
output. By design, this ripple contributes less total
ripple than the even-order products that are
naturally and inevitably present at the output of a
perfectly balanced detector.
As in the VCA, input signals are currents to the
RMS
IN
pin (pin 2). This input is a virtual ground, so a
resistor (R1 in Figure 2) is normally used to convert
input voltages to the desired current. The level detec-
tor is capable of accurately resolving signals well
below 10 mV (with a 5 kΩ input resistor). However,
if the detector is to accurately track such low-level
signals, ac coupling (C1 in Figure 2) is required to
prevent dc offsets from causing a dc current to flow
in the detector’s input, which would obscure
low-level ac signal currents.
The log-domain filter cutoff frequency is usually
placed well below the frequency range of interest. For
an audio-band detector, a typical value would be
5 Hz, or a 32 ms time constant (τ). The filter's time
constant is determined by an external timing capaci-
tor (C
TIME
) attached to the C
T
pin (pin 4), and an
internal current source (I
T
) connected to C
T
. The
current source is internally fixed at 7.5
μA.
The
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Copyright © 2015, THAT Corporation; All rights reserved.
Document 600067 Rev 02
Page 5 of 20
THAT4305 Pre-trimmed
Analog Engine
®
Dynamics Prcoessor IC
resulting time constant in seconds is approximately
equal to 3467 times the value of the timing capacitor
in Farads. Note that, as a result of the mathematics
of RMS detection, the attack and release time
constants are fixed in their relationship to each
other.
The RMS detector is capable of driving large
spikes of current into C
TIME
, particularly when the
audio signal input to the RMS detector increases
suddenly. This current is drawn from V
CC
(pin 9),
fed through C
TIME
at pin 4, and returns to the power
supply through the ground end of C
TIME
. If not
handled properly through layout and bypassing,
these currents can mix with the audio in the circuit’s
ground structure with unpredictable and undesirable
results. As noted in the Applications section, local
bypassing from the V
CC
pin to the ground end of C
TIME
is strongly recommended in order to keep these
currents out of the ground structure of the circuit
(see C4 in Figure 2.)
The dc output of the detector is scaled with the
same constant of proportionality as the VCA gain
control: 6.2 mV/dB. The detector's 0 dB reference
(i
in0
, the input current which causes the detector's
output to equal 0V), is trimmed during wafer probe
to equal approximately 7.5
μA.
The RMS detector
output stage is capable of sinking or sourcing
125
μA.
It is also capable of driving up to 150 pF of
capacitance.
Frequency response of the detector extends
across the audio band for a wide range of input
signal levels. Note, however, that it does fall off at
high frequencies at low signal levels like THAT’s
other RMS detectors.
Differences between the 4305's RMS level detec-
tor circuitry and that of the THAT 2252 RMS detec-
tor include the following.
1. The rectifier in the 4305 RMS Detector is
internally balanced by design, and cannot be
balanced via an external control. The 4305 will
typically balance positive and negative halves of the
input signal within 10% but in extreme cases the
mismatch may reach +40% or -30% (±3dB).
However, even such extreme-seeming mismatches
will not significantly increase ripple-induced distor-
tion in dynamics processors over that caused by
balanced signal ripple alone.
2. The time constant of the 4305's RMS detector
is determined by the combination of an external
capacitor C
TIME
and an internal current source. The
internal current source is set to about 7.5
μA.
A
resistor is not normally connected directly to the C
T
pin on the 4305.
3. The 0 dB reference point, or level match, is
also set to approximately 7.5
μA.
However, as in the
2252, the level match will be affected by any
additional currents drawn from the C
T
pin.
Compressor (or Limiter) Configurations
The 4305 provides the two essential building
blocks required for a wide variety of dynamics
processing applications. The part may be configured
into practically any type of dynamics processor
system.
Perhaps the most common application for the
4305 is as a compressor or limiter. These circuits
are intended to reduce gain above some determined
signal level in order to prevent subsequent stages
from being overloaded by too high a signal.
Compressors generally have low to moderate
compression ratios, while limiters have high ratios.
In such applications, the signal path has static gain
so long as the input signal remains below some
threshold, but gain is reduced when the signal rises
above the threshold. Compression ratio is defined as
the number of dB the input signal increases for a
1 dB increase in output signal.
Feedforward Topologies
To make a compressor or limiter with a 4305,
typically, the input signal is applied to both the VCA
and the RMS detector. The RMS output signal is fed
forward to the VCA's negative control port (E
C-
) via a
dc-coupled op-amp based stage. This stage has gain
above some dc level (the threshold), and no trans-
mission below that level. This path, called the
"sidechain," — from detector output to VCA control
port — determines the compression behavior of the
circuit. As signal level rises, the dc voltage at the
RMS' output rises. Once the dc level exceeds the
threshold, the rms output signal is transmitted
through the sidechain and presented to the VCA
control port, lowering the gain to signals passing
through the VCA. As a result, the output signal level
is reduced, or compressed, relative to rising input
signal levels.
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Copyright © 2015, THAT Corporation; All rights reserved.
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