THAT4311 Datasheet, PDF (9/12 Pages) List of Unclassifed Manufacturers – Low-voltage, Low-power Analog Engine Dynamics Processor

Rev. 08/30/01
Preliminary Information
Page 9
+5
R5
R4
20k
33k
R3
51R
R2
20k
C2
47p
Vref _{15 14 13}
In
C1
+
47u
R1
17
EC+ SYM
IN VCA OUT
20k
EC-
U1A
Out
OA3 ¹²
R10
R17
R16 ¹⁶
THAT4311
82k
R9
+5
82k
4k99
Vref
+5
Threshold
R12
R11
20k
51k
C7 ^10k
CR1 ¹⁰⁰ⁿ
Gain
R18
50k
C5
100n
Vin0=-10dBu
C3
+
R6
47u ^28k7
R7
267k
U1B
1 _IN
OUT ⁴
2 ^RMS
5
Iset TC
THAT 4311
R8
4k99
+ _C4
10u
1N4148
R14
R15
6
1k43
10k
3
CR2
R13
OA2 ⁷
10k
U1C ^1N4148
Vref _{THAT4311 +5}
Vref
Compression
Ratio
C6
U1E
11 _{Vcc Vref} 9
+
10u
10 ^Vref 8
Vee Cap
19 ^U1D
20 ^{OA1 18}
THAT4311
Vref
THAT 4311
+ _C8
10u
+ _C9
22u
Fig 19. Simple compressor / limiter using the THAT 4311
most out of the 4311, it is useful to know the major
differences among these opamps.
OA3, being internally connected to the output of
the VCA. is intended for current-to-voltage conver-
sion. Its input noise performance, at 7.5nV / Hz,
complements that of the VCA, adding negligible noise
at unity gain. Its output section is capable of driving
1mA into a 2kW load.
OA1 is the quietest opamp of the three, and with
its typical input referred noise of 6.5nV / Hz, is the
opamp of choice for input stages. Its output section
is nominally capable of driving 3mA into a 5kW load.
OA2 is best suited for control voltage processing,
though is does have anti-paralleled diodes that can
be used to fashion it into a clipper. (However in most
applications where a clipper is needed, it’s preferable
to place it around OA3). OA2’s input noise is compa-
rable to OA3 and its output drive is comparably to
OA1.
The Reference Voltage - In Brief
THAT Corporation’s log/anti-log VCAs and RMS`
detectors require a reference voltage between the pos-
itive and negative power supplies, and to supply this,
the THAT 4311 provides an on-chip, 2V reference
about which the VCA, the RMS detector, and OA2 are
biased. This reference is a buffered band-gap refer-
ence that is amplified to 2V. Pins are provided for fil-
ter capacitors at both the input and the output of the
buffer, which are labeled CAP and VREF respectively.
Application Information
As noted previously, the THAT 4311 was origi-
nally designed for noise reduction systems, hence the
emphasis on those parameters in the specifications.
Its low power consumption, integration, and similar-
ity to the THAT 4301, however, extend its utility to a
variety of other products and applications. The cir-
cuit of Fig 19, shows a typical application for the
THAT 4311. This simple compressor/limiter design
features adjustable hard-knee threshold, compres-
sion ratio, and static gain. The applications discus-
sion in this data sheet will center on this circuit for
the purpose of illustrating important design issues.
Signal Path
As mentioned in the section on theory, the VCA
input pin is a virtual ground with negative feedback
provided internally. An input resistor (R1, 20kW) is
required to convert the AC input voltage to a current
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com