THAT4311 Datasheet, PDF (8/12 Pages) List of Unclassifed Manufacturers – Low-voltage, Low-power Analog Engine Dynamics Processor

Page 8
Low-voltage, Analog Engine^® Dynamics Processor
Preliminary Information
N/C
N/C
U1B
1 _IN
OUT ⁴ _N/C
2 ^RMS
Iset TC
5
N/C
R5 ⁺⁵
50k
THAT 4311
Control Port Drive
R4
51k
R3
R2
20k
C2
51R
Signal In C1
+
47u
15 14 13 ^47p
R1
17
EC+ SYM
IN VCA OUT
20k
EC-
U1A
OA3 ¹²
Signal Out
16
THAT 4311
Vref ^Vref
+5
C7
U1E
11 _{Vcc Vref} 9
100n 10 ^Vref 8
Vee Cap
THAT 4311 + _C5
10u
U1C
U1D
6
N/C
+ _C8
22u
3
OA2 ⁷ N/C
THAT 4311
Vref
N/C _{19 OA1} 18 _N/C
N/C
20 _{THAT 4311}
Fig 18. Circuit showing gain control at EC+
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
5Hz, or a 32ms time constant (t). The filter’s time
constant is determined by an external capacitor at-
tached to the CT pin, and an internal current source
(I_TIME) connected to CT. The current source is pro-
grammed via the IT pin: current in IT is mirrored to
ITIME with a gain of approximately one. The resulting
time constant t is approximately equal to
(0.026 ´ CT) / I_T. Note that, as a result of the mathe-
matics of rms detection, the attack and release time
constants are fixed in their relationship to each other.
The DC output of the detector is scaled with the
same constant of proportionality as the VCA gain
control: 6.1mV/dB. The detector’s zero dB reference
(Iin0, the input current which causes zero volts out-
put), is determined by IT as follows: Iin0=IT. The
detector output stage is capable of sinking or sour-
cing l00mA.
Differences between the 4311’s RMS-Level Detec-
tor circuitry and that of the THAT 2252 RMS Detec-
tor are as follows:
1. The rectifier in the 4311 RMS Detector is inter-
nally balanced by design, and cannot be balanced via
an external control. The 4311 will typically balance
positive and negative halves of the input signal within
±1.5%, but in extreme cases the mismatch may
reach +20%. However, a 20% mismatch will not sig-
nificantly increase ripple-induced distortion in dy-
namics processors over that caused by signal ripple
alone.
2. The time constant of the 4311’s RMS detector
is determined by the combination of an external ca-
pacitor (connected to the CT pin) and an internal,
programmable current source. The current source is
equal to IT. Normally, a resistor is not connected di-
rectly to the CT pin on the 4311.
3. The zero dB reference point, or level match, is
not adjustable via an external current source. How-
ever, as in the 2252, the level match is affected by the
timing current, which, in this case, is drawn from the
IT pin and mirrored internally to CT.
4. The input stage of the 4311 RMS detector uses
integrated P-channel FETs rather than a bias-current
corrected bipolar differential amplifier. Input bias
currents are therefore negligible, improving perfor-
mance at low signal levels.
The Opamps - in Brief
The three opamps in the 4311 are intended for
general purpose applications. All are 5MHz opamps
with slew rates of approximately 2V/ms. All use bipo-
lar PNP input stages. However, the design of each is
optimized for its expected use. Therefore, to get the
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