THAT4311 Datasheet, PDF (7/12 Pages) List of Unclassifed Manufacturers – Low-voltage, Low-power Analog Engine Dynamics Processor

Rev. 08/30/01
Preliminary Information
Page 7
R5 ⁺⁵
20k
Signal In
C3
+
R6
47u ^10k
R7
264k
R2
R4
51k
20k
R3
C2
51R
15 14 13 ^47p
U1B
1 _IN
OUT
2 ^RMS
Iset TC
4
5
RMS
C1
+
Out _47u
R1 ^{Vref EC+ SYM}
IN VCA OUT
20k ¹⁷
EC-
16
U1A
Signal
OA3 ¹²
THAT 4311
Control Port Drive
Vref
Out
THAT 4311 _+C4
+5
10u
C6
+
10u
U1C
U1D
C7
100n
U1E
11 _{Vcc Vref} 9
10 Vref 8
Vee Cap
THAT 4311
+ _C5
10u
+ _C8
22u
6
3
OA2 ⁷
THAT 4311
Vref
19
20 ^{OA1 18}
THAT 4311
Vref
Fig 17. Circuit showing gain control at EC-
As mentioned, for proper operation, the same
voltage must be applied to EC+ and SYM, except for
a small (±2.5 mV) DC bias applied between these
pins. This bias voltage adjusts for internal mis-
matches in the VCA gain cell which would otherwise
cause small differences between the gain of positive
and negative half-cycles of the signal. The voltage is
usually applied via an external trim potentiometer
(R7 in Fig 2), which is adjusted for minimum signal
distortion at unity (zero dB) gain.
The VCA may be controlled via EC-, as shown in
Fig 17, or via the combination of EC+ and SYM.
This connection is illustrated in Fig 18. Note that
this latter figure shows only that portion of the cir-
cuitry needed to drive the positive VCA control port;
circuitry associated with OA1, OA2 and the RMS de-
tector has been omitted.
While the 4311’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 VCA is fixed internally.
Approximately 500mA is available for the sum of in-
put and output signal currents.
2. The signal current output of the VCA is inter-
nally connected to the inverting input of an on-chip
opamp. In order to provide external feedback
around this opamp, this node is brought out to a pin.
3. The input stage of the 4311 VCA uses inte-
grated P-channel FETs rather than a bias-current
corrected bipolar differential amplifier. Input bias
currents have therefore been reduced.
The RMS Detector - in Brief
The THAT 4311’s detector computes RMS level
by rectifying input current signals, converting the rec-
tified 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) remains su-
perimposed on the DC output. The AC signal is at-
tenuated by a log-domain filter, which constitutes a
single-pole roll-off with cutoff determined by an ex-
ternal capacitor and a programmable DC current.
As in the VCA, input signals are currents to the
RMS IN pin. This input is a virtual ground biased at
VREF, so a resistor (R11 in Fig 2) is normally used to
convert input voltages to the desired current. The
level detector is capable of accurately resolving sig-
nals well below 10mV (with a 10kW input resistor).
However, if the detector is to accurately track such
low-level signals, AC coupling is normally required.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com