THAT4311 ETC, THAT4311 Datasheet - Page 7

• Manufacturer Part Number: THAT4311
• Description: Low-voltage/ Low-power Analog Engine Dynamics Processor
• Manufacturer: ETC

Rev. 08/30/01
Preliminary Information
voltage must be applied to EC+ and SYM, except for
a small (±2.5 mV) DC bias applied between these
pins.
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.
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.
that of the THAT 2180 Series VCAs, there are several
important differences, as follows:
Approximately 500mA is available for the sum of in-
put and output signal currents.
nally connected to the inverting input of an on-chip
As mentioned, for proper operation, the same
The VCA may be controlled via EC-, as shown in
While the 4311’s VCA circuitry is very similar to
1. Supply current for the VCA is fixed internally.
2. The signal current output of the VCA is inter-
Signal In
This bias voltage adjusts for internal mis-
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
+
47u
C3
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
264k
10k
R7
R6
100n
C7
1
2
U1B
THAT 4311
+5
IN
Iset
10
11
RMS
THAT 4311
U1E
Vcc
Vee
Fig 17. Circuit showing gain control at E
+
10u
C6
OUT
Vref
TC
Vref
4
5
Cap
RMS Out
+
10u
C4
9
8
+
10u
C5
Control Port Drive
+
47u
C1
+
opamp.
around this opamp, this node is brought out to a pin.
grated P-channel FETs rather than a bias-current
corrected bipolar differential amplifier.
currents have therefore been reduced.
The RMS Detector - in Brief
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.
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.
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.
22u
C8
20k
R1
3. The input stage of the 4311 VCA uses inte-
The THAT 4311’s detector computes RMS level
As in the VCA, input signals are currents to the
Vref
U1C
Vref
17
3
IN
In order to provide external feedback
OA2
51R
EC+
C-
R3
15
THAT 4311
VCA
51k
EC-
R4
16
14
20k
SYM
R5
6
7
OUT
13
Vref
+5
OA3
C2
47p
U1A
THAT 4311
20k
R2
Vref
U1D
19
20
12
OA1
THAT 4311
Signal Out
18
Input bias
Some AC
Page 7