that4320 THAT Corporation, that4320 Datasheet - Page 8

• Manufacturer Part Number: that4320
• Description: Pre-trimmed Low-voltage Low-power Analog Engine Dynamics Processor Ic
• Manufacturer: THAT Corporation

Page 8
tially-controlled VCAs and log-responding level de-
tectors is covered in THAT Corporation’s application
note AN101, The Mathematics of Log-Based Dy-
namic Processors.
The VCA — in Brief
poration’s
log-antilog gain cell topology -- The Blackmer VCA --
as used in THAT 2180-Series IC VCAs. VCA symme-
try is trimmed during wafer probe for minimum dis-
tortion. No external adjustment is allowed. See
Figures 3 ~ 6, page 6 for the representative THD
data.
This pin is a virtual ground with dc level approxi-
mately equal to V
put voltage is converted to input current via an
appropriately sized resistor (R
Because the currents associated with dc offsets pres-
ent at the input pin and any dc offset in preceding
stages will be modulated by gain changes (thereby
becoming audible as thumps), the input pin is nor-
mally ac-coupled (C
with respect to the input current. In normal opera-
tion, the output current is converted to a voltage via
inverter OA
termined by the feedback resistor (R
Figure 2) connected between OA
verting input. The signal path through the VCA and
OA
applied between E
ternal node connected to the Vref generator. Gain (in
decibels) is proportional to (E
[page
6.0 mV/dB for the voltage at E
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.
noise (in dBV — referenced to 1 V — in a 22 kHz
bandwidth) at the output of OA
mands over a range of -100 dB to +30 dB gain. At
large attenuation, the noise floor of ~-109 dBV is
limited by the input noise of OA
resistor. At 0 dB gain, the noise floor is ~-98 dBV as
specified. In the vicinity of 0 dB gain, the noise in-
creases more slowly than the gain: approximately
5 dB noise increase for every 10 dB gain increase.
3
The VCA in THAT 4320 is based on THAT Cor-
Input signals are currents in the VCA’s IN pin.
The VCA output signal is also a current, inverted
The gain of the VCA is controlled by the voltage
The VCA’s noise performance varies with gain in
is noninverting.
7].
3
, where the ratio of the conversion is de-
The
highly
REF
C+
constant
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
19
, so in normal operation an in-
and E
in Figure 2).
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
successful
C-
Figure 8 [page 7] plots
. Note that E
of
C+
C+
44
3
3
in Figure 2, Page 6).
(relative to V
‘s output and its in-
– E
proportionality
3
vs. VCA gain com-
and its feedback
C-
complementary
). See Figure 7
Copyright © 2007, THAT Corporation
46
C-
is an in-
or R
REF
).
47
is
,
Finally, as gain approaches 30dB, output noise be-
gins to increase directly with gain.
that of the THAT 2180 Series VCAs, there are sev-
eral important differences, as follows.
At +5 V V
the sum of input and output signal currents. This in-
creases to about 1 mA at +15 V V
to ~1.8 mA for a 2180 Series VCA when biased as
recommended. This is appropriate given the lower
supply voltage for the 4320.)
nally connected to the inverting input of on-chip
opamp OA
around this opamp, this node is brought out to a pin.
trol.
2180 VCA) exists, but is driven from an internally
trimmed current generator.
port (E
6.0 mV/dB, due primarily to the lower internal oper-
ating temperature of the 4320 compared to that of
the 2180 Series (and the 4301).
the 4320 uses less emitter degeneration resistance
in its output than that of the 2180 VCA. This re-
quires that the source impedance at the VCA’s input
(which is a summing junction) must be under 5 kΩ
at frequencies over 1 MHz. In Figure 2, C
accomplish this. See the applications section for an
alternative on how to address this issue.
The RMS Detector — in Brief
tifying input current signals, converting the rectified
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 com-
ponent (at twice the input frequency) remains super-
imposed on the dc output. The ac signal is attenu-
ated by a log-domain filter, which constitutes a sin-
gle-pole rolloff with cutoff determined by an external
capacitor and a programmable dc current.
RMS IN pin. This input is a virtual ground with dc
level equal to V
normally used to convert input voltages to the desired
current. The level detector is capable of accurately re-
While the 4320’s VCA circuitry is very similar to
1) Supply current for the VCA depends on V
2) The signal current output of the VCA is inter-
3) Only the E
4) The control-voltage constant is approximately
5) The OTA used for the VCA’s internal opamp in
The 4320’s detector computes RMS level by rec-
As in the VCA, input signals are currents to the
A SYM control port (similar to that on the
C-
THAT4320 Pre-trimmed Low-voltage Low-power
) is internally connected to V
CC
3
. In order to provide external feedback
, approximately 500 μA is available for
REF
Analog Engine® Dynamics Processor IC
C+
, so a resistor (R
node is available for gain con-
The negative control
CC
24
. (Compare this
REF
in Figure 2) is
.
16
and R
CC
57
.