THAT4305 Datasheet, PDF (4/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Page 4
THAT4305 Pre-trimmed Analog Engine^
current and operating over a wide range of power
supply voltages.
For details of the theory of operation of the VCA
and RMS Detector, we refer the interested reader to
THAT Corporation's data sheets on the 2180-Series
VCAs and the 2252 RMS Level Detector. Theory of
the interconnection of exponentially controlled VCAs
and log-responding level detectors is covered in
THAT Corporation's application note AN101A, The
Mathematics of Log-Based Dynamic Processors.
The VCA - in Brief
The VCA in the 4305 is based on THAT Corpora-
tion's highly successful complementary log-antilog
gain cell topology (the Blackmer™ VCA) as used in
THAT 2180-Series IC VCAs. VCA symmetry is
trimmed during wafer probe for minimum distor-
tion. No external adjustment is allowed.
Input signals are currents in the VCA's VCA_IN pin
(pin 15). This pin is a virtual ground with a small dc
offset, so in normal operation an input voltage is
converted to input current via an appropriately sized
resistor (R3 in Figure 2). Because the dc current as-
sociated with dc offsets present at the input pin plus
any dc offset in the preceding stages will be modu-
lated by gain changes (thereby becoming audible as
thumps), the input pin is normally ac-coupled. This
blocks such offset currents and reduces dc offset
variation with gain.
The VCA output signal, VCA_OUT (pin 13), is also
a current, inverted with respect to the input current.
In normal operation, the output current is converted
to a voltage via an external op-amp, where the cur-
rent-to-voltage conversion ratio is determined by the
feedback resistor connected between the op-amp's
output and its inverting input (R2 in Figure 2). The
resulting signal path through the VCA plus op-amp
is noninverting.
The VCA gain is controlled by the voltage applied
between E_C+ (pin 11) and E_C- (pin 12). Note that any
unused control port should be connected to ground
(as E_C+ is in Figure 2). The gain (in decibels) is pro-
portional to (E_C+ - E_C-). The constant of proportion-
ality is 6.2 mV/dB for the voltage at E_C+ (relative to
E_C-). Note that neither E_C+ or E_C- should be driven
more than ±0.6 V away from ground.
The VCA's noise performance varies with gain in
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. At large attenuation, the
noise floor is usually limited by the input noise of
the output op-amp and its feedback resistor. At 0 dB
gain, the noise floor of ~ -97.5 dBV is the result of
the VCA’s output noise current, converted to a volt-
age by the typical 20k I-V converter resistor (R2 in
Figure 2). 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.
Finally, as gain approaches 30 dB, output noise be-
gins to increase directly with gain.
While the 4305's VCA circuitry is very similar to
that of the THAT 2180 Series VCAs, there are sev-
eral important differences, as follows.
1. Supply current for the 4305 VCA depends on
the supply voltage. At ±5 V, approximately 800 uA is
available for the sum of input and output signal cur-
rents. This increases to about 1.8 mA at ±15 V.
(Compare this to ~1.8 mA for a 2180 Series VCA
when biased as recommended.)
2. The SYM control port (similar to that on the
2180 VCA) is not brought out to an external pin; it is
driven from an internally trimmed current genera-
tor.
3. The control-voltage constant is approximately
6.2 mV/dB, due primarily to the higher internal op-
erating temperature of the 4305 compared to that of
the 2180 Series.
The RMS Detector - in Brief
The 4305's detector computes RMS level by rec-
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 plus higher-
order even harmonics) remains superimposed on
the dc output. The ac signal is attenuated by a log
domain filter, which constitutes a single-pole rolloff
with cutoff determined by an external capacitor (C4
in Figure 2).
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com