THAT4305 Datasheet, PDF (5/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Document 600067 Rev 00
Page 5
The rectifier is balanced to within ±3 dB, so a
small amount of fundamental (and higher odd-order
harmonics) ripple can be present at the detector out-
put. By design, this ripple contributes less total rip-
ple than the even-order products that are naturally
and inevitably present at the output of a perfectly
balanced detector.
As in the VCA, input signals are currents to the
RMS_IN pin (pin 2). This input is a virtual ground, so
a resistor (R1 in Figure 2) is normally used to con-
vert input voltages to the desired current. The level
detector is capable of accurately resolving signals
well below 10 mV (with a 5 kΩ input resistor). How-
ever, if the detector is to accurately track such
low-level signals, ac coupling (C1 in Figure 2) is re-
quired to prevent dc offsets from causing a dc cur-
rent to flow in the detector’s input, which would
obscure low-level ac signal currents.
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
5 Hz, or a 32 ms time constant (τ). The filter's time
constant is determined by an external timing capaci-
tor (C_TIME) attached to the C_T pin (pin 4), and an in-
ternal current source (I_T) connected to C_T. The
current source is internally fixed at 7.5 µA. The re-
sulting time constant in seconds is approximately
equal to 3467 times the value of the timing capacitor
in Farads. Note that, as a result of the mathematics
of RMS detection, the attack and release time con-
stants are fixed in their relationship to each other.
The RMS detector is capable of driving large
spikes of current into C_TIME, particularly when the
audio signal input to the RMS detector increases
suddenly. This current is drawn from V_CC (pin 9),
fed through C_TIME at pin 4, and returns to the power
supply through the ground end of C_TIME. If not han-
dled properly through layout and bypassing, these
currents can mix with the audio in the circuit’s
ground structure with unpredictable and undesir-
able results. As noted in the Applications section,
local bypassing from the V_CC pin to the ground end
of C_TIME is strongly recommended in order to keep
these currents out of the ground structure of the cir-
cuit (see C4 in Figure 2.)
The dc output of the detector is scaled with the
same constant of proportionality as the VCA gain
control: 6.2 mV/dB. The detector's 0 dB reference
(iin0, the input current which causes the detector's
output to equal 0V), is trimmed during wafer probe
to equal approximately 7.5 µA. The RMS detector
output stage is capable of sinking or sourcing
125 µA. It is also capable of driving up to 150 pF of
capacitance.
Frequency response of the detector extends
across the audio band for a wide range of input sig-
nal levels. Note, however, that it does fall off at high
frequencies at low signal levels like THAT’s other
RMS detectors.
Differences between the 4305's RMS level
detector circuitry and that of the THAT 2252 RMS
detector include the following.
1. The rectifier in the 4305 RMS Detector is in-
ternally balanced by design, and cannot be balanced
via an external control. The 4305 will typically bal-
ance positive and negative halves of the input signal
within 10 %, but in extreme cases the mismatch may
reach +40 % or -30 % (±3 dB). However, even such
extreme-seeming mismatches will not significantly
increase ripple-induced distortion in dynamics pro-
cessors over that caused by balanced signal ripple
alone.
2. The time constant of the 4305's RMS detector
is determined by the combination of an external ca-
pacitor C_TIME and an internal current source. The in-
ternal current source is set to about 7.5 µA. A
resistor is not normally connected directly to the C_T
pin on the 4305.
3. The 0 dB reference point, or level match, is
also set to approximately 7.5 µA. However, as in the
2252, the level match will be affected by any addi-
tional currents drawn from the C_T pin.
Compressor (or Limiter) Configurations
The 4305 provides the two essential building
blocks required for a wide variety of dynamics pro-
cessing applications. The part may be configured
into practically any type of dynamics processor sys-
tem.
Perhaps the most common application for the
4305 is as a compressor or limiter. These circuits
are intended to reduce gain above some determined
signal level in order to prevent subsequent stages
from being overloaded by too high a signal. Com-
pressors generally have low to moderate compres-
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