that4320 THAT Corporation, that4320 Datasheet - Page 11

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

Document 600045 Rev 04
itself.
should take care to minimize currents, especially
signal currents, that flow through the V
signal currents should return to the real circuit
ground (GND); V
atively high impedance loads (e.g., the positive input
of opamps).
otherwise) must be delivered at the V
opamp should be used to buffer the V
is to operate the 4320 from symmetrical split sup-
plies (e.g., ±5 V and ground). In such cases, the
center-tap of the resistive divider (pin 13) should be
grounded. This will force V
(within the offset of the V
nections is that both of the 4320’s two GND pins (1
and 14) must be tied together for proper operation
of the device. While these pins are tied together in-
ternally on the chip, due to the large size of the die
inside the part, the resistance and inductance of the
internal connection is not as low as an external PCB
trace can provide. The 4320 may not meet all its
specifications unless a short PCB connection is
made between these two pins.
PTAT Voltage Generator
output both share a fundamental temperature drift
proportional to absolute temperature.
perature is approximately 300 ºK (or 27 ºC), so near
room temperature the drift amounts to +0.33 %/ºC.
The drift is expressed in percent per degree Celsius
because the magnitude of the change with tempera-
ture depends on the gain control command or de-
tected
temperature drift at 0 dB gain, or at the RMS’ refer-
ence level.
points, the scale factor of these parameters varies by
0.33 % for each degree Celsius of temperature
change.
that varies directly with absolute temperature.
25 ºC, it’s output is 72mV. One end of the generator
is connected to V
fered and brought out at V
plication for the voltage on this pin might be to read
the temperature of the IC, it has many important
practical uses in audio applications based on the
4320.
used, after appropriate scaling, to supply any gain
Another approach to power supply arrangements
A final note on the subject of power supply con-
The VCA control port and the RMS-level detector
The PTAT voltage generator produces an output
Basically, it provides a voltage that can be
However, in any such uses, the designer
level
But, away from either of these 0 dB
Where significant currents (signal or
being
REF
REF
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
, the other (negative end) is buf-
should be connected only to rel-
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
presented.
CC
PTAT
/2 buffer).
REF
to very nearly ground
(pin 9). While one ap-
There
REF
REF
REF
Copyright © 2007, THAT Corporation
dc level, an
line itself.
Room tem-
line. Any
is
no
At
controls or offsets used to condition the RMS detec-
tor output and/or the VCA gain control signals.
a designer wants to provide a potentiometer to con-
trol signal gain through the VCA. If the desired gain
range is 0 to +20 dB, the VCA control port must be
driven from 0 mV (for 0 dB gain) to +120 mV (for
+20 dB gain), but only at room temperature. (At
room temperature, the gain control constant is
6.0 mV/dB.) If the temperature increases by 10 ºC,
the voltage for 0 dB gain remains the same, but that
for 20 dB gain increases by 3.3 %, to 124 mV. If the
same 120 mV gain command is applied (because it
comes from a source that does not vary with temper-
ature), the gain will be 19.35 dB, not 20 dB.
rives from a stable voltage source, the commanded
gain will drift with temperature. Alternatively, if the
supply can be made to vary with temperature just as
the control port’s sensitivity drifts, the two can com-
pensate each other and the result will be stable.
That is the purpose of the 4320’s PTAT voltage gen-
erator: to supply a voltage that drifts exactly as the
VCA and the RMS detector drifts. The PTAT voltage
can be used, with appropriate scaling, to reference
all gain controls, gain offsets, and threshold setting
amplifiers throughout the level-processing side
chain. And, because the PTAT generator is inte-
grated on the same IC as its VCA and RMS detector,
temperature tracking between these three compo-
nents is excellent.
The No Connection Pins
and 24) are labeled "No Connection" (N/C). These
pins are not internally connected to the 4320 die, so
it is acceptable to leave these pins unconnected or to
connect these pins to some external circuit nodes. In
fact, the placement of the N/C pins was chosen
partly to facilitate passive guarding to certain pins
which are sensitive to low-level leakage currents
(e.g., the RMS and VCA inputs).
circuit nodes is very close to V
tion recommends that all the N/C pins be connected
to V
straints may preclude such a connection. In this
case, either leave the pins open, or choose a slow
moving (dc) signal that is close in dc potential to
V
GND -- not recommended -- will guard against AC
signals, but runs the risk of generating unantici-
REF
An example may help make this clear. Suppose
If the supply that feeds the gain-control pot de-
Six pins on the THAT4320 (5, 10, 12, 19, 22,
Because the dc potential at the most sensitive
, such as V
REF
wherever possible. However, layout con-
PTAT
. Tying the N/C pins to V
REF
, THAT Corpora-
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CC
or