THAT2252 THAT, THAT2252 Datasheet - Page 5

• Manufacturer Part Number: THAT2252
• Description: IC RMS-Level Detector
• Manufacturer: THAT

600032 Rev 01
lows the (
transformation from the linear to the log world
steepens the apparent attack shape.
The time constant, , also determines the amount
of ripple (at frequency 2f
given input frequency, f
ripple at the expense of longer attack and release
times. For f
put is given by: V
ripple voltage.
Taking the Square Root
The square root portion of the Root-Mean Square
is implied by the constant of proportionality for
the output voltage: it is not computed explicitly.
This is because, in the log representation, taking
the square root is equivalent to division by two.
The voltage at pin 6 is proportional to the mean of
the square at approximately 3 mV/dB, and propor-
tional to the square root of the mean of the square
at approximately 6 mV/dB.
Figure 6. 2252 DC Output Vs. AC Input Level
1 e
Figure 5. Tone Burst Response
in
t
)
4
R
shape of the attack curve. The
1
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
, the ripple voltage at the out-
4
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
in
V
. Larger values of
2
T
in
f
in
) in the output for any
, where V
R
is the rms
reduce
Output Buffering and Level Shifting
The voltage at pin 6 is buffered by OA3, and level
shifted down by the bias voltage V3. Level shifting
is required so that the output voltage will be zero
when the rms input current reaches a predeter-
mined value, I
match, and represents the 0 dB reference of the
circuit.
The various level shifts throughout the 2252 are as
follows: V2 represents one diode drop, so the volt-
age at the emitter of Q4 is +1V
OA2 is two diode drops higher than this, or
+3V
output of OA2, so the voltage at pin 6 will be
+2V
setting the voltage at pin 7 to 0 V.
Of course, the actual value of all these level shifts
is dependent on the currents through the transis-
tors responsible for each V
turn, are dependent on the bias programming cur-
rent in pin 2 (I
from pin 6 (I
follows:
I
causing 0 V output, I
I
from the geometry of the transistors involved.
Figure 6 plots output voltage versus input level for
a 2252 in its recommended circuit configuration
(Figure 4). In this plot, 0 dBr
plots output voltage for several different con-
stant-amplitude frequency sweeps for the same cir-
cuit. The vertical divisions are 60 mV apart,
representing approximately 10 dB increments. Full
audio bandwidth is maintained over a 60 dB dy-
namic range.
Current Programming
All the internal current sources in the 2252 are
slaved to the current in pin 2, I
above, the choice of this current affects I
in
BIAS
0
BE
BE
is the current in pin 2. The factor 2.9 derives
Figure 7. 2252 DC Output Vs. Frequency at
. Q6 will subtract one diode drop from the
. Finally, V3 represents two diode drops,
I
BIAS T
2 9 .
I
T
,
in0
). This dependence may be given as
BIAS
. This current is often called level
where I
Various Levels
) and the timing current pulled
T
is the current in pin 6, and
in0
BE
is the input current
. These currents, in
BIAS
BE
43 mV. Figure 7
. The output of
. As mentioned
in0
. I
Page 5
BIAS