THAT2252 THAT, THAT2252 Datasheet - Page 3

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

600032 Rev 01
The THAT 2252 RMS-Level Detector is designed
for high performance in audio-frequency applica-
tions requiring logarithmic output, rms response,
and wide dynamic range. The parts compute rms
level by rectifying input current signals, converting
the resulting current waveform to a logarithmic
voltage, and applying this voltage to a log-domain
filter.
Current Rectification
Figure 3 presents a simplified internal circuit dia-
gram of the 2252. The input signal current, I
flows in pin 1, the input pin. OA1 drives the base
of Q3 and the emitter of Q1 (through V1) to main-
tain pin 1 at virtual ground potential. A negative
input current (flowing out of pin 1) will tend to
drive the inverting input of OA1 negative, driving
OA1’s output positive, turning on Q3. V1 is de-
signed to cut off Q1 while Q3 is on. Therefore, neg-
ative input currents are forced to flow through the
collector-emitter of Q3.
Positive I
off Q3 and turning on diode-connected transistor
Q1. Positive input current is thereby forced to flow
through the collector-emitter of Q1. Pin 4 is nor-
mally connected through a 20
(see Figure 4, Typical Application Circuit, Page 4,
and Symmetry Adjustment, Page 6), so the
base-emitter potential of Q2 is the same as that of
Q1. Therefore, the current in the collector of Q2
(I
which equals the positive input current.
Since the input impedance of OA2 is high, the cur-
rent in the emitter of Q4 (I
currents I
C2
) will mirror that in the collector of Q1 (I
1
4
in
Iin
C2
will drive OA1’s output negative, cutting
and I
20
C3
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
. The mirror action of Q1/Q2
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
+
-
IC1
OA1
C4
Q1
), is the sum of the
resistor to ground
Q3
Figure 3. Simplified Internal Schematic
+
-
Theory of Operation
V1
IC2
IC3
Q2
C1
IC4
in
),
,
Q4
+
-
V2
reverses the positive input currents so that they
add to the negative input currents in Q4. The cur-
rent in Q4, therefore, is equal to the absolute value
of the input current.
Logging Action
OA2, together with Q4 and Q5, forms a log ampli-
fier. By using two diode-connected transistors in
the feedback loop of OA2, the 2252 produces a
voltage proportional to twice the log of IC4 at the
output of OA2. This voltage, V
portional to the log of the square of the input cur-
rent, plus a bias voltage (V2).
Where V
reverse-saturation current of Q4 and Q5 (assumed
to be the same in each).
+
-
OA2
I
But, I
Mathematically,
See Figure 3 for definitions of these currents.
Mathematically,
Q5
C
V
3
log
T
C
0
4
2
is the thermal voltage,
I
,
in
V n
I
2
V
V
in
,
T
Vlog
T
T
I
I
V
1
in
C
Q6
T
1
1
I
3
0
I
in
n I
n I
in
1
I
0
I
in
n I
,
I
C
.
,
S
I
I
,
4
and I
in
C
in
C
I
in
4
2
C
6
2
4
0
2
V
0
I
V6
2
C
2
C
I
2
1
T
3
2
V
V
T
V
T
+
-
T
1
I
I
C
1
C
n I
1
log
OA3
2
n I
1
n I
V3
, is therefore pro-
S
kT
S
q
S
-
0
I
, and I
in
,
I
,
V
in
I
V
2
V
in
2
,
Vout
2
0
7
0
S
.
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is the