T H AT
Corporation
IC RMS-Level Detector
THAT
2252
FEATURES
·
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·
·
True RMS Response
Wide Dynamic Range: >80 dB
High Crest Factor: 8 (1 dB error)
Wide Bandwidth: to > 20 kHz
Logarithmic Output Scaling
Low Cost: $2.20 in ’000s
Single In-Line Package
Matches 2180 and 2181 Series
VCAs
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APPLICATIONS
Meters
Spectrum Analyzers
Compressors
Expanders
Oscillators
Psychoacoustic Modeling
Noise Measurement
Description
The THAT 2252 integrated-circuit rms-level detec-
tor is designed to convert an ac input current into a
dc output voltage. The output is proportional to the
log of the true rms value of the input signal. The
parts are housed in a space-efficient plastic 8-pin
single-in-line (SIP) package, and require minimal
support circuitry. Based on dbx technology and fab-
ricated in a super low-noise process, the 2252 com-
bines wide dynamic range with frequency response
to beyond 20 kHz. The logarithmic output is espe-
cially convenient for audio applications requiring
decibel-linear scaling. The integration time is ad-
justable via an external R/C pair. With some exter-
nal circuitry, response to dc is also possible.
PIN 1
MODEL NO.
8
THAT
BIAS
CURRENT
COMP
1
BIAS
CURRENT
COMP
2
H
J
N
B
D
C
E
TYP.
G
F
M
L
K
I
A
1
3
4
2
+
V
-
7
ITEM
A
B
C
D
E
F
G
H
I
J
K
L
M
N
MILLIMETERS
20.32 MAX.
1.1 MIN.
0.5 ± 0.1
0.25
2.54
1.27 MAX.
0.51 MIN.
5.08 MAX.
2.8 ± 0.2
5.75 MAX.
1.5 MAX.
0.25 +0.10 -0.04
3.2 ± 0.5
1.1 MIN.
INCHES
0.8 MAX.
0.043 MIN.
0.02 ± 0.004
0.01
0.1
0.05 MAX.
0.02 MIN.
0.2 MAX.
0.11 ± 0.008
0.227 MAX.
0.058 MAX.
0.01 +0.004 -0.002
0.126 ± 0.02
0.043 MIN.
5
6
Figure 1. 2252 Equivalent Circuit Diagram
Figure 2. 2252 Physical Outline
dbx
is a registered trademark of Carillon Electronics Corporation
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Page 2
THAT 2252 RMS-Level Detector
SPECIFICATIONS
1
Absolute -Maximum Ratings (TA = 25°C)
Positive Supply Voltage (VCC)
Negative Supply Voltage (VEE)
Supply Current (ICC)
+18
-18
10
V
V
mA
Power Dissipation (PD)
Operating Temperature Range (TOP)
Storage Temperature Range (TST)
330
mW
-20 to +75°C
-40 to +125°C
Recommended Operating Conditions
Parameter
Positive Supply Voltage
Negative Supply Voltage
Bias Set Current
Signal Current
Timing Current
Symbol
V
CC
V
EE
I
BIAS
I
in
I
T
I
BIAS
= 24
mA
Conditions
Min
+4
-4
15
—
1
Typ
+12
-12
24
—
7.5
Max
+15
-15
50
1
50
Units
V
V
mA
mA
mA
Electrical Characteristics
2
Parameter
Supply Current
Equiv. Input Bias Current
Input Offset Voltage
Symmetry Voltage
Output Scale Factor
Symbol
I
CC
I
B
V
OFF(IN)
V
SYM
E
O
/20log(I
in
/
I
in0
) 31.6nA<I
IN
<1mA
T
A
=25°C (T
CHIP
»35°C)
I
in0
f
IN
= 1kHz
1mA <
I
in
< 100mA
100nA <
I
in
< 316mA
31.6nA <
I
in
< 1mA
1ms pulse repetition rate
0.2 dB error
0.5 dB error
1.0 dB error
I
in
³
100mA
I
in
³
10mA
I
in
³
1mA
I
in
³
100nA
Conditions
No Signal
No Signal
No Signal
Min
—
—
0
-2
Typ
1
5
+8
8
Max
3
8
+16
+18
Units
mA
nA
mV
mV
6.0
I
BIAS
×
I
T
3.5
6.1
I
BIAS
×
I
T
2.9
6.2
I
BIAS
×
I
T
2.4
mV/dB
Input Current for 0V Output
Output Linearity
—
—
—
0.1
0.5
1.0
—
—
—
dB
dB
dB
Crest Factor
—
—
—
—
3.5
5
8
80
74
30
4
—
—
—
—
—
—
—
C
T
I
T
—
kHz
kHz
kHz
kHz
s
%/°C
Maximum Frequency for 1 dB additional error
Filtering Time Constant
Output TempCo
D
E
0
/
D
T
CHIP
Re: T
CHIP
= 27°C
—
(
.026
)
0.33
1. All specifications subject to change without notice.
2. Unless otherwise noted, T
A
=25°C, V
CC
= +15V, V
EE
= -15V. Test circuit is as shown in Figure 4. SYM is adjusted for
minimum ripple at V
out
with V
in
=1 V
RMS
, 100 Hz.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
600032 Rev 01
Page 3
Theory of Operation
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.
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.
Mathematically,
I
C
3
=
{
-
I
in
,
I
in
£
0
0,
I
in
>
0
,
and
I
C
1
=
I
C
2
=
{
0,
I
in
£
0
I
in
,
I
in
>
0
.
Current Rectification
Figure 3 presents a simplified internal circuit dia-
gram of the 2252. The input signal current,
I
in
,
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
in
will drive OA1’s output negative, cutting
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
W
resistor to ground
(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
C2
) will mirror that in the collector of Q1 (I
C1
),
which equals the positive input current.
Since the input impedance of OA2 is high, the cur-
rent in the emitter of Q4 (I
C4
), is the sum of the
currents
I
C2
and
I
C3
. The mirror action of Q1/Q2
But,
I
C
4
=
I
C
3
+
I
C
2
=
I
C
3
+
I
C
1
=
{
-
I
in
,
I
in
<
0
I
in
,
I
in
>
0
=
I
in
.
See Figure 3 for definitions of these currents.
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
log
, is therefore pro-
portional to the log of the square of the input cur-
rent, plus a bias voltage (V2).
Mathematically,
æ
I
C
V
log
=
2
V
T
1
n
ç
I
4
ç
S
è
ö+
V
÷
2
ø
=
2
V
T
1
n
(
I
C
4
)
-
2
V
T
1
n
(
I
S
)
+
V
2
=
V
T
1
n
(
I
C
4
)
-
2
V
T
1
n
(
I
S
)
+
V
2
2
=
V
T
1
n I
in
2
-
2
V
T
1
n
(
I
S
)
+
V
2
,
kT
,
q
Where V
T
is the thermal voltage,
and I
S
is the
reverse-saturation current of Q4 and Q5 (assumed
to be the same in each).
-
+
OA1
IC3
+
V2
-
Q3
Q4
IC2
IC4
-
+
OA2
Vlog
V3
+
-
1
Iin
IC1
Q5
Q6
I
-
+
OA3
T
7
Vout
20
Q1
+
V1
-
Q2
6 V6
4
Figure 3. Simplified Internal Schematic
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Page 4
THAT 2252 RMS-Level Detector
Computing the Mean
In the classic mathematical definition of rms
value, the time integral of the square of the signal
must be evaluated over infinite time. Obviously, for
a practical measurement, only a finite time is
available, which leads to the question of how to
weight events occuring at various times. Tradi-
tionally, the simplest and most meaningful weight-
ing is exponential in time, giving highest weight to
the most recent history, and exponentially less
weight to increasingly older events. This weighting
corresponds to convolution in time with the famil-
iar exponential weighting function,
e
t
.
To accomplish this weighting, Pin 6 is normally
connected to a capacitor and a negative current
source. (Refer to the
Typical Application Circuit
in
Figure 4. In this circuit, C
T
is the capacitor and R
T
together with V- form the current source.) This
current source establishes a quiescent dc bias
current, I
T
, through Q6. Over time, the capacitor
charges to 1 V
BE
below V
log
(the potential at the
output of OA2).
The instantaneous emitter current in Q6 is propor-
tional to the antilog of its V
BE
, which is the differ-
ence between Q6’s base voltage and the voltage at
pin 6. The potential at the base of Q6 represents
the log of the
square
of the input current, while
the emitter of Q6 is held at ac ground via the ca-
pacitor. Since Q6’s emitter current is proportional
to the antilog of its V
BE
, the current in Q6 is pro-
portional to the square of the instantaneous input
current.
Note that this antilogging only takes place for
dy-
namic
signals. For a dc input, the output of OA2
represents the square of the input current. After
charging, the external timing capacitor voltage
again approaches one diode drop below V
log
. The
exact value of the diode drop will be determined by
V+
1k
10u
4
1
IN
the bias current I
T
. However, for sudden increases
in the input current
I
in
, the current available to
charge the capacitor C
T
is proportional to the
square of the short-term increase in input current.
The “dynamic” antilogging causes the capacitor
voltage to represent the log of the mean of the
square of the input current.
Time Constants
Another way of looking at this situation is to con-
sider the action of Q6 and C
T
as a first-order filter
in the log domain. Q6 and C
T
establish a single
pole at a frequency determined by a) the imped-
ance of Q6 at the bias current I
T
and b) the value
of C
T
. The time constant
t
is given below.
t =
C
T
=
C
T
V
T
I
T
0.0259
I
T
-
t
, at 300° Kelvin.
The result is that the voltage at pin 6 represents
the average (or mean) of the square of the input
signal, averaged over the time constant
t.
The av-
eraging corresponds to convolution with the time
weighting of a simple RC circuit. Mathematically,
this is as follows:
-
t
æ
ö
2
V
6
a
1
n
ç
1
2
ò
T
I
in
e
t
dt
÷
, where T is the time at
0
ç
T
÷
ø
è
which the average level is computed. Note that
æ
-
t
ö
ç
e
t
÷
represents the exponential time weighting
ç
÷
ø
è
imposed by the log-domain filter.
How fast the 2252 acquires a signal (the “attack”),
and how fast it returns to rest following a signal
(the “release”), are locked in relationship to each
other by the nature of the exponential
time-weighting imposed by this log-domain filter.
Separate attack and release adjustments are not
possible within the constraint of rms response.
The time response for typical values of I
T
and C
T
(the circuit of Figure4 ) is shown in Figure 5,
which shows the 2252’s response to a 100 ms,
1 kHz tone burst at ~+10 dBV followed by
~500 ms of 1 kHz at ~–30 dBV. The top trace is
the input tone burst (at 10 V/div), the bottom trace
is the output at 50 mV/div. The time scale is
50 ms/div.
The shape of the attack and release waveforms is
determined by the interaction of the exponential
response of the log-domain filter with the
log-representation of the signal. The straight-line
decay follows from the fact that the natural release
of the exponential time weighting is a decaying ex-
ponential in the linear world. This maps to a
straight line in the log representation. The attack
in the photo appears exponential, but actually fol-
V-
1u
SYM
50k
V+
47k
20
24k
Rb
560k
2
2252
3
8
OUT
SYM IBIAS V+
V- GND CAP
7
OUT
5
6
IN
Cin
20u
Rin
10k
V-
RT
2M2
CT
10u
Rf 22M
Figure 4. Typical Application Circuit (±15V)
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
600032 Rev 01
Page 5
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
in0
. This current is often called
level
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
BE
. The output of
OA2 is two diode drops higher than this, or
+3V
BE
. Q6 will subtract one diode drop from the
output of OA2, so the voltage at pin 6 will be
+2V
BE
. Finally, V3 represents two diode drops,
setting the voltage at pin 7 to 0 V.
Figure 5. Tone Burst Response
lows the (1-
e
-
t
) shape of the attack curve. The
transformation from the linear to the log world
steepens the apparent attack shape.
The time constant,
t,
also determines the amount
of ripple (at frequency 2f
in
) in the output for any
given input frequency, f
in
. Larger values of
t
reduce
ripple at the expense of longer attack and release
times. For
f
in
>>
4 1
t
, the ripple voltage at the out-
p
put is given by:
V
R
»
ripple voltage.
4
p
V
T
,
2
f
in
t
Of course, the actual value of all these level shifts
is dependent on the currents through the transis-
tors responsible for each V
BE
. These currents, in
turn, are dependent on the bias programming cur-
rent in pin 2 (I
BIAS
) and the timing current pulled
from pin 6 (I
T
). This dependence may be given as
follows:
I
in
0
=
I
BIAS
I
T
,
2.9
where
I
in0
is the input current
where V
R
is the rms
causing 0 V output, I
T
is the current in pin 6, and
I
BIAS
is the current in pin 2. The factor 2.9 derives
from the geometry of the transistors involved.
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 7. 2252 DC Output Vs. Frequency at
Various Levels
Figure 6 plots output voltage versus input level for
a 2252 in its recommended circuit configuration
(Figure 4). In this plot, 0 dBr
»
43 mV. Figure 7
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
Figure 6. 2252 DC Output Vs. AC Input Level
All the internal current sources in the 2252 are
slaved to the current in pin 2, I
BIAS
. As mentioned
above, the choice of this current affects
I
in0
. I
BIAS
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
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