THAT2252 THAT, THAT2252 Datasheet - Page 10

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

Page 10
R
is intended to buffer V
additional current from the pin 1 node. For the
output of OA2 to be an accurate representation of
V
current opamp, and should have either low input
offset voltage (< 0.1 mV) or be trimmed to have
low input offset. Typical choices would include an
OP07 or an LM108A with a trim for offset voltage.
The voltage at OA2’s output is divided by 2
through the two 10 k
1 f capacitor bypasses any ac signal at the output
of OA2 so that only dc is present at the
noninverting input of OA1. Assuming that the in-
put to the entire circuit is connected to a 0
age source, the voltage gain from the noninverting
input of OA1 to its output will be 2.0, so the com-
bined gain from the output of OA2 to the output of
OA1 is unity.
In this fashion, the dc voltage at the output of OA1
is equal to V
no-signal conditions, no current flows in R
signal, this current is
also be a low-offset opamp, though its input cur-
rent is not as critical as that for OA2. Typical
choices here would include an OP27 or an
LM308A type with offset-voltage trimming.
An additional consideration is that R
R
age applied to R
V
measured will also affect the gain to V
well-defined, but non-zero source impedances, re-
duce the value of R
uncontrolled, non-zero source impedances, the in-
put signal should be buffered.
With this technique, the frequency response is ex-
tended to dc, while a slight loss in low-level accu-
racy results from the additional bias current
drawn by OA2.
Temperature-Compensated Measurements
In certain applications, the 2252 may be used for
high-precision measurements where the 0.33% / °C
variation in output scaling may be unacceptable.
For these situations, the output should be temper-
ature
3300ppm / °C resistor. Such resistors are available
from RCD Components, Inc., 3301 Bedford St.,
Manchester,
KOA/Speer Electronics, PO Box 547, Bradford, PA,
16701 USA [(814)-362-5536]. Figure 13 shows a
typical circuit. For good results, R1 should be lo-
OFF(IN)
OFF(IN)
in
4
should be precision tolerance types, or the volt-
and the loop gain of the internal amplifier. OA2
, OA2 must be a low drift, low input bias
. The source impedance of the voltage to be
compensated
OFF(IN)
NH,
in
3
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
plus the input voltage. Under
will not accurately represent
by the value of the source. For
USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
OFF(IN)
through
R
V
resistors R
in
in
. Of course, OA1 must
[(603)-669-0054]
without drawing any
the
1
1
and R
, R
use
OFF(IN)
2
, R
in
2
. With
3
of
. The
. For
volt-
and
and
a
cated physically close to the 2252 to optimize
temperature tracking.
Unusual Applications
Since the 2252 contains a precision rectifier fol-
lowed by a logging stage, it is possible to think of
other ways to apply the part beyond its conven-
tional use as an rms-level detector.
Log Amplifier
In applications where precision logging is re-
quired, the rectifier can be entirely bypassed by
driving the input with a single-sided dc signal, and
the logged output used directly. In such cases, the
timing capacitor (C
should be reduced to a small value (1~10 nf) to
quiet noise in the output while avoiding significant
delay in the response.
Log-Responsive Peak Detector
Where peak detection is desired, the 2252 may be
configured with very fast time constants (as in the
Log Amplifier above), and followed by a peak hold
amplifier. The advantage of using the 2252 for this
application is that the 2252 provides rectification
and logging before the peak detector, which allows
extremely wide dynamic range in the resulting sig-
nal.
Closing Thoughts
THAT Corporation welcomes comments, questions
and suggestions regarding this device, its design
and application. Our engineering staff has exten-
sive experience in applying this part to commercial
circuits. We are pleased to offer assistance in opti-
mizing your circuitry to adapt the 2252 to your
particular application. Please feel free to contact
us with your thoughts and questions.
SYM
50k
V+
IN
Figure 13. Temperature Compensating the 2252
Cin
20u
V-
24k
47k
20
10k
Rin
1u
1
560k
IN
SYM
V-
Rb
V-
5
2252
4
R T
2M2
GND
IBIAS
T
2
3
Rf 22M
THAT 2252 RMS-Level Detector
) connected to the pin 6
CAP
OUT
V+
6
8
7
1k
10u
+3300ppm
C T
10u
R1 2k
V+
R2
2k
OA1
OUT