THAT2252 Datasheet, PDF (10/10 Pages) List of Unclassifed Manufacturers – IC RMS-Level Detector

Page 10
THAT 2252 RMS-Level Detector
Rin and the loop gain of the internal amplifier. OA2
is intended to buffer VOFF(IN) without drawing any
additional current from the pin 1 node. For the
output of OA2 to be an accurate representation of
VOFF(IN), OA2 must be a low drift, low input bias
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 kW resistors R1 and R2. The
1 mf 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 W volt-
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_OFF(IN) plus the input voltage. Under
no-signal conditions, no current flows in Rin. With
signal,
this
current
is
V_in
R _in
.
Of
course,
OA1
must
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 R1, R2, R3 and
R4 should be precision tolerance types, or the volt-
age applied to Rin will not accurately represent
VOFF(IN). The source impedance of the voltage to be
measured will also affect the gain to VOFF(IN). For
well-defined, but non-zero source impedances, re-
duce the value of R₃ by the value of the source. For
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 compensated through the use of a
3300ppm / °C resistor. Such resistors are available
from RCD Components, Inc., 3301 Bedford St.,
Manchester, NH, USA [(603)-669-0054] and
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-
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-
SYM
50k
V+
V-
1u ^Rb
560k
V+
1k
24k
10u
42 8
47k
1
SYM IBIAS V+
IN 2252 OUT ⁷
20
V- GND CAP
R1 2k
5 3 6 ^+3300ppm
R2
2k
OA1
OUT
IN
Cin
Rin
20u ^10k
RT
CT
2M2
10u
V-
Rf 22M
Figure 13. Temperature Compensating the 2252
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 (CT) connected to the pin 6
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.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com