THAT2252 Datasheet, PDF (9/10 Pages) List of Unclassifed Manufacturers – IC RMS-Level Detector

600032 Rev 01
Page 9
In the section on Input Bias Currents and Volt-
ages (page 6), it was noted that the 2252 output is
generally proportional to the log of the ac rms in-
put current. But, at low levels (<20 nA input cur-
rent), the input bias compensation in internal
opamp OA2 in Figure 3 (page 3) can cause a sharp
increase in the output change with input, as shown
in Figure 8, (page 6). Since this effect is dependent
on the accuracy of the bias compensation, some
2252s will exhibit this behavior, but most will not.
For most applications, where consistent low-level
response is desireable, the 2252 should be config-
ured to supply its own “correction” signal to pre-
vent this discontinuity from occuring, as shown in
the circuit of Figure 4, (page 4). In this circuit, a
22 MW resistor (Rf) is connected from the output
of the 2252 back to the input.
When the output voltage is near 0 V, Rf contributes
negligible current to the input of the detector. For
positive output voltages, the current in Rf is very
small compared to the ac input signal which
causes a positive output, so its effect is again negli-
gible. However, as the ac input signal drops in
level, the dc output goes negative, supplying more
and more input current through Rf, which tends to
drive the dc output positive. The point at which
the two effects will balance is dependent on IBIAS,
IT, and the size of the feedback resistor. In the cir-
cuit of Figure 4, the output will “bottom out” at ap-
proximately -300 mV, where Rf supplies ~14 nA of
dc current to the input.
Rf establishes a stable “floor” in the 2252’s re-
sponse. However, this self-supplied lower limit will
obscure very low-level input signals which could
be measured by those 2252s with good input-bias
correction at OA2 of Figure 3. If accurate low-level
response is quite important, the 2252s may be se-
lected, and Rf may be omitted, as shown in Fig-
ure 11. The 2252s should be selected on the basis
of accurate low-level, low-frequency performance
(see Figure 8, page 6).
The circuit of Figure 11 may also be used with un-
selected 2252s for applications where accurate
low-level performance is unnecessary. However,
the relatively low cost of a 5% 22 MW resistor usu-
ally argues in favor of its use.
DC Measurements
As noted earlier, the 2252 is primarily intended for
measuring the rms value of ac signals. This follows
from the 2252’s relatively high input offset voltage
(typically +8 mV). If the input is dc coupled, the
input offset voltage will cause a dc current to flow
in the input, which will interfere with precise
low-level rectification, ultimately producing a
“floor” below which input signals will be obscured.
SYM
50k
V+
V-
1u
24k
Rb
560k
V+
1k
10u
4 28
47k
SYM IBIAS V+
1
IN
2252 OUT
7
OUT
20
V- GND CAP
536
IN
Cin
20u
Rin
10k
RT
2M2
CT
10u
V-
Figure 11. Extended Low-Level Range with Selected
2252s
For typical values of Rin » 10 kW, this dc input cur-
rent would be 800 nA, upsetting rectification for ac
signals below approximately 8 mA, and almost
completely obscuring signals below 800 nA peak.
Cin in Figures 4 and 11 blocks this dc current, ex-
tending low-level performance to well below
100 nA.
However, as shown in Figure 12, with a few added
parts, it is possible to extend response to dc with
little loss in low-level accuracy.
The essential purpose of the added circuitry (OA1,
OA2 and associated parts) is to buffer the input
offset voltage (at pin 1) and add it to the input sig-
nal. The circuit of Figure 12 operates as follows.
IN
SYM
50k
R₃
V+
10k
R₄
10k
-
OA1
R₂
+ _OA1
10k
R₁
1u
10k
V-
1u
24k
47k
20
Rin
10k
OA2 ₊
OA2
-
Rb
560k
V+
1k
10u
428
SYM IBIAS V+
1 _{IN 2252 OUT} 7
V- GND CAP
5 36
OUT
RT
CT
10u
2M2
V-
Rf 22M
Figure 12. Measuring DC and AC Signals
As has been noted, pin 1 is a virtual ground. Theo-
retically, the voltage at pin 1 is only the dc offset of
the 2252’s internal input amplifier, V_OFF(IN). Prac-
tically, the voltage at pin 1 will consist of this offset
plus a small ac signal determined by the value of
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