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 M
W 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 M
W 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.
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.
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, VOFF(IN). Prac-
tically, the voltage at pin 1 will consist of this offset
plus a small ac signal determined by the value of
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 9
V+
V+
OUT
IN
7
2
6
3
8
1
5
4
2252
V+
OUT
CAP
V-
SYM
GND
IBIAS
IN
V-
V-
T
C
T
R
SYM
50k
24k
47k
20
1k
2M2
1u
Rb
560k
10u
Cin
20u
10u
Rin
10k
Figure 11. Extended Low-Level Range with Selected
2252s
V+
V+
OUT
IN
7
2
6
3
8
1
5
4
2252
V+
OUT
CAP
V-
SYM
GND
IBIAS
IN
V-
V-
+
-
+
-
T
C
T
R
Rin
R1
10k
R
10k
2
R4
10k
R
10k
3
OA2
OA1
OA2
OA1
1u
10k
SYM
50k
24k
47k
20
Rf 22M
1k
2M2
1u
Rb
560k
10u
10u
Figure 12. Measuring DC and AC Signals
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