THAT2252 THAT, THAT2252 Datasheet - Page 9

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

600032 Rev 01
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
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 I
I
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
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
“floor” below which input signals will be obscured.
T
, and the size of the feedback resistor. In the cir-
resistor (Rf) is connected from the output
rectification,
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
ultimately
producing
resistor usu-
BIAS
a
,
For typical values of R
rent would be 800 nA, upsetting rectification for ac
signals below approximately 8
completely obscuring signals below 800 nA peak.
C
tending
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, V
tically, the voltage at pin 1 will consist of this offset
plus a small ac signal determined by the value of
in
IN
Figure 11. Extended Low-Level Range with Selected
SYM
in Figures 4 and 11 blocks this dc current, ex-
50k
IN
V+
R
10k
2
R
10k
Figure 12. Measuring DC and AC Signals
1u
3
low-level
+
Cin
20u
10k
-
R
OA1
4
SYM
50k
V+
10k
R
OA1
1
V-
24k
47k
20
Rin
10k
performance
1u
OA2
in
2252s
10k
V-
24k
47k
Rin
20
OA2
1
10 k , this dc input cur-
+
-
560k
IN
4
1u
SYM
V-
V-
Rb
2252
5
2M2
2
GND
R
IBIAS
1
T
3
560k
SYM
IN
4
8
V-
V-
CAP
Rb
OUT
2252
5
V+
to
2
2M2
6
GND
A, and almost
IBIAS
R
T
3
8
Rf 22M
7
CAP
OUT
OFF(IN)
V+
1k
10u
well
6
7
C
10u
1k
10u
T
V+
10u
C
. Prac-
OUT
Page 9
below
T
V+
OUT