THAT2252 Datasheet, PDF (6/10 Pages) List of Unclassifed Manufacturers – IC RMS-Level Detector

Page 6
THAT 2252 RMS-Level Detector
is normally set by a resistor from pin 2 to ground
(VCC would do also). Note that the voltage at pin 2
is normally VEE + 2.1 V, where VEE is the negative
supply voltage.
Symmetry Adjustment
The rectifier (Q1 through Q3 and OA1) depends on
the matching between Q1 and Q2 for accurate re-
production of positive-half input signals. The base
of Q2 is brought out to pin 4 in order to allow ad-
justment for mismatches between these transis-
tors and for the input offset voltage of OA1. Pin 4
should be connected to a variable low-impedance
voltage source capable of supplying a few millivolts
(VSYM). The base of Q1 is connected to its collector
through a 20W resistor to allow matching between
the base impedances of Q1 and Q2. A 20W source
should be used at pin 4 for optimum matching.
Input Bias Currents and Voltages
OA1 will rectify any currents flowing in its feed-
back network, including the bias current of OA1 it-
self. Input signals below OA1’s bias current will be
obscured, with a steady dc output “floor” the re-
sult. The input bias compensation for OA1 largely
cancels OA1’s bias current, improving low-level
performance. Even if overcompensated, any uncor-
rected bias current (positive or negative) in OA1
results in a limit to the low-level resolution of the
2252.
Any dc offset voltage at pin 1 will cause dc input
currents to flow if the input is dc coupled. The dc
offset at pin 1 is typically 8 mV, so the input
should be ac coupled (as shown in Figure 4) for
most applications. If low-level performance is not
required, dc coupling may be used without further
modifications to the application circuits. However,
dc coupling and good low-level performance are
possible with additional external circuitry, as de-
scribed in the section on DC Measurements on
Page 9.
Positive equivalent input bias current in OA2 (after
correction) will also add to the output level read-
ing. This is because the equivalent input bias cur-
rent is supplied through Q4 and Q5, which adds to
the dc level at the output of OA2. OA2, like OA1,
has input bias compensation. While generally ex-
tending low-level performance, the compensation
can cause OA2’s net input bias requirement to be-
come negative, tending to reverse bias Q4 and Q5.
The circuit of Figure 4 uses a 22 MW resistor, Rf,
to supply increasing dc input current as the 2252
output drops. This forces current to flow in Q2,
which overwhelms any negative input bias at OA2
and prevents the current in Q4 and Q5 from re-
versing.
Rf is responsible for the flattening of the low-level
response shown in Figure 6 (Page 5). The feed-
back current provided enforces a lower limit to the
resolvable input current. Without this feedback,
those 2252s where the OA2 input bias is
overcorrected will exhibit a sharp downward swing
in output level at low input signals (see Figure 8).
For many applications, such low-level response is
not necessary. In such cases, Rf may be deleted, as
shown in Figure 11. See further discussion of this
subject in Output Considerations, (Page 8).
Figure 8. Low Level Output from Several 2252s
(no Rf)
V-
1u
20
Rin
10k
IN
Cin
20u
Rb
750k
V+
1k
10u
428
SYM IBIAS V+
1
7
IN 2252 OUT
V- GND CAP
536
OUT
RT
CT
2M2
10u
V-
Rf 22M
Figure 9. Application Circuit Without Symmetry
Control
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com