THAT2252 Datasheet, PDF (7/10 Pages) List of Unclassifed Manufacturers – IC RMS-Level Detector

600032 Rev 01
Page 7
Applications
The intended purpose of the 2252 is to compute
the log of the time-weighted root-mean-square of
an input current signal. Several practical consider-
ations apply when attempting to make full use of
the 100 dB dynamic range of the device.
Input Considerations
The 2252 input is intended to accept a current: as
is clear from Figure 3 (page 3), pin 1 is a summing
junction supplied with internal negative feedback.
Maximum input current is on the order of 1 mA,
limited by internal device characteristics. The min-
imum practical input current is determined by the
effects of internal bias currents (whether from
OA1, OA2, or the effect of V1). At currents as low
as 31 nA, approximately 1 dB error in reading will
result from interal bias currents.
The gain of the input op amp (OA1) is sufficient to
provide effective logging for Rin ³ 10 kW. However,
with R_in < 10 kW., low level and high frequency
performance will suffer due to the finite open-loop
gain of OA1. This is because OA1 is required to
swing across a “dead zone” between turning on Q1
and Q3. The dead zone is reduced by V1. But, V1
is small (»0.5 V) to maintain low-level tracking, so
OA1 must still swing through several tenths of a
volt at each reversal of the input polarity. The
smaller the value of Rin, the higher the loop gain
demanded from OA1 for accurate rectification.
Therefore, for good high-frequency performance,
R_in should be 10 kW or larger. Otherwise, choose
Rin based on the desired input voltage at level
match,
Ein0,
and
Iin0
as
follows:
R_in
=
E_{in 0}
I_{in 0}
The negative input of OA1 typically rests at +8 mV.
If dc coupled, this will cause an input current to
flow which will effectively set a low-level “floor” be-
low which readings will be obscured. Therefore, ac
coupling is required if low level signals are to be
accurately observed. Choose the value of the ac
coupling capacitor (Cin in Figure 4) based on the
value of Rin and the desired low-frequency limit.
C_in
=
2p
1
R_in
F_C ^,
where
F_C
is
the
desired
3
dB-down
point. (For dc coupling, see the section on page 9,
DC Measurements.)
Symmetry Adjustment
As noted earlier, the rectifier relies on the match-
ing of Q1 and Q2 for precise reproduction of posi-
tive-half input currents. Q2’s base is brought out
to pin 4 to allow adjustment of this match. Pin 4
should be connected to a 20 W voltage source capa-
ble of supplying from –4 mV to +20 mV. The appli-
cation circuits in Figure 4 (page 4) and Figure 11
(page 6) are typical.
To set the symmetry, apply a low-frequency sine
wave to the input. Neither the frequency nor the
level are critical: 100 Hz at near level match is
usually a good choice. Observe the output wave-
form with a ‘scope while adjusting the symmetry
trim. With proper adjustment, the ripple in the
output will be almost pure second harmonic of the
input frequency. No fundamental frequency should
be present in the output. Another method would
be to sense ripple in the output via a narrow
bandpass filter centered at the fundamental feed-
ing an ac voltmeter: adjust the trim for minimum
voltage reading.
The actual voltage required for proper symmetry
depends on the input offset voltage of OA1 (typi-
cally +8 mV), and the VBE mismatch between Q1
and Q2 (< ±6 mV). For less critical applications
where precise rectification is not required, pin 4
may be connected to a voltage matching the input
offset voltage of OA1, preferably through a 20 W re-
sistor (to match the 20 W in Q1’s base). The sim-
plest circuit connects pin 4 to pin 1 through a
20 W resistor, as shown in Figure 9. This connec-
tion ensures that the VBE of Q1 will equal that of
Q2, but does not allow for adjustment for any mis-
match in the two devices. When using this configu-
ration, one should try to keep the bias
programming current below 20 mA, to ensure sta-
ble operation.
Time Constants
Both the capacitor (C_T) and current source (I_T)
connected to pin 6 control the time constant over
which the rms value of the input current is evalu-
ated. Either may be varied, but a few practical con-
siderations influence the choice of values. First,
the input bias current of OA3 in Figure 3 will add
to the charging current IT. For small values of IT,
this will affect the accuracy of the resulting time
constant. The input bias current for OA3 is typi-
cally 100nA, so IT should be kept above 1 mA.
At the other extreme, I_T flows from OA2 and
through Q6 under steady-state conditions. Dy-
namically, as was mentioned on page 4 in Com-
puting the Mean, the current which charges CT is
proportional to the square of the short-term in-
crease in Iin. A sudden 30 dB increase in input
causes a 60 dB increase in charging current. For
example, if IT is 10 mA, the peak charging current
will be 10 mA. However, if I_T were 100 mA, the
peak charging current called for would be 100 mA.
The devices within the 2252 will not support this
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