THAT2252 Datasheet, PDF (8/10 Pages) List of Unclassifed Manufacturers – IC RMS-Level Detector

Page 8
THAT 2252 RMS-Level Detector
high a current demand, so timing will be less accu-
rate under this condition. (The steady-state out-
put voltage will be accurate, since over time the
error will be “forgotten” due to the time-weighting
e
-t
t
.
However,
the
dynamic
response
will
be
incor-
rect.)
The best performance normally results when
5 mA < IT < 50 mA. CT is usually chosen to be a
convenient value which results in the desired time
constant t, and an IT within these limits. See Time
Constants (page 4) for the relationship between t,
I_T and C_T.
Since C_T is commonly 1mf or larger, this capacitor
is often a polarized electrolytic or tantalum. V6 is
normally positive approximately 2Vbe (»1.4 V),
which determines the polarity of CT. Leakage in CT
will add to IT, so low-leakage electrolytics or
tantalums are preferred. Tantalum, once preferred
for long-term stability and low leakage, has been
largely superceded by premium electrolytic types
which are available in low-leakage, high-stability
versions. When using an electrolytic, however, it is
good practice to select a voltage rating higher than
absolutely necessary (at least 15 V is recom-
mended).
When a resistor to the negative supply is used as
the IT current source (as in the circuits shown
here), determine its value by the equation:
R_T
=
V_EE +
I_T
1.4_,
where
V_EE
is
the
negative
supply
voltage.
Bias Programming
As shown in Figure 1 (page 1), pin 2 is connected
to the negative supply through three diodes in se-
ries. Its typical voltage is V_EE + 2.1 V. The current
in these diodes, IBIAS, controls the current sources
throughout the 2252, and therefore affects the
bandwidth of the device, as well as Iin0. Because it
is responsible for programming so many currents,
IBIAS should be restricted to a relatively narrow
range about the nominal 24 mA.
Normally, IBIAS is chosen to fine-tune Iin0, the 0 dB
reference
point.
I_BIAS
=
8.4
I_in0
I_T
2
.
The
factor
8.4
de-
rives from the geometry of the devices in the 2252.
As with the value of RT, Rb is determined by the de-
sired IBIAS and the negative supply voltage, as fol-
lows: R_b = ^VE_I^EBIA-_S^2.1.
Power Supply Bypassing
The 2252 is not particularly sensitive to power
supply impedances, but the high charging currents
which are possible in CT must be kept out of the
PC board ground and voltage supply system for
good results. The application circuits show a 1 kW
resistor in series with the positive supply pin
(pin 8), and a 10 mf capacitor bypassing this pin to
ground. The ground end of this capacitor is in-
tended to be connected directly to the ground end
of CT. One (and only one) point on this trace
should be connected to the circuit ground system,
since high currents flow in this trace during input
signal transients.
Figure 8 illustrates the flow of current, ICHARGE,
during an input transient. Note that ICHARGE is
1k
V+
confined to a local
loop, flowing from
8
CB through OA2
and Q6 (within the
PART OF
OA2
I _CHARGE
2252) into CT, and
CB _{returning directly}
WITHIN
2252
Q6
6
CT
to CB. By keeping
the return trace
short, and con-
necting it to sys-
Figure 10.
Decoupling Supply Transients
tem ground at
only one point,
high currents are
kept out of the
overall ground system, while CT maintains its ref-
erence to ground potential through the single con-
nection to ground.
Another bypass capacitor is required at pin 2 (the
IBIAS pin). Typically, with IBIAS » 24 mA, the imped-
ance at pin 2 is approximately 3 kW to V–. If the in-
put signal capacitively couples into pin 2, IBIAS will
be modulated at the input signal frequency. This
modulation adds signal voltage (as ripple) to the
output at pin 7. (Recall that, ideally, the 2252 pro-
duces ripple only at the second harmonic of the in-
put signal.) When the 2252 is used to control the
gain of a voltage-controlled amplifier (VCA) such
as the THAT 2180 or 2181, this fundamen-
tal-frequency ripple will cause second harmonic
distortion in the VCA. Bypassing pin 2 to the nega-
tive supply shunts any signal-related currents
around the 2252 bias system, preventing this rip-
ple from occuring.
Output Considerations
The output of the 2252 (pin 7) presents a low
source impedance, but it is somewhat limited in
available current drive. Referring to Figure 1
(page 1), the negative current source at the output
pin is approximately 10 IBIAS. Therefore, for the
typical I_BIAS of 24 mA, the maximum current sink
at pin 7 is 240 mA. (The 2252 will source consider-
ably more than this current if required.) Since the
output voltage normally swings between ±300 mV,
the 2252 will directly drive loads of greater than
1.5 kW without difficulty.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com