THAT2252 THAT, THAT2252 Datasheet - Page 8

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

Page 8
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
rect.)
The best performance normally results when
5
convenient value which results in the desired time
constant , and an I
Constants (page 4) for the relationship between ,
I
Since C
is often a polarized electrolytic or tantalum. V6 is
normally positive approximately 2V
which determines the polarity of C
will add to I
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largely superceded by premium electrolytic types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 I
here),
R
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
in these diodes, I
throughout the 2252, and therefore affects the
bandwidth of the device, as well as I
is responsible for programming so many currents,
I
range about the nominal 24 A.
Normally, I
reference point. I
rives from the geometry of the devices in the 2252.
As with the value of R
sired I
lows: R
Power Supply Bypassing
The 2252 is not particularly sensitive to power
supply impedances, but the high charging currents
which are possible in C
PC board ground and voltage supply system for
T
BIAS
T
and C
t
. However, the dynamic response will be incor-
T
V
should be restricted to a relatively narrow
BIAS
EE
current source (as in the circuits shown
b
T
determine
T
I
.
is commonly 1 f or larger, this capacitor
T
< 50
BIAS
1.4
and the negative supply voltage, as fol-
V
EE
I
BIAS
, where V
T
is chosen to fine-tune I
, so low-leakage electrolytics or
2.1
BIAS
BIAS
.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
its
T
, controls the current sources
C
T
within these limits. See Time
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
, R
T
8.4
EE
T
value
is usually chosen to be a
b
must be kept out of the
is determined by the de-
EE
I
is the negative supply
in0
I
T
+ 2.1 V. The current
2
. The factor 8.4 de-
by
T
. Leakage in C
the
in0
be
in0
. Because it
, the 0 dB
( 1.4 V),
equation:
T
good results. The application circuits show a 1 k
resistor in series with the positive supply pin
(pin 8), and a 10 f capacitor bypassing this pin to
ground. The ground end of this capacitor is in-
tended to be connected directly to the ground end
of C
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, I
during an input transient. Note that I
overall ground system, while C
erence to ground potential through the single con-
nection to ground.
Another bypass capacitor is required at pin 2 (the
I
ance at pin 2 is approximately 3 k
put signal capacitively couples into pin 2, I
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 I
typical I
at pin 7 is 240 A. (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 k
BIAS
PART OF
Decoupling Supply Transients
OA2
WITHIN
2252
T
pin). Typically, with I
. One (and only one) point on this trace
without difficulty.
BIAS
8
Figure 10.
Q6
of 24
I
1k
CHARGE
6
A, the maximum current sink
THAT 2252 RMS-Level Detector
C
V+
T
BIAS
BIAS
C
B
. Therefore, for the
T
confined to a local
loop, flowing from
C
and Q6 (within the
2252) into C
returning directly
to C
the
short,
necting it to sys-
tem
only
high currents are
kept out of the
24 A, the imped-
maintains its ref-
B
to V–. If the in-
B
through OA2
return
. By keeping
ground
one
CHARGE
and
BIAS
CHARGE
T
point,
, and
trace
con-
will
is
at
,