2181-DEMO THAT Corporation, 2181-DEMO Datasheet - Page 5

• Manufacturer Part Number: 2181-DEMO
• Description: Audio Modules & Development Tools Trimmable VCA Demo Board
• Manufacturer: THAT Corporation

Specifications of 2181-DEMO

• Description/function: Audio Amplifiers
• Operating Supply Voltage: 15 V
• Product: Audio Modules
• For Use With/related Products: THAT2181
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

THAT 2181 Series
Blackmer® Trimmable IC VCAs
negative control port. Because of the exponential
characteristic, the control voltage sets gain linearly
in decibels. Figure 6 shows the decibel current gain
of a 2181 versus the voltage at Ec+, while Figure 7
shows gain versus the Ec-.
Temperature Effects
on the logarithmic relationship between voltage and
current in a semiconductor junction (in particular,
between a transistor's V
this relationship is temperature dependent. There-
fore, the gain of any log-antilog VCA depends on its
temperature.
negative control port. (The positive control port
behaves in the same manner.) Note that the gain at
Ec = 0 V is 0 dB, regardless of temperature. Chang-
ing temperature changes the scale factor of the gain
by 0.33%/°C, which pivots the curve about the 0 dB
point.
temperature (25°C) and the actual temperature, and
Gain is the gain in decibels. At room temperature,
this reduces to
becomes
becomes
DC Bias Currents
the resistor between pin 5 (V-) and the negative
supply voltage (V
resistor is 5.1 kΩ, which provides approximately
2.4 mA. This current is split into two paths: 570 μA
is used for biasing the IC, and the remainder
becomes I
transistors (Q1 through Q4), the rest is available for
input and output signal current.
The logging and antilogging in the VCA depends
Figure 8 shows the effect of temperature on the
Mathematically, the 2181's gain characteristic is
Gain =
where ∆T is the difference between room
Gain =
If only the positive control port is used, this
Gain =
If only the negative control port is used, this
Gain =
The 2181 current consumption is determined by
er split in two parts: about 20 μA biases the core
CELL
(0.0061)(1+0.0033 T)
E
0.0061
0.0061
C+
0.0061
E
E
as shown in Figure 5. I
C+
C−
− E
E
EE
C+
C−
). Typically, with 15V supplies, the
,
,
− E
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
,
C−
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
be
and I
,
C
). As is well known,
Eq. 1
Eq. 2
Eq. 3
Eq. 4
CELL
Copyright © 2008, THAT Corporation
is furth
Page 5 of 12
Trimming
adjusted for minimum distortion by applying a small
variable offset voltage to pin 4, the SYM pin. Note
that there is a 25 Ω resistor internal to the 2181
between pin 4 and pin 2. As shown in Figure 2,
Page 3, the usual method of applying this offset is to
use the internal 25 Ω resistor along with a larger
value resistor to form a voltage divider connected to
the wiper of a trim pot across the supply rails.
harmonic distortion. This is usually done by applying
a middle-level, middle-frequency signal (e.g. 1 kHz at
1 V) to the audio input, setting the VCA to 0 dB gain,
and adjusting the SYM trim while observing THD at
the output. In the 2181, this adjustment coincides
closely with the setting which produces minimum
control-voltage feedthrough, though the two settings
are not always identical.
DC Feedthrough
(the output). When the gain is changed, the dc term
changes. This control-voltage feedthrough is more
pronounced with gain; the –A version of the part
produces the least feedthrough, the –C version the
most. See Figure 9 for typical curves for dc offset vs.
gain
Audio Performance
testing ensure extremely good audio performance
when used as recommended. The 2181 maintains
low distortion over a wide range of gain, cut and
signal levels. Figures 10 through 12 show typical
distortion performance for representative samples of
each grade of the part. At or near unity gain, the
2181 behaves much like a good opamp, with low
distortion over the entire audio band. Figure
shows typical THD for a 2181A over frequency at 0
dB gain, with a 1 V input signal, while Figure 14
details the harmonic content of the distortion in a
typical A–grade part.
The 2181-Series VCAs are intended to be
This trim should be adjusted for minimum
Normally, a small dc error term flows in pin 8
The 2181-Series VCA design, fabrication and
Figure 9. Representative DC Offset vs. Gain
Document 600030 Rev 02
13