THAT2181 Datasheet, PDF (5/10 Pages) List of Unclassifed Manufacturers – Trimmable IC Voltage Controlled Amplifiers

600030 Rev 01
Page 5
ship is temperature dependent. Therefore, the gain of
any log-antilog VCA depends on its temperature.
Figure 8 shows the effect of temperature on the nega-
tive control port. (The positive control port behaves in the
same manner.) Note that the gain at Ec = 0 V is 0 dB, re-
gardless of temperature. Changing temperature changes
the scale factor of the gain by 0.33%/°C, which pivots the
curve about the 0 dB point.
Mathematically, the 2181's gain characteristic is
Gain =
E_C+ - E_C-
,
(0.0061)(1 + 0.0033DT)
Eq. 1
where DT is the difference between room temperature
(25°C) and the actual temperature, and Gain is the
gain in decibels. At room temperature, this reduces to
Gain = ^{EC+ - EC-} ,
0.0061
Eq. 2
If only the positive control port is used, this becomes
Gain = ^EC+ ,
0.0061
Eq. 3
If only the negative control port is used, this becomes
Gain = ^-EC- ,
0.0061
Eq. 4
DC Bias Currents
The 2181 current consumption is determined by the
resistor between pin 5 (V-) and the negative supply voltage
(V_EE). Typically, with 15V supplies, the resistor is 5.1 kW,
which provides approximately 2.4 mA. This current is
split into two paths: 570 mA is used for biasing the IC,
and the remainder becomes Icell as shown in Figure 5.
Icell is further split in two parts: about 20 mA biases the
core transistors (Q1 through Q4), the rest is available for
input and output signal current.
Trimming
The 2181-Series VCAs are intended to be adjusted for
minimum distortion by applying a small variable offset
voltage to pin 4, the SYM pin. Note that there is a 25 W re-
sistor 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 W resistor along with a
larger value resistor to form a voltage divider connected
to the wiper of a trim pot across the supply rails.
This trim should be adjusted for minimum harmonic
distortion. This is usually done by applying a mid-
dle-level, middle-frequency signal (e.g. 1 kHz at 1 V) to
the audio input, setting the VCA to 0 dB gain, and adjust-
ing the SYM trim while observing THD at the output. In
the 2181, this adjustment coincides closely with the set-
ting which produces minimum control-voltage
feedthrough, though the two settings are not always iden-
tical.
DC Feedthrough
Normally, a small dc error term flows in pin 8 (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
Figure 10. 1 kHz THD+Noise Vs. Input Level, 0 dB Gain
Figure 11. 1 kHz THD+Noise Vs. Input Level, +15 dB
Gain
Figure 9. Representative DC Offset Vs. Gain
Figure 12. 1 kHz THD+Noise Vs. Input Level, -15 dB
Gain
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