that2181a THAT Corporation, that2181a Datasheet - Page 5

• Manufacturer Part Number: that2181a
• Description: Trimmable Ic Voltage Controlled Amplifiers
• Manufacturer: THAT Corporation

600030 Rev 01
ship is temperature dependent. Therefore, the gain of
any log-antilog VCA depends on its temperature.
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.
DC Bias Currents
resistor between pin 5 (V-) and the negative supply voltage
(V
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
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
EE
Figure 8 shows the effect of temperature on the nega-
Mathematically, the 2181's gain characteristic is
Gain
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
If only the positive control port is used, this becomes
Gain
If only the negative control port is used, this becomes
Gain
The 2181 current consumption is determined by the
The 2181-Series VCAs are intended to be adjusted for
). Typically, with 15V supplies, the resistor is 5.1 kW,
Figure 9. Representative DC Offset Vs. Gain
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0.0061
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C
0.0061
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C
C
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C
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THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
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Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
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-
D
,
Eq. 1
Eq. 2
Eq. 3
Eq. 4
larger value resistor to form a voltage divider connected
to the wiper of a trim pot across the supply rails.
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
feedthrough, though the two settings are not always iden-
tical.
DC Feedthrough
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
Figure 12. 1 kHz THD+Noise Vs. Input Level, -15 dB
This trim should be adjusted for minimum harmonic
Normally, a small dc error term flows in pin 8 (the
which
produces
Gain
Gain
minimum
control-voltage
Page 5