THAT2181 Datasheet, PDF (6/10 Pages) List of Unclassifed Manufacturers – Trimmable IC Voltage Controlled Amplifiers

Page 6
THAT2181 Series IC VCAs
Audio Performance
The 2181-Series VCA design, fabrication and 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 rep-
resentative 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
13 shows typical THD for a 2181A over frequency at
0 dB gain, with a 1 V input signal, while Figure 14 de-
tails the harmonic content of the distortion in a typical
A–grade part.
Figure 13. 2181A THD+N vs. Frequency, 0 dB Gain, 1 V
Figure 14. FFT of THD, Typical 2181A,
0dB Gain, 1V, 1kHz
Applications
Input
As mentioned above, input and output signals are
currents, not voltages. While this often causes some
conceptual difficulty for designers first exposed to
this convention, the current input/output mode pro-
vides great flexibility in application.
The Input pin (pin 1) is a virtual ground with neg-
ative feedback provided internally (see Figure 5,
Page 4). The input resistor (shown as 20 kW in Fig-
ure 2, Page 3) should be scaled to convert the avail-
able ac input voltage to a current within the linear
range of the device. Generally, peak input currents
should be kept under 1 mA for best distortion perfor-
mance.
Refer to Figures 10 through 12 to see how distor-
tion varies with signal level for the three parts in the
2181 Series for 0 dB, +15 dB and -15 dB gain. The
circuit of Figure 2, Page 3 was used to generate these
curves.
For a specific application, the acceptable distor-
tion will usually determine the maximum signal cur-
rent level which may be used. Note that, with 20 kW
current-to-voltage converting resistors, distortion re-
mains low even at 10 V rms input at 0 dB or -15 dB
gain, and at 1.7 V rms input at +15 dB gain
(~10 V rms output). This is especially true in the –A
and –B grades of the part.
Distortion vs. Noise
A designer may trade off noise for distortion by
decreasing the 20 kW current-to-voltage converting re-
sistors used at the input and output in Figure 2,
Page 3. For every dB these resistor values are de-
creased, the voltage noise at the output of the OP275
is reduced by one dB. For example, with 10 kW resis-
tors, the output noise floor drops to –104 dBV (typi-
cal) at 0 dB gain — a 6 dB reduction in noise because
10 kW is 1/2 of (6 dB lower than) 20 kW.
Conversely, if THD is more important than noise
performance, increasing these resistors to 40 kW will
increase the noise level by 6 dB, while reducing dis-
tortion at maximum voltage levels. Furthermore, if
maximum signal levels are higher (or lower) than the
traditional 10 V rms, these resistors should be scaled
to accommodate the actual voltages prevalent in the
circuit. Since the 2181 handles signals as currents,
these ICs can even operate with signal levels far ex-
ceeding the 2181's supply rails, provided appropri-
ately large resistors are used.
High-Frequency Distortion
The choice of input resistor has an additional,
subtle effect on distortion. Since the feedback imped-
ances around the internal opamp (essentially Q1/D1
and Q3/D3) are fixed, low values for the input resistor
will require more closed-loop gain from the opamp.
Since the open-loop gain naturally falls off at high fre-
quencies, asking for too much gain will lead to in-
creased high-frequency distortion. For best results,
this resistor should be kept to 10 kW or above.
Stability
An additional consideration is stability: the inter-
nal op amp is intended for operation with source im-
pedances of less than 60 kW at high frequencies. For
most audio applications, this will present no problem
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