THAT2180A Datasheet, PDF (6/8 Pages) List of Unclassifed Manufacturers – Pre-Trimmed IC Voltage Controlled Amplifiers

Page 6
THAT2180 Series IC VCAs
Applications
Input
As mentioned above, input and output signals are
currents, not voltages. While this often causes some con-
ceptual difficulty for designers first exposed to this con-
vention, the current input/output mode provides great
flexibility in application.
The Input pin (pin 1) is a virtual ground with nega-
tive feedback provided internally (see Figure 5, Page 4).
The input resistor (shown as 20 kW in Figure 2, Page 3)
should be scaled to convert the available ac input voltage
to a current within the linear range of the device. Gen-
erally, peak input currents should be kept under 1 mA
for best distortion performance.
Figures 10 through 12 show distortion vs. Signal
level for the three parts in the 2180 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 distortion
will usually determine the maximum signal current level
which may be used. Note that, with 20 kW cur-
rent-to-voltage converting resistors, distortion remains
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 de-
creasing the 20 kW current-to-voltage converting resis-
tors used at the input and output in Figure 2, Page 3. For
every dB these resistor values are decreased, the voltage
noise at the output of the OP275 is reduced by one dB.
For example, with 10 kW resistors, the output noise floor
drops to –104 dBV (typical) at 0 dB gain — a 6 dB re-
duction in noise because 10 kW is 1/2 of (6 dB lower
than) 20 kW.
Conversely, if THD is more important than noise per-
formance, increasing these resistors to 40 kW will in-
crease the noise level by 6 dB, while reducing distortion
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 accommo-
date the actual voltages prevalent in the circuit. Since the
2180 handles signals as currents, these ICs can even op-
erate with signal levels far exceeding the 2180's supply
rails, provided appropriately large resistors are used.
High-Frequency Distortion
The choice of input resistor has an additional, subtle
effect on distortion. Since the feedback impedances
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 frequencies,
asking for too much gain will lead to increased
high-frequency distortion. For best results, this resistor
should be kept to 10 kW or above.
Stability
An additional consideration is stability: the internal
op amp is intended for operation with source imped-
ances of less than 60 kW at high frequencies. For most
audio applications, this will present no problem
DC Coupling
The quiescent dc voltage level at the input (the input
offset voltage) is approximately +0 mV, but, as in many
general-purpose opamps, this is not well controlled. Any
dc input currents will cause dc in the output which will
be modulated by gain; this may cause audible thumps. If
the input is dc coupled, dc input currents may be gener-
ated due to the input offset voltage of the 2180 itself, or
due to offsets in stages preceeding the 2180. Therefore,
capacitive coupling is almost mandatory for quality au-
dio applications. Choose a capacitor which will give ac-
ceptable low frequency performance for the application.
Summing Multiple Input Signals
Multiple signals may be summed via multiple resis-
tors, just as with an inverting opamp configuration. In
such a case, a single coupling capacitor may be located
next to pin 1 rather than multiple capacitors at the
driven ends of the summing resistors. However, take
care that the capacitor does not pick up stray signals.
Output
The Output pin (pin 8) is intended to be connected
to a virtual ground node, so that current flowing in it
may be converted to a voltage (see Figures 2 & 14).
Choose the external opamp for good audio performance.
The feedback resistor should be chosen based on the de-
sired current-to-voltage conversion constant. Since the
input resistor determines the voltage-to-current conver-
sion at the input, the familiar ratio of R_f /R_i for an invert-
ing opamp will determine the overall voltage gain when
the 2180 is set for 0 dB current gain. Since the VCA per-
forms best at settings near unity gain, use the input and
feedback resistors to provide design-center gain or loss,
if necessary.
A small feedback capacitor around the output
opamp is needed to cancel the output capacitance of the
VCA. Without it, this capacitance will destabilize most
opamps. The capacitance at pin 8 is typically 15 pf.
Power Supplies
Positive
The positive supply is connected directly to V+
(pin 7). No special bypassing is necessary, but it is good
practice to include a small (~1 mf) electrolytic or
(~0.1 mf) ceramic capacitor close to the VCA IC on the
PCB. Performance is not particularly dependent on sup-
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