VCA2613 Datasheet, PDF (5/20 Pages) Burr-Brown (TI) – Dual, VARIABLE GAIN AMPLIFIER with Low-Noise Preamp

THEORY OF OPERATION
The VCA2613 is a dual-channel system consisting of three
primary blocks: a Low Noise Preamplifier (LNP), a Voltage
Controlled Attenuator (VCA), and a Programmable Gain
Amplifier (PGA). For greater system flexibility, an onboard
multiplexer is provided for the VCA inputs, selecting either
the LNP outputs or external signal inputs. Figure 1 shows a
simplified block diagram of the dual-channel system.
op amp. The V_CM node shown in the drawing is the V_CM
output (pin 19). Typical R and C values are shown, yielding
a high-pass time constant similar to that of the LNP. If a
different common-mode referencing method is used, it is
important that the common-mode level be within 10mV of the
V_CM output for proper operation.
1kΩ
External
In_A
Channel A
Input
LNP
VCA
PGA
Channel A
Output
VCA
Control
Analog
Control
Maximum
Gain
Select
MGS
Input
Signal
47nF
1kΩ
To VCA_IN
V_CM (+2.5V)
FIGURE 2. Recommended Circuit for Coupling an External
Signal into the VCA Inputs.
Channel B
Input
LNP
VCA
PGA
Channel B
Output
External
In_B
FIGURE 1. Simplified Block Diagram of the VCA2613.
LNP—OVERVIEW
The LNP input may be connected to provide active-feedback
signal termination, achieving lower system noise perfor-
mance than conventional passive shunt termination. Even
lower noise performance is obtained if signal termination is
not required. The unterminated LNP input impedance is
600kΩ. The LNP can process fully differential or single-
ended signals in each channel. Differential signal processing
results in significantly reduced 2nd-harmonic distortion and
improved rejection of common-mode and power-supply noise.
The first gain stage of the LNP is AC-coupled into its output
buffer with a 44µs time constant (3.6kHz high-pass charac-
teristic). The buffered LNP outputs are designed to drive the
succeeding VCA directly or, if desired, external loads as low
as 135Ω with minimal impact on signal distortion. The LNP
employs very low impedance local feedback to achieve
stable gain with the lowest possible noise and distortion.
Four pin-programmable gain settings are available: 5dB,
17dB, 22dB, and 25dB. Additional intermediate gains can be
programmed by adding trim resistors between the Gain Strap
programming pins.
The common-mode DC level at the LNP output is nominally
2.5V, matching the input common-mode requirement of the
VCA for simple direct coupling. When external signals are
fed to the VCA, they should also be set up with a 2.5VDC
common-mode level. Figure 2 shows a circuit that demon-
strates the recommended coupling method using an external
VCA—OVERVIEW
The magnitude of the differential VCA input signal (from the
LNP or an external source) is reduced by a programmable
attenuation factor, set by the analog VCA Control Voltage
(VCA_CNTL) at pin 43. The maximum attenuation factor is
further programmable by using the three MGS bits (pins 40-
42). Figure 3 illustrates this dual-adjustable characteristic.
Internally, the signal is attenuated by having the analog
VCA_CNTL vary the channel resistance of a set of shunt-
connected FET transistors. The MGS bits effectively adjust
the overall size of the shunt FET by switching parallel
components in or out under logic control. At any given
maximum gain setting, the analog variable gain characteris-
tic is linear in dB as a function of the control voltage, and is
created as a piecewise approximation of an ideal dB-linear
transfer function. The VCA gain control circuitry is common
to both channels of the VCA2613.
0
Minimum Attenuation
–24
–45
0
Maximum Attenuation
Control Voltage
3.0V
FIGURE 3. Swept Attenuator Characteristic.
VCA2613
5
SBOS179D
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