op amp. The VCM node shown in Figure 2 is the VCM 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 VCM
output for proper operation.
THEORY OF OPERATION
The VCA2616 and VCA2611 are dual-channel systems con-
sisting of three primary blocks: an LNP, a 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.
1kΩ
External
InA
To VCAIN
47nF
Input
Signal
Channel A
Input
Channel A
Output
LNP
VCA
PGA
1kΩ
VCM (+2.5V)
Maximum
Gain
Analog
Control
VCA
Control
MGS
Select
FIGURE 2. Recommended Circuit for Coupling an External
Signal into the VCA Inputs.
Channel B
Input
Channel B
Output
LNP
VCA
PGA
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
(VCACNTL) 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 charac-
teristic. Internally, the signal is attenuated by having the
analog VCACNTL 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 VCA2616 and VCA2611.
External
InB
FIGURE 1. Simplified Block Diagram of the VCA2616.
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. Further
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 4.8µs time constant (33kHz 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.
0
Minimum Attenuation
–24
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
Maximum Attenuation
–45
0
3.0
Control Voltage (V)
FIGURE 3. Swept Attenuator Characteristic.
VCA2616, VCA2611
9
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