VCA2615 Datasheet, PDF (15/29 Pages) Texas Instruments – Dual, Low-Noise Variable-Gain Amplifier with Preamp

VCA2615
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THEORY OF OPERATION
The VCA2615 is a dual-channel system consisting of two
primary blocks: a low noise preamplifier (LNP) and a
variable gain amplifier (VGA), which is driven from the
LNP. The LNP is very flexible; both the gain and input
impedance can be programmed digitally without using
external components. The LNP is coupled to the VGA
through a multiplexer to facilitate interfacing with an
external signal processor. The VGA is a true variable-gain
amplifier, achieving lower noise output at lower gains. The
output amplifier has two gains, allowing for further
optimization with different analog-to-digital converters.
Figure 53 shows a simplified block diagram of a single
channel of the dual-channel system. Both the LNP and the
VGA can be powered down together or separately in order
to conserve system power when necessary.
LNP
VGA
Figure 53. Simplified Block Diagram of VCA2615
LNP—OVERVIEW
The LNP has differential input and output capability. It also
has exceptionally low noise voltage and input current
noise. At the highest gain setting (of 22dB), the LNP
achieves 0.7nV/√Hz voltage noise and typically 1pA/√Hz
current noise. The LNP can process fully differential or
single-ended signals in each channel. Differential signal
processing reduces second harmonic distortion and offers
improved rejection of common-mode and power-supply
noise. The LNP gain can be electronically programmed to
have one of four values that can be selected by a two-bit
word (see Table 2). The gain of the LNP when driving the
VGA is approximately 1dB higher because of the loss in
the buffer.
The LNP also has four programmable feedback resistors
that can be selected by a four-bit word to create 16 different
values in order to facilitate the easy use of active feedback.
With this combination of both programmable gain and
feedback resistors, as many as 61 different values of input
impedance can be created to provide a wide variety of
input-matching resistors (see Table 5). By using active
SBOS316C − JULY 2005 − REVISED SEPTEMBER 2005
feedback with this wide selection of feedback resistors, the
user is able to provide a low-noise means of terminating
input signal while incurring only a 3dB loss in
signal-to-noise ratio (SNR), instead of a 6dB loss in SNR
which is usually associated with the conventional type of
signal termination. More information is given in the section
of this document that provides a detailed description of the
LNP.
The LNP output drives a buffer that in turn drives the
feedback network and supplies the LNP to a multiplexer.
The multiplexer can be configured to supply the signal
off-chip for further processing, or can be set to drive the
internal VGA directly from the LNP. An external coupling
capacitor is not required to couple the LNP to the VGA.
VGA—OVERVIEW
The VGA that is used with the VCA2615 is a true
variable-gain amplifier; as the gain is reduced, the noise
contribution from the VGA itself is also reduced. A block
diagram of the VGA is shown in Figure 53. This design is
in contrast with another popular device architecture (used
by the VCA2616), where an effective VCA characteristic
is obtained by a voltage variable-attenuator succeeded by
a fixed-gain amplifier. At the highest gain, systems with
either architecture are dominated by the noise produced
by the LNP. At low gains, however, the noise output is
dominated by the contribution from the VGA. Therefore,
the overall system with lower VGA gain will produce less
noise.
The following example will illustrate this point. Figure 53
shows a block diagram of an LNP driving a variable-gain
amplifier; Figure 54 shows a block diagram of an LNP
driving a variable attenuation attenuator followed by a
fixed gain amplifier. For purposes of this example, let us
assume the performance characteristics shown in Table 1;
these values are the typical performance data of the
VCA2615 and the VCA2616.
LNP
ATTENUATOR
Amplifier
Figure 54. Block Diagram of Older VCA Models
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