VCA2613 Datasheet, PDF (6/16 Pages) Burr-Brown (TI) – Dual, VARIABLE GAIN AMPLIFIER with Low-Noise Preamp

PGA OVERVIEW AND OVERALL DEVICE
CHARACTERISTICS
The differential output of the VCA attenuator is then amplified
by the PGA circuit block. This post-amplifier is programmed
by the same MGS bits that control the VCA attenuator,
yielding an overall swept-gain amplifier characteristic in which
the VCA • PGA gain varies from 0dB (unity) to a program-
mable peak gain of 24, 27, 30, 33, 36, 39, 42, or 45dB.
The GAIN vs VCA_CNTL curve in the typical characteristics
shows the composite gain control characteristic of the entire
VCA2613. Setting VCA_CNTL to 3.0V causes the digital MGS
gain control to step in 3dB increments. Setting VCA_CNTL to 0V
causes all the MGS-controlled gain curves to converge at
one point. The gain at the convergence point is the LNP gain
less 6dB, because the measurement setup looks at only one
side of the differential PGA output, resulting in 6dB lower
signal amplitude.
ADDITIONAL FEATURES—OVERVIEW
Overload protection stages are placed between the attenua-
tor and the PGA, providing a symmetrically clipped output
whenever the input becomes large enough to overload the
PGA. A comparator senses the overload signal amplitude
and substitutes a fixed DC level to prevent undesirable
overload recovery effects. As with the previous stages, the
VCA is AC-coupled into the PGA. In this case, the coupling
time constant varies from 5µs at the highest gain (45dB) to
59µs at the lowest gain (25dB).
The VCA2613 includes a built-in reference, common to both
channels, to supply a regulated voltage for critical areas of
the circuit. This reduces the susceptibility to power supply
variation, ripple, and noise. In addition, separate power
supply and ground connections are provided for each chan-
nel and for the reference circuitry, further reducing interchannel
cross-talk.
Further details regarding the design, operation and use of
each circuit block are provided in the following sections.
LOW NOISE PREAMPLIFIER (LNP)—DETAIL
The LNP is designed to achieve a low noise figure, especially
when employing active termination. Figure 4 is a simplified
schematic of the LNP, illustrating the differential input and
output capability. The input stage employs low resistance
local feedback to achieve stable low noise, low distortion
performance with very high input impedance. Normally, low
noise circuits exhibit high power consumption due to the
large bias currents required in both input and output stages.
The LNP uses a patented technique that combines the input
and output stages such that they share the same bias
current. Transistors Q₄ and Q₅ amplify the signal at the gate-
source input of Q₄, the +IN side of the LNP. The signal is
further amplified by the Q₁ and Q₂ stage, and then by the final
Q₃ and R_L gain stage, which uses the same bias current as
the input devices Q₄ and Q₅. Devices Q6 through Q₁₀ play
the same role for signals on the –IN side.
The differential gain of the LNP is given in Equation (1):
Gain
=
2
•
_RRSL



(1)
COMP2A
V_DD
COMP1A
C_COMP
4.7pF
(External
Capacitor)
Q₂
Q₃
R_W
Q₄
LNP_INP
Q₁
R_L
93Ω
LNP_OUTN LNP_OUTP
Buffer
Buffer
R_S1
105Ω
R_S2
34Ω
R_S3
17Ω
LNP_GS2
LNP_GS1
LNP_GS3
Q₅
R_L
93Ω
Q₉
Q₈
To Bias
Circuitry
Q₇
R_W
LNP_INN
Q₁₀
To Bias
Circuitry
Q₆
FIGURE 4. Schematic of the Low Noise Preamplifier (LNP).
6
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VCA2613
SBOS179D