VCA2613Y BURR-BROWN [Burr-Brown Corporation], VCA2613Y Datasheet - Page 6

• Manufacturer Part Number: VCA2613Y
• Description: Dual, VARIABLE GAIN AMPLIFIER with Low-Noise Preamp
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

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
shows the composite gain control characteristic of the entire
VCA2613. Setting VCA
gain control to step in 3dB increments. Setting VCA
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 5 s at the highest gain (45dB) to
59 s at the lowest gain (25dB).
FIGURE 4. Schematic of the Low Noise Preamplifier (LNP).
6
Capacitor)
(External
C
LNP
4.7pF
COMP
IN
P
CNTL
R
W
CNTL
Q
curve in the typical characteristics
1
Q
2
to 3.0V causes the digital MGS
COMP2A
Q
Q
3
4
Q
5
93
R
L
Buffer
34
17
R
R
CNTL
S2
S3
105
R
LNP
S1
to 0V
www.ti.com
OUT
V
N
LNP
LNP
DD
LNP
GS2
GS3
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 Q
source input of Q
further amplified by the Q
Q
the input devices Q
the same role for signals on the –IN side.
The differential gain of the LNP is given in Equation (1):
3
OUT
and R
LNP
P
GS1
Buffer
L
gain stage, which uses the same bias current as
4
, the +IN side of the LNP. The signal is
4
4
and Q
Gain
Q
and Q
6
93
R
1
COMP1A
L
and Q
Q
Q
5
8
7
. Devices Q6 through Q
5
2
amplify the signal at the gate-
2
Q
stage, and then by the final
R
R
9
Q
10
L
S
R
W
Circuitry
Circuitry
To Bias
LNP
To Bias
VCA2613
SBOS179D
IN
N
10
play
(1)