VCA2613 Datasheet, PDF (7/20 Pages) Burr-Brown (TI) – Dual, VARIABLE GAIN AMPLIFIER with Low-Noise Preamp

where R_L is the load resistor in the drains of Q₃ and Q₈, and
R_S is the resistor connected between the sources of the input
transistors Q₄ and Q₇. The connections for various R_S com-
binations are brought out to device pins LNPG_S1, LNPG_S2,
and LNPG_S3 (pins 13-15 for channel A, 22-24 for channel B).
These Gain Strap pins allow the user to establish one of four
fixed LNP gain options as shown in Table I.
LNP PIN STRAPPING
LNPG_S1, LNPG_S2, LNPG_S3 Connected Together
LNPG_S1 Connected to LNPG_S3
LNPG_S1 Connected to LNPG_S2
All Pins Open
LNP GAIN (dB)
25
22
17
5
TABLE I. Pin Strappings of the LNP for Various Gains.
It is also possible to create other gain settings by connecting
an external resistor between LNPG_S1 on one side, and
LNPG_S2 and/or LNPG_S3 on the other. In that case, the
internal resistor values shown in Figure 4 should be com-
bined with the external resistor to calculate the effective
value of R_S for use in Equation (1). The resulting expression
for external resistor value is given in Equation (2).
R_EXT
=
2R_S1R_L
+ 2R_FIXR_L – Gain • R_S1R_FIX
Gain • R_S1 – 2R_L
(2)
where R_EXT is the externally selected resistor value needed
to achieve the desired gain setting, R_S1 is the fixed parallel
resistor in Figure 4, and R_FIX is the effective fixed value of the
remaining internal resistors: R_S2, R_S3, or (R_S2 || R_S3) depend-
ing on the pin connections.
Note that the best process and temperature stability will be
achieved by using the pre-programmed fixed gain options of
Table I, since the gain is then set entirely by internal resistor
ratios, which are typically accurate to ±0.5%, and track quite
well over process and temperature. When combining exter-
nal resistors with the internal values to create an effective R_S
value, note that the internal resistors have a typical tempera-
ture coefficient of +700ppm/°C and an absolute value toler-
ance of approximately ±5%, yielding somewhat less predict-
able and stable gain settings. With or without external resis-
tors, the board layout should use short Gain Strap connec-
tions to minimize parasitic resistance and inductance effects.
The overall noise performance of the VCA2613 will vary as
a function of gain. Table II shows the typical input- and
output-referred noise densities of the entire VCA2613 for
maximum VCA and PGA gain; i.e., VCA_CNTL set to 3.0V and
all MGS bits set to 1. Note that the input-referred noise
values include the contribution of a 50Ω fixed source imped-
ance, and are therefore somewhat larger than the intrinsic
input noise. As the LNP gain is reduced, the noise contribu-
tion from the VCA/PGA portion becomes more significant,
resulting in higher input-referred noise. However, the output-
referred noise, which is indicative of the overall SNR at that
gain setting, is reduced.
To preserve the low noise performance of the LNP, the user
should take care to minimize resistance in the input lead. A
parasitic resistance of only 10Ω will contribute 0.4nV/√Hz.
LNP GAIN (dB)
25
22
17
5
NOISE (nV/√Hz)
Input-Referred
Output-Referred
1.54
2260
1.59
1650
1.82
1060
4.07
597
TABLE II. Noise Performance for MGS = 111 and VCA_CNTL = 3.0V.
The LNP is capable of generating a 2V_PP differential signal.
The maximum signal at the LNP input is therefore 2V_PP
divided by the LNP gain. An input signal greater than this
would exceed the linear range of the LNP, an especially
important consideration at low LNP gain settings.
ACTIVE FEEDBACK WITH THE LNP
One of the key features of the LNP architecture is the ability
to employ active-feedback termination to achieve superior
noise performance. Active-feedback termination achieves a
lower noise figure than conventional shunt termination, es-
sentially because no signal current is wasted in the termina-
tion resistor itself. Another way to understand this is as
follows: Consider first that the input source, at the far end of
the signal cable, has a cable-matching source resistance of
R_S. Using conventional shunt termination at the LNP input, a
second terminating resistor of value R_S is connected to
ground. Therefore, the signal loss is 6dB due to the voltage
divider action of the series and shunt R_S resistors. The
effective source resistance has been reduced by the same
factor of 2, but the noise contribution has been reduced by
only the √2, only a 3dB reduction. Therefore, the net theoreti-
cal SNR degradation is 3dB, assuming a noise-free amplifier
input. (In practice, the amplifier noise contribution will de-
grade both the unterminated and the terminated noise fig-
ures, somewhat reducing the distinction between them.)
See Figure 5 for an amplifier using active feedback. This
diagram appears very similar to a traditional inverting ampli-
fier. However, the analysis is somewhat different because
the gain A in this case is not a very large open-loop op amp
gain; rather it is the relatively low and controlled gain of the
LNP itself. Thus, the impedance at the inverting amplifier
terminal will be reduced by a finite amount, as given in the
familiar relationship of Equation (3):
R_IN
=
R_F
(1+ A)
(3)
where R_F is the feedback resistor (supplied externally be-
tween the LNP_INP and FB terminals for each channel), A is
the user-selected gain of the LNP, and R_IN is the resulting
amplifier input impedance with active feedback. In this case,
unlike the conventional termination above, both the signal
voltage and the R_S noise are attenuated by the same factor
VCA2613
7
SBOS179D
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