VCA2613

7

SBOS179D

www.ti.com

LNP GAIN (dB)

Input-Referred

Output-Referred

25

1.54

2260

22

1.59

1650

17

1.82

1060

5

4.07

597

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

ground. Therefore, the signal loss is 6dB due to the voltage

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

R

1A

IN

F

=

+

(

)

amplifier input impedance with active feedback. In this case,

unlike the conventional termination above, both the signal

It is also possible to create other gain settings by connecting

internal resistor values shown in Figure 4 should be com-

bined with the external resistor to calculate the effective

for external resistor value is given in Equation (2).

R

RR

R

R

Gain RR

Gain R

R

EXT

S

L

FIX

L

S

FIX

S

L

=

+•

•

22

2

11

1

–

–

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-

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

all MGS bits set to

1. Note that the input-referred noise

values include the contribution of a 50

Ω 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.

NOISE (nV/

√Hz)

LNP PIN STRAPPING

LNP GAIN (dB)

25

22

17

All Pins Open

5

TABLE I. Pin Strappings of the LNP for Various Gains.

(3)

(2)

These Gain Strap pins allow the user to establish one of four

fixed LNP gain options as shown in Table I.

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 10