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VCA2616Y

Dual, Variable-Gain Amplifier with Low-Noise Preamp

TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS 

where RL is the load resistor in the drains of Q3 and Q8, and

RS is the resistor connected between the sources of the input

transistors Q4 and Q7. The connections for various RS com-

binations are brought out to device pins LNPGS1, LNPGS2,

and LNPGS3 (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

LNPGS1, LNPGS2, LNPGS3 Connected Together

LNPGS1 Connected to LNPGS3

LNPGS1 Connected to LNPGS2

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 LNPGS1 on one side, and

LNPGS2 and/or LNPGS3 on the other. In that case, the

internal resistor values (see Figure 4) should be combined

with the external resistor to calculate the effective value of RS

for use in Equation 1. The resulting expression for external

resistor value is given in Equation 2:

REXT

=

2RS1RL

+ 2RFIXRL – Gain × RS1RFIX

Gain × RS1 – 2RL

(2)

where REXT is the externally selected resistor value needed

to achieve the desired gain setting, RS1 is the fixed parallel

resistor in Figure 4, and RFIX is the effective fixed value of the

remaining internal resistors: RS2, RS3, or (RS2 || RS3), de-

pending 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 RS

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 VCA2616 and VCA2611

will vary as a function of gain. Table II shows the typical input-

and-output-referred noise densities of the entire VCA2616 and

VCA2611 for maximum VCA and PGA gain; that is, VCACNTL

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 impedance, and are therefore somewhat larger than

the intrinsic input noise. As the LNP gain is reduced, the noise

contribution from the VCA/PGA portion becomes more signifi-

cant, 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

1.35

2260

1.41

1650

1.63

1060

4.28

597

TABLE II. Equivalent Noise Performance for MGS = 111 and

VCACNTL = 3.0V with 50Ω source impedance.

The LNP is capable of generating a 2VPP differential signal.

The maximum signal at the LNP input is therefore 2VPP

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.

The VCA2611 is an upgraded version of the VCA2616. The

only difference between the VCA2616 and the VCA2611 is the

input structure to the LNP. The VCA2616 is limited to –0.3V

negative-going input spikes; the VCA2611 is limited to –2.0V

negative-going input spikes. This change allows the user to

use slower and less expensive input clamping diodes prior to

the LNA input. In some designs, input clamping may not be

required.

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 to

consider first that the input source, at the far end of the signal

cable, has a cable-matching source resistance of RS. Using

conventional shunt termination at the LNP input, a second

terminating resistor of value RS is connected to ground.

Therefore, the signal loss is 6dB due to the voltage divider

action of the series and shunt RS 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 theoretical SNR

degradation is 3dB, assuming a noise-free amplifier input. (In

practice, the amplifier noise contribution will degrade both

the unterminated and the terminated noise figures, some-

what 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:

RIN

=

RF

(1+ A)

(3)

where RF is the feedback resistor (supplied externally be-

tween the LNPINP and FB terminals for each channel), A is

VCA2616, VCA2611

11

SBOS234D

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