VCA2611 데이터 시트보기 (PDF) - TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS

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VCA2611

Dual, Variable-Gain Amplifier with Low-Noise Preamp

TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS 

THEORY OF OPERATION

The VCA2616 and VCA2611 are dual-channel systems con-

sisting of three primary blocks: an LNP, a VCA, and a

Programmable Gain Amplifier (PGA). For greater system

flexibility, an onboard multiplexer is provided for the VCA

inputs, selecting either the LNP outputs or external signal

inputs. Figure 1 shows a simplified block diagram of the dual-

channel system.

External

InA

Channel A

Input

LNP

VCA

PGA

Channel A

Output

VCA

Control

Analog

Control

Maximum

Gain

Select

MGS

Channel B

Input

LNP

VCA

PGA

Channel B

Output

External

InB

FIGURE 1. Simplified Block Diagram of the VCA2616.

LNP—OVERVIEW

The LNP input may be connected to provide active-feedback

signal termination, achieving lower system noise perfor-

mance than conventional passive shunt termination. Further

lower noise performance is obtained if signal termination is

not required. The unterminated LNP input impedance is

600kΩ. The LNP can process fully differential or single-

ended signals in each channel. Differential signal processing

results in significantly reduced 2nd-harmonic distortion and

improved rejection of common-mode and power-supply noise.

The first gain stage of the LNP is AC-coupled into its output

buffer with a 4.8µs time constant (33kHz high-pass charac-

teristic). The buffered LNP outputs are designed to drive the

succeeding VCA directly or, if desired, external loads as low

as 135Ω with minimal impact on signal distortion. The LNP

employs very low impedance local feedback to achieve

stable gain with the lowest possible noise and distortion.

Four pin-programmable gain settings are available: 5dB,

17dB, 22dB, and 25dB. Additional intermediate gains can be

programmed by adding trim resistors between the Gain Strap

programming pins.

The common-mode DC level at the LNP output is nominally

2.5V, matching the input common-mode requirement of the

VCA for simple direct coupling. When external signals are

fed to the VCA, they should also be set up with a 2.5VDC

common-mode level. Figure 2 shows a circuit that demon-

strates the recommended coupling method using an external

op amp. The VCM node shown in Figure 2 is the VCM output

(pin 19). Typical R and C values are shown, yielding a high-

pass time constant similar to that of the LNP. If a different

common-mode referencing method is used, it is important

that the common-mode level be within 10mV of the VCM

output for proper operation.

1kΩ

Input

Signal

47nF

1kΩ

To VCAIN

VCM (+2.5V)

FIGURE 2. Recommended Circuit for Coupling an External

Signal into the VCA Inputs.

VCA—OVERVIEW

The magnitude of the differential VCA input signal (from the

LNP or an external source) is reduced by a programmable

attenuation factor, set by the analog VCA Control Voltage

(VCACNTL) at pin 43. The maximum attenuation factor is

further programmable by using the three MGS bits

(pins 40-42). Figure 3 illustrates this dual-adjustable charac-

teristic. Internally, the signal is attenuated by having the

analog VCACNTL vary the channel resistance of a set of

shunt-connected FET transistors. The MGS bits effectively

adjust the overall size of the shunt FET by switching parallel

components in or out under logic control. At any given

maximum gain setting, the analog variable gain characteris-

tic is linear in dB as a function of the control voltage, and is

created as a piecewise approximation of an ideal dB-linear

transfer function. The VCA gain control circuitry is common

to both channels of the VCA2616 and VCA2611.

0

Minimum Attenuation

–24

–45

Maximum Attenuation

0

3.0

Control Voltage (V)

FIGURE 3. Swept Attenuator Characteristic.

VCA2616, VCA2611

9

SBOS234D

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