VCA8613 datasheet(13/21 Pages) TI

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VCA8613

SBOS200C

www.ti.com

TYPICAL CHARACTERISTICS (Cont.)

At TA = +25°C, fIN = 2MHz, ATN = 00, PG = 01, VCNTRL = 1.7V, unless otherwise noted. Differential output, 750mVPP, and AVDD = DVDD = 3.0V.

OVERLOAD RECOVERY vs TIME

(ATN = 00, PG = 00, V

CNTRL = 1V)

Time (400ns/div)

The signal is greater than 2V

PP input, so the LNA is

severely overloaded. Overload recovery time is 192ns.

Channel 1

(Output)

(2V/div)

Channel 2

(Input)

(1V/div)

OVERLOAD RECOVERY vs TIME

(ATN = 00, PG = 01, V

CNTRL = 2V)

Time (400ns/div)

The signal is greater than 40mV

PP input, so the LNA is in the linear region

and the output amplifier is overloaded. Overload recovery time is 584ns.

Channel 1

(Output)

(2V/div)

Channel 2

(Input)

(20mV/div)

APPLICATION INFORMATION

INPUT CIRCUIT

The input of the VCA8613 integrates several commonly used

elements. Prior to reaching the input of the VCA, the receive

signal should be coupled with a capacitor of at least 1nF,

preferably more. When this AC coupling element is inserted,

the LNA input bias point is held to a common-mode value of

2.4V by an integrated 4.5k

Ω resistor. This common-mode

value will change with temperature and may also vary from

chip to chip, but for each chip, it will be held constant. In

parallel with this resistor are two back-to-back clipping di-

odes. These diodes prevent excessive input voltages from

passing through to the LNA input, preventing deep saturation

effects in the LNA itself.

LOW-NOISE PRE-AMPLIFIER (LNA)

The VCA8613 integrates a low-noise pre-amplifier. Because

of the high level of integration in the system, noise perfor-

mance was traded for power consumption, resulting in an

extremely low-power pre-amplifier, with 1.2nV/

√Hz noise

performance at 5MHz. The LNA is configured as a fixed-gain

25dB amplifier. Of this total gain, 6dB results from the single-

ended to differential conversion accomplished within the LNA

itself. The output of the LNA is limited to a little over 2V

differential swing. This implies a maximum input voltage

swing of approximately 110mV to be operating in the linear

range at 5MHz. Larger input signals can be accepted by the

LNA, but distortion performance will degrade with high-level

input signals.

CW DOPPLER PROCESSOR

The VCA8613 integrates many of the elements necessary to

allow for the implementation of a simple CW Doppler pro-

cessing circuit. One circuit that was integrated was a V/I

converter following the LNA (see Figure 1). The V/I converter

converts the LNA voltage output to a current which is then

passed through an 8x10 switch matrix (see Figure 2). Within

this switch matrix, any of the eight LNA outputs can be

connected to any of ten CW output pins. This is a simple

current-summing circuit such that each CW output can rep-

resent the sum of any or all the channel currents. The output

current for each LNA is equal to the single-ended LNA output

voltage swing divided by an internally integrated 700

Ω resis-

tor. This resistor value may change

±5% from chip to chip.

The CW output pins need a compliance voltage between 3V

to 3.3V. The 3V to 3.3V can be applied through either an

inductor (see Figure 3) tied to the 3V to 3.3V source or from

the inverting input of an op amp circuit (see Figure 4). The

architecture of the V/I converter requires a 2mA to 2.5mA

current that is generated by the compliance voltage.

The CW outputs are typically routed to a passive delay line,

allowing coherent summing of the signals. After summing, IQ

separation and down conversion to base-band precedes a

pair of high-resolution, low sample rate ADCs.

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