AD9277 datasheet(23/49 Pages) AD

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AD9277

Rev. 0 | Page 22 of 48

POST

AMP

LNA

GAIN–

GAIN+

SERIAL

LVDS

AAF

X-AMP VGA

ATTENUATOR

–42dB TO 0dB

GAIN

INTERPOLATOR

PIPELINE

ADC

LOSW-x

LO-x

LI-x

LG-x

RESET

4LO+

4LO–

RFB2

GENERATION

15.6dB,

17.9dB,

21.3dB

21dB,

24dB,

27dB,

30dB

CWI+

CWI–

CWQ+

CWQ–

DOUTx+

DOUTx–

RFB1

CLG

CSH

TRANSDUCER

T/R

SWITCH

Figure 46. Simplified Block Diagram of a Single Channel

CHANNEL OVERVIEW

Each channel contains both a TGC signal path and a CW Doppler

signal path. Common to both signal paths, the LNA provides user-

adjustable input impedance termination. The CW Doppler path

includes an I/Q demodulator. The TGC path includes a differen-

tial X-AMP® VGA, an antialiasing filter, and an ADC. Figure 46

shows a simplified block diagram with external components.

The signal path is fully differential throughout to maximize signal

swing and reduce even-order distortion; however, the LNA is

designed to be driven from a single-ended signal source.

Low Noise Amplifier (LNA)

Good noise performance relies on a proprietary ultralow noise

LNA at the beginning of the signal chain, which minimizes the

noise contribution in the following VGA. Active impedance

control optimizes noise performance for applications that

benefit from input impedance matching.

A simplified schematic of the LNA is shown in Figure 47. LI-x

is capacitively coupled to the source. An on-chip bias generator

establishes dc input bias voltages of around 0.9 V and centers the

output common-mode levels at 1.5 V (AVDD2 divided by 2). A

capacitor, CLG, of the same value as the input coupling capacitor,

CS, is connected from the LG-x pin to ground.

It is highly recommended that the LG-x pins form a Kelvin type

connection to the input or probe connection ground. Simply

connecting the LG-x pin to ground near the device can allow

differences in potential to be amplified through the LNA. This

generally shows up as a dc offset voltage that can vary from

channel to channel and part to part, depending on the appli-

cation and the layout of the PCB.

CSH

TRANSDUCER

CLG

LI-x

VCM

RFB2

RFB1

CFB

VO+

VO–

T/R

SWITCH

VCM

LG-x

LOSW-x

LO-x

Figure 47. Simplified LNA Schematic

The LNA supports differential output voltages as high as 4.4 V p-p

with positive and negative excursions of ±1.1 V from a common-

mode voltage of 1.5 V. The LNA differential gain sets the maximum

input signal before saturation. One of three gains is set through the

SPI. The corresponding full-scale input for the gain settings of

15.6 dB, 17.9 dB, and 21.3 dB is 733 mV p-p, 550 mV p-p, and

367 mV p-p, respectively. Overload protection ensures quick

recovery time from large input voltages. Because the inputs are

capacitively coupled to a bias voltage near midsupply, very large

inputs can be handled without interacting with the ESD

protection.

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