AD9271 datasheet(25/58 Pages) AD

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Preliminary Technical Data

AD9271

Rev. PrA | Page 25 of 58

Crosspoint Switch

Each LNA is followed by a transconductance amp for V/I con-

version. Currents can be routed to one of six pairs of differential

outputs or to 12 single-ended outputs for summing. Each CWD

output pin sinks 2.4 mA dc current, and the signal has a full-scale

of ±2 mA for each channel selected by the cross-point switch.

For example, if four channels were to be summed on one CWD

output, the output would sink 9.6 mA dc and have a full-scale

current output of ±8 mA. The maximum number of channels

combined must be considered in setting the load impedance for

I/V conversion to ensure that the full-scale swing and common-

mode voltage are within the operating limits of the AD9271.

When interfacing to the AD8339, a common-mode voltage of

2.5 V and a full-scale swing of 2.8 V p-p are desired This can be

accomplished by connecting an inductor between each CWD

output and a 2.5 V supply, and then connecting either a single-

ended or differential load resistance to the CWD outputs. The

value of resistance should be calculated based on the maximum

number of channels that can be combined.

CWD outputs are required under full-scale swing to be within

1.5 V and CWVDD (3.3 V supply).

TGC OPERATION

The signal path is fully differential throughout to maximize

signal swing and reduce even-order distortion; however, the

LNAs are designed to be driven from a single-ended signal

source. Gain values are referenced from the single-ended LNA

input to the differential output of the LNA. A simple exercise in

understanding the maximum and minimum gain requirements

is shown in Figure 41.

Table 8. LNA Specifications

LNA Parameters

Specifications

BW (MHz)

FS/FSrms (mVpp/mV)

333/118

SNR (dB)

88.1

ENOB (Bits)

14.3

Noise (rms uV/nV/rt(Hz))

4.65/1.2

ADC EQUIVALENT

DYNAMIC RANGE

LNA FS

(0.333V p-p SE)

VGA NOISE FLOOR

(4.7µV rms)

ADC FS (2V p-p)

ADC NOISE FLOOR

(194µV rms)

MINIMUM GAIN

MAXIMUM GAIN

88dB

70dB

Figure 41. Gain Requirements of TGC for a 12-Bit, 40 MSPS ADC

Table 9. ADC Specifications

ADC Parameters

Specifications

FS/FSrms (Vpp/mV)

2/707

SNR (dB)

ENOB (Bits)

11.3

SFDR (dB)

−82

Noise (rms uV/nV/rt(Hz))

194/50

In summary, the maximum gain required is determined by

(ADC Noise Floor/VGA Input Noise Floor) + Margin =

20 log(194/4.7) + 10 dB = 42.3 dB

The minimum gain required is determined by

(ADC Input FS/VGA Input FS) + Margin =

20 log(2/0.333) – 6 dB = 9.6 dB

Therefore, a 12-bit, 40 MSPS ADC with 15 MHz of bandwidth

should suffice in achieving the dynamic range required for most

ultrasound systems today.

The system gain is distributed as listed in Table 7.

Table 10. Channel Gain Distribution

Section

Nominal Gain (dB)

LNA

14/15.6/18

Attenuator

0 to −30

VGA Amp

Filter

ADC

Total

9 to 39/10.6 to 40.6/13 to 43

The linear-in-dB gain range of the TGC path is 30 dB, extending

from 9.6 dB to 39.6 dB. The slope of the gain control interface is

30 dB/V, and the gain control range is 0 V to 1 V. Equation 1 is

the expression for gain.

ICPT

Gain

GAIN +

)

(

(1)

where ICPT is the intercept point of the LNA gain.

In its default condition, the LNA has a gain of 15.6 dB (6×) and

the VGA gain is −6 dB if the voltage on the VGAIN pin is 0 V.

This gives rise to a total gain (or ICPT) of 9.6 dB through the

TGC path if the LNA input is unmatched, or of 3.6 dB if the

LNA is matched to 50 Ω (RFB = 200 Ω). If the voltage on the

VGAIN pin is 1 V, however, the VGA gain is 24 dB. This gives rise

to a total gain of 39.6 dB through the TGC path if the LNA

input is unmatched, or of 33.5 dB if the LNA input is matched.

Each of the LNA outputs is dc-coupled to a VGA input. The

VGA consists of an attenuator with a range of 30 dB followed by

an amplifier with 24 dB of gain for a net gain range of −5 dB to

+25 dB. The X-AMP gain-interpolation technique results in low

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