AD9277 datasheet(30/49 Pages) AD

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AD9277

Rev. 0 | Page 29 of 48

TGC OPERATION

The TGC signal path is fully differential throughout to maxi-

mize 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 ADC input. A simple exercise in under-

standing the maximum and minimum gain requirements is

shown in Figure 55.

The maximum gain required is determined by

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

20 log(224/3.9) + 11 dB = 46 dB

The minimum gain required is determined by

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

20 log(2/0.55) − 10 dB = 3 dB

Therefore, 42 dB of gain range for a 14-bit, 50 MSPS ADC with

15 MHz of bandwidth should suffice in achieving the dynamic

range required for most of today’s ultrasound systems.

The system gain is distributed as listed in Table 9.

Table 9. Channel Gain Distribution

Section

Nominal Gain (dB)

LNA

15.6/17.9/21.3

Attenuator

−42 to 0

VGA Amplifier

21/24/27/30

Filter

ADC

The linear-in-dB gain (law conformance) range of the TGC path

is 42 dB. The slope of the gain control interface is 28.5 dB/V,

and the gain control range is −0.8 V to +0.8 V. Equation 3 is the

expression for the differential voltage VGAIN, and Equation 4 is

the expression for the channel gain.

VGAIN (V) = (GAIN+) – (GAIN−)

(3)

Gain (dB) = 28.5 dB/V × VGAIN + ICPT

(4)

where ICPT is the intercept point of the TGC gain.

In its default condition, the LNA has a gain of 21.3 dB (12×), and

the VGA postamp gain is 24 dB if the voltage on the GAIN+ pin

is 0 V and the voltage on the GAIN− pin is 0.8 V (42 dB attenua-

tion). This results in a total gain (or ICPT) of 3.6 dB through

the TGC path if the LNA input is unmatched, or a total gain of

−2.4 dB if the LNA is matched to 50 Ω (RFB = 350 Ω). However,

if the voltage on the GAIN+ pin is 1.6 V and the voltage on the

GAIN− pin is 0.8 V (0 dB attenuation), the VGA gain is 24 dB.

This results in a total gain of 45 dB through the TGC path if the

LNA input is unmatched or in a total gain of 39 dB if the LNA

input is matched.

Each LNA output is dc-coupled to a VGA input. The VGA

consists of an attenuator with a range of −42 dB to 0 dB followed

by an amplifier with 21 dB/24 dB/27 dB/30 dB of gain. The

X-AMP gain interpolation technique results in low gain error

and uniform bandwidth, and differential signal paths minimize

distortion.

LNA FULL SCALE

(0.55V p-p SINGLE-ENDED)

LNA INPUT-REFERRED

NOISE FLOOR

(3.9µV rms) @ AAF BW = 15MHz

LNA + VGA NOISE = 1.0nV/ Hz

ADC FULL SCALE (2V p-p)

~10dB MARGIN

>11dB MARGIN

ADC NOISE FLOOR

(224µV rms)

MINIMUM GAIN

MAXIMUM GAIN

LNA

ADC

73dB

VGA GAIN RANGE > 42dB

MAX CHANNEL GAIN > 48dB

94dB

Figure 55. Gain Requirements of TGC Operation for a 14-Bit, 50 MSPS ADC

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