AD9278BBCZ datasheet(27/45 Pages) AD

27 / 45 page

AD9278

Data Sheet

Rev. A | Page 26 of 44

Variable Gain Amplifier (VGA)

The differential X-AMP VGA provides precise input attenu-

ation and interpolation. It has a low input-referred noise of

3.5 nV/√Hz and excellent gain linearity. The VGA is driven by

a fully differential input signal from the LNA. The X-AMP archi-

tecture produces a linear-in-dB gain law conformance and low

distortion levels—only deviating ±0.5 dB or less from the ideal.

The gain slope is monotonic with respect to the control voltage

and is stable with variations in process, temperature, and supply.

The resulting total gain range is 45 dB, which allows for range

loss at the endpoints.

The X-AMP inputs are part of a programmable gain feedback

amplifier (PGA) that completes the VGA. The PGA in the VGA

can be programmed to a gain of 21 dB, 24 dB, 27 dB, or 30 dB.

This allows for optimization of channel gain for different imaging

modes in the ultrasound system. The VGA bandwidth is

approximately 100 MHz. The input stage is designed to ensure

excellent frequency response uniformity across the gain setting.

For TGC mode, this minimizes time delay variation across the

gain range.

Gain Control

The gain control interface, GAIN±, is a differential input. VGAIN

varies the gain of all VGAs through the interpolator by selecting

the appropriate input stages connected to the input attenuator.

For GAIN− at 0.8 V, the nominal GAIN+ range for 28 dB/V is

0 V to 1.6 V, with the best gain linearity from approximately

0.16 V to 1.44 V, where the error is typically less than ±0.5 dB.

For GAIN+ voltages greater than 1.44 V and less than 0.16 V,

the error increases. The value of GAIN+ can exceed the supply

voltage by 1 V without gain foldover.

Gain control response time is less than 750 ns to settle within 10%

of the final value for a change from minimum to maximum gain.

There are two ways in which the GAIN+ and GAIN− pins can

be interfaced. Using a single-ended method, a Kelvin type of

connection to ground can be used, as shown in Figure 48. For

driving multiple devices, it is preferable to use a differential

method, as shown in Figure 49. In either method, the GAIN+

and GAIN− pins should be dc-coupled and driven to accom-

modate a 1.6 V full-scale input.

GAIN+

GAIN–

100

Ω

AD9278

0V TO 1.6V DC

0.01µF

KELVIN

CONNECTION

Figure 48. Single-Ended GAIN± Pin Configuration

AD8138

499Ω

AD9278

499Ω

±0.4V DC

AT 0.8V CM

±0.8V DC

±0.4V DC

AT 0.8V CM

499Ω

AVDD2

0.8V CM

523Ω

100Ω

0.01µF

GAIN+

GAIN–

0.01µF

100Ω

31.3kΩ

10kΩ

Figure 49. Differential GAIN± Pin Configuration

VGA Noise

In a typical application, a VGA compresses a wide dynamic

range input signal to within the input span of an ADC. The

input-referred noise of the LNA limits the minimum resolvable

input signal, whereas the output-referred noise, which depends

primarily on the VGA, limits the maximum instantaneous

dynamic range that can be processed at any one particular gain

control voltage. This latter limit is set in accordance with the

total noise floor of the ADC.

Output-referred noise as a function of GAIN+ is shown in

Figure 7, Figure 8, and Figure 10 for the short-circuit input

conditions. The input noise voltage is simply equal to the output

noise divided by the measured gain at each point in the control

range.

The output-referred noise is a flat 50 nV/√Hz (postamp gain =

24 dB) over most of the gain range because it is dominated by

the fixed output-referred noise of the VGA. At the high end of

the gain control range, the noise of the LNA and of the source

prevail. The input-referred noise reaches its minimum value

near the maximum gain control voltage, where the input-

referred contribution of the VGA is miniscule.

At lower gains, the input-referred noise and, therefore, the

noise figure, increases as the gain decreases. The instantaneous

dynamic range of the system is not lost, however, because the

input capacity increases as the input-referred noise increases.

The contribution of the ADC noise floor has the same depen-

dence. The important relationship is the magnitude of the VGA

output noise floor relative to that of the ADC.

Gain control noise is a concern in very low noise applications.

Thermal noise in the gain control interface can modulate the

channel gain. The resultant noise is proportional to the output

signal level and is usually evident only when a large signal is

present. The gain interface includes an on-chip noise filter,

which significantly reduces this effect at frequencies above

5 MHz. Care should be taken to minimize noise impinging at

the GAIN± inputs. An external RC filter can be used to remove

VGAIN source noise. The filter bandwidth should be sufficient to

accommodate the desired control bandwidth.

Antialiasing Filter (AAF)

The filter that the signal reaches prior to the ADC is used to

reject dc signals and to band limit the signal for antialiasing.

The antialiasing filter is a combination of a single-pole high-

pass filter and a second-order low-pass filter. The high-pass

filter can be configured at a ratio of the low-pass filter cutoff.

This is selectable through the SPI.

The filter uses on-chip tuning to trim the capacitors and, in

turn, to set the desired cutoff frequency and reduce variations.

The default −3 dB low-pass filter cutoff is 1/3 or 1/4.5 the ADC

sample clock rate. The cutoff can be scaled to 0.7, 0.8, 0.9, 1, 1.1,

1.2, or 1.3 times this frequency through the SPI. The cutoff

tolerance is maintained from 8 MHz to 18 MHz.

AD9278BBCZ 数据表(PDF) 27 Page - Analog Devices

AD9278BBCZ 数据表(HTML) 27 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

数据表下载

链接网址

AD9278BBCZ 数据表(PDF) 27 Page - Analog Devices

AD9278BBCZ 数据表(HTML) 27 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

数据表 下载

链接网址

数据表下载