AD9671EBZ datasheet(25/61 Pages) AD

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AD9671

Data Sheet

Rev. A| Page 24 of 60

Table 10. High-Pass Filter Cutoff Frequency, fHP, for CLG = 10 nF

Address

0x120[4:3]

(mS)

LNAGAIN =

15.6 dB

LNAGAIN =

17.9 dB

LNAGAIN =

21.6 dB

00 (default)

0.5

41 kHz

55 kHz

83 kHz

1.0

83 kHz

110 kHz

167 kHz

1.5

133 kHz

178 kHz

267 kHz

2.0

167 kHz

220 kHz

330 kHz

For other values of CLG, determine the high-pass filter cutoff

frequency by scaling the values from Table 10 or calculating

based on CLG, LNAGAIN, and gm, as shown in Equation 6.

GAIN

LNA

)

(













(6)

where fHP is the high-pass filter cutoff frequency (see Table 10).

Variable Gain Amplifier (VGA)

The differential X-AMP VGA provides precise input attenu-

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

2.5 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—deviating only ±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 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, allowing optimization of channel gain for

different imaging modes in the ultrasound system. The VGA

bandwidth is greater than 100 MHz. The input stage is designed

to ensure excellent frequency response uniformity across the gain

setting. For TGC mode, this input stage minimizes time delay

variation across the gain range.

Gain Control

The analog 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. The nominal VGAIN range is 14 dB/V from −1.6 V to

+1.6 V, with the best gain linearity from approximately −1.44 V

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

VGAIN voltages of greater than 1.44 V and less than −1.44 V, the

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

voltage by 1 V without gain foldover.

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

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

The differential input pins, GAIN+ and GAIN−, can interface

to an amplifier, as shown in Figure 38. Decouple and drive the

GAIN+ and GAIN− pins to accommodate a 3.2 V full-scale input.

ADA4938-x1

1ADA4938-x REFERS TO THE ADA4938-1 AND ADA4938-2 DEVICES

249Ω

AD9671

249Ω

±0.8V DC

AT 0.8V CM

±0.8V DC

AT 0.8V CM

±1.6V

249Ω

0.8V CM

249Ω

100Ω

0.01µF

GAIN+

GAIN–

0.01µF

100Ω

31.3kΩ

10kΩ

Figure 38. Differential GAIN± Pin Configuration

Use Address 0x011, Bits[7:4], to disable the analog gain control

and to control the attenuator digitally. The control range is

45 dB and the step size is 3.5 dB.

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.

The output referred noise is a flat 40 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 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 increase 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 dependence. 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 resulting noise is proportional to the output

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

present. Take care to minimize noise impinging at the GAIN±

inputs. Use an external RC filter to remove VGAIN source noise.

Ensure that the filter bandwidth is sufficient to accommodate the

desired control bandwidth and attenuate unwanted switching noise

from the external DACs used to drive the gain control.

The AD9671 can bypass the GAIN± inputs and control the gain

of the attenuator digitally (see the Gain Control section). This

mode removes any external noise contributions when active gain

control is not needed.

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