AD9670EBZ datasheet(24/48 Pages) AD

24 / 48 page

Data Sheet

AD9674

Rev. A | Page 23 of 47

The feedback acts as a high-pass filter providing dynamic correction

of the dc offset. The cutoff frequency of the high-pass filter response

is dependent on the value of the CLG capacitor, the gain of the

LNA (LNAGAIN), and the gm of the feedback transconductance

amplifier. The gm value is programmed in Address 0x120, Bits[4:3].

It is required that CS be equal to CLG for proper operation.

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

Addr.

0x120[4:3]

gm (mS)

LNAGAIN =

15.6 dB

LNAGAIN =

17.9 dB

LNAGAIN =

21.6 dB

00 (default)

0.5 mS

41 kHz

55 kHz

83 kHz

1.0 mS

83 kHz

110 kHz

167 kHz

1.5 mS

133 kHz

178 kHz

267 kHz

2.0 mS

167 kHz

220 kHz

330 kHz

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

be determined by scaling the values from Table 10 or by calculating

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

GAIN

Table

LNA

)

(

)

(

(6)

Variable Gain Amplifier (VGA)

The differential X-AMP VGA provides precise input attenuation

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 architecture 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, allowing range loss at the endpoints.

The X-AMP inputs are part of a programmable gain 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, allowing

optimization of the 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, the

design of the 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 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.

The 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.

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.

249Ω

AD9674

249Ω

±0.8V DC

AT 0.8V CM

±1.6V

±0.8V DC

AT 0.8V CM

249Ω

AVDD2

0.8V CM

249Ω

100Ω

0.01µF

GAIN+

GAIN–

0.01µF

100Ω

31.3kΩ

10kΩ

ADA4938-1/

ADA4938-2

Figure 38. Differential GAIN± Pin Configuration

The analog gain control can be disabled and the attenuator can be

controlled digitally using Address 0x011, Bits[7:4]. 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 (postamplifier

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. An external RC filter can be used to remove VGAIN source

noise. The filter bandwidth must be sufficient to accommodate the

desired control bandwidth and attenuate unwanted switching noise

from the external digital-to-analog converters used to drive the

gain control.

The AD9674 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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