AD9670EBZ datasheet(24/53 Pages) AD

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Data Sheet

AD9670

Rev. A | Page 23 of 52

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

Reg. 0x120,

Bits[4:3]

(mS)

LNAGAIN =

15.6 dB (kHz)

LNAGAIN =

17.9 dB

(kHz)

LNAGAIN =

21.6 dB (kHz)

00 (default)

0.5

1.0

110

167

1.5

133

178

267

2.0

167

220

330

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.

fHP (CLG) =





× LNAGAIN ×

= fHP (see Table 10) ×

(6)

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 by 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

range loss at the endpoints. The X-AMP inputs are part of a PGA

that completes the VGA. The PGA in the VGA is programmable

to a gain of 21 dB, 24 dB, 27 dB, or 30 dB. This allows the

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 uniformity

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 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 be interfaced

as shown in Figure 38. DC couple the GAIN+ and GAIN− pins,

and drive them to accommodate a 3.2 V full-scale input.

249Ω

AD9670

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

Disable the analog gain control and digitally control the

attenuator using SPI Register 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 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 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. The

filter bandwidth must be sufficient to accommodate the desired

control bandwidth and attenuate unwanted switching noise from

the external DACs used to drive the gain control.

The AD9670 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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