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
01 1.0 mS 83 kHz 110 kHz 167 kHz
10 1.5 mS 133 kHz 178 kHz 267 kHz
11 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.
LG
HP
LG
m
GAINLG
HP
C
10Tablef
C
g
LNACf nF10
)(
π2
1
)( ×=××
×
=
(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
11293-027
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.