Analog Devices AD604 - Manual (Page 22)

AD604
MEDICAL ULTRASOUND TGC DRIVING THE
AD9050, A 10-BIT, 40 MSPS ADC
The AD604 is an ideal candidate for the time gain control (TGC)
amplifier that is required in medical ultrasound systems to limit
the dynamic range of the signal that is presented to the ADC.
Figure 52 shows a schematic of an AD604 driving an AD9050
in a typical medical ultrasound application.
The gain is controlled by means of a digital byte that is input to
an AD7226 DAC that outputs the analog gain control signal.
The output common-mode voltage of the AD604 is set to VPOS/2
by means of an internal voltage divider. The VOCM pin is
bypassed with a 0.1 pF capacitor to ground.
The DSX output is optionally filtered and then buffered by
an AD9631 op amp, a low distortion, low noise amplifier. The
op amp output is ac-coupled into the self-biasing input of an
AD9050 ADC that is capable of outputting 10 bits at a 40 MSPS
sampling rate.
0.1MF
0.1MF
ANALOG
INPUT
500
-w.-----
O.iMFij:: —
-------- Mi
0.1 m F
------ ----------
-DSXI VGN1
+DSX1 vref
PAO1 OUT1
FBK1 GND1
PAII VPOS
COMI VNEG
COM2 VNEG
PAI2 VPOS
FBK2 GND2
PAO2 OUT2
+DSX2 VOCM
-DSX2 VGN2
AD6O4
_ 1Oo
0 ]
v out B
VoutA
IB
^ il J
V OuT C
VoutD
V DD
.. AD7226
Vref AO
AGND A1
DGND WR
DB7 DB0
(MSB) (LSB)
DB6 DB1
DB5 DB2
DB4 DB3
AO 17|—I
A1 l e T-l
WR 1 5 1-4
I------
DIGITAL GAIN CONTROL
Figure 52. TGC Circuit for Medical Ultrasound Application
Rev. E | Page 22 of 32
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