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AD8600 Datasheet(PDF) 13 Page - Analog Devices

Part No. AD8600
Description  16-Channel, 8-Bit Multiplying DAC
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD8600 Datasheet(HTML) 13 Page - Analog Devices

 
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AD8600
REV. 0
–13–
staa
ddrc
*
ldx
#$00
set x to point to the DAC address
ldaa
0,x
load the address from 68HC11 mem
staa
portb
set the address on portb
*
LD, CS, EN, R/W are all low for
*
transparent DAC loading
ldab
#$ff
set accumulator b to 255
*
loop
ldaa
#$00
start the triangle wave at zero
staa
portc
write the data to the AD8600
*
load
inca
increase the data by one
staa
portc
send the new data to the AD8600
cba
compare a to b
bne
load
we haven’t reached 255 yet
jmp
loop
we have reached 255, so start over
Figure 22. 68HC11 Microcode to Interface to the AD8600.
Time Dependent Variable Gain Amplifier Using the AD600
The AD8600 is ideal for generating a control signal to set the
gain of the AD600, a wideband, low noise variable gain ampli-
fier. The AD600 (and similar parts such as the AD602 and
AD603) is often used in ultrasound applications, which require
the gain to vary with time. When a burst of ultrasound is ap-
plied, the reflections from near objects are much stronger than
from far objects. To accurately resolve the far objects, the gain
must be greater than for the near objects. Additionally, the sig-
nals take longer to reach the ultrasound sensor when reflected
from a distant object. Thus, the gain must increase as the time
increases.
The AD600 requires a dc voltage to adjust its gain over a
40 dB range. Since it is a dual, the two variable gain amplifiers
can be cascaded to achieve 80 dB of gain. The AD8600 is used
to generate a ramped output to control the gain of the AD600.
The slope of the ramp should correspond to the time delay
of the ultrasound signal. Since ultrasound applications often
require multiple channels, the AD8600 is ideal for this
application.
The circuit to achieve a time dependent variable gain amp is
shown in Figure 23. The AD600’s gain is controlled by differ-
ential inputs, C1LO and C1HI, with a gain constant of
32 dB/V. Thus for 40 dB of gain, the differential control input
needs to be 1.25 V. In this application, the C1LO input is set at
the midscale voltage of 0.625 V, which is generated by a simple
voltage divider from the REF43. The AD8600’s output is di-
vided in half, generating a 0 V to 1.25 V ramp, and then applied
to C1HI. This ramp sweeps the gain from 0 dB to 40 dB.
O
ø
+5V
–5V
V
OUT
R1
10k
V
IN
V
REF
V
CC
, V
DD1
, V
DD2
AD8600
2
13
R2
10k
+5V
C1
100pF
0V – 1.25V
46
REF43
+5V
2
+2.5V
DIGITAL
CONTROL
C1HI
16
2
3
A1HI
A1LO
4
GAT1
1
C1LO
V
POS
13
12
15
A1CM
14
A1OP
AD600
R3
30k
R4
10k
(FROM
ULTRASOUND
SENSOR)
0.625V
Figure 23. Ultrasound Amplifier with Digitally Controlled
Variable Gain


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