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