AD844
1
2
3
4
5
MSB
+15V
24
*
*
*
*
+15V
REFCOM
23
–15V
22
I
BPD 21
I
OUT 20
–15V
AD844
R
I
V
OUT
DIGITAL
INPUTS
6
7
8
9
10
11
12
AD568
R
L
19
ACOM
18
LCOM
17
SPAN
16
SPAN
15
THCOM
14
LSB
V
TH 13
ANALOG
SUPPLY
GROUND
GROUND
100pF
*0.22
F
POWER SUPPLY
BYPASS CAPACITORS
–5V
DIGITAL
SUPPLY
TOP VIEW
(Not to Scale)
Figure 12. High Speed DAC Amplifier
+V
S
TYP+6V
@15 A
10
0.22 F
INPUTS
V
X
0 TO 3V
V
Y
2V FS
3nF
1
16
10
0.22 F
TOP VIEW
(Not to Scale)
AD844
AD539
OUTPUT
V
W
V
W
=
–V
X
V
Y
2V
I/P
GND
0.22 F
8
9
10
–V
S
TYP+6V
@15 A
Figure 13. DAC Amplifier Full-Scale Transient Response
20 MHz Variable Gain Amplifier
0.22 F
10
*V
X
AND V
Y
INPUTS MAY OPTIONALLY
BE TERMINATED – TYPICALLY BY USING
A 50 OR 75 RESISTOR TO GROUND.
The AD844 is an excellent choice as an output amplifier for the
AD539 multiplier, in all of its connection modes. (See AD539
data sheet for full details.) Figure 14 shows a simple multiplier
providing the output:
V V
V
W
=
–
X Y
2
V
where
V
X
is the “gain control” input, a positive voltage of from
0 V to 3.2 V (max) and
V
Y
is the “signal voltage,” nominally
±
2 V FS but capable of operation up to
±
4.2 V. The peak out-
put in this configuration is thus
±
6.7 V. Using all four of the
internal application resistors provided on the AD539 in parallel
results in a feedback resistance of 1.5 kΩ, at which value the
bandwidth of the AD844 is about 22 MHz, and is essentially
independent of V
X
. The gain at V
X
= 3.16 V is +4 dB.
Figure 14. 20 MHz VGA Using the AD539
Figure 15 shows the small signal response for a 50 dB gain
control range (V
X
= 10 mV to 3.16 V). At small values of V
X
,
capacitive feedthrough on the PC board becomes troublesome,
and very careful layout techniques are needed to minimize this
problem. A ground strip between the pins of the AD539 will be
helpful in this regard. Figure 16 shows the response to a 2 V
pulse on V
Y
for V
X
= 1 V, 2 V, and 3 V. For these results, a load
resistor of 500
Ω
was used and the supplies were
±
9 V. The
multiplier will operate from supplies between
±4.5
V and
±16.5
V.
Disconnecting Pins 9 and 16 on the AD539 alters the denomi-
nator in the above expression to 1 V, and the bandwidth will be
approximately 10 MHz, with a maximum gain of 10 dB. Using
only Pin 9 or Pin 16 results in a denominator of 0.5 V, a band-
width of 5 MHz and a maximum gain of 16 dB.
–12–
REV. D
AD [ ANALOG DEVICES ]
