AD844
Rev. F | Page 17 of 20
TOP VIEW
(Not to Scale)
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13
1
2
3
4
5
6
7
8
9
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11
12
AD568
+15V
(V
CC
)
REFCOM
–15V
(V
EE
)
I
BPO
I
OUT
R
L
ACOM
LCOM
SPAN
SPAN
THCOM
V
TH
DIGITAL
INPUTS
+15V
–15V
V
OUT
ANALOG
SUPPLY
GROUND
DIGITAL
SUPPLY
GROUND
–5V
100pF
0.22µF*
0.22µF*
0.22µF*
0.22µF*
*POWER SUPPLY BYPASS CAPACITORS.
R
I
AD844
MSB
LSB
6
7
4
3
2
0897-042
Figure 42. High Speed DAC Amplifier
HIGH SPEED DAC BUFFER
The AD844 performs very well in applications requiring current-
to-voltage conversion. Figure 42 shows connections for use with
the AD568 current output DAC. In this application, the bipolar
offset is used so that the full-scale current is ±5.12 mA, which
generates an output of ±5.12 V using the 1 kΩ application resistor
on the AD568. Figure 43 shows the full-scale transient response.
Care is needed in power supply decoupling and grounding
techniques to achieve the full 12-bit accuracy and realize the
fast settling capabilities of the system. The AD568 data sheet
should be consulted for more complete details about its use.
0897-043
2V
10
50ns
100
0
90
Figure 43. DAC Amplifier Full-Scale Transient Response
20 MHZ VARIABLE GAIN AMPLIFIER
The AD844 is an excellent choice as an output amplifier for the
AD539 multiplier, in all of its connection modes. (See the
AD539 data sheet for full details.) Figure 44 shows a simple
multiplier providing the output:
V
VV
V
YX
W
2
−= (1)
where V
X
is the gain control input, a positive voltage from 0 V
to 3.2 V (maximum), and V
Y
is the signal voltage, nominally
±2 V full scale but capable of operation up to ±4.2 V.
The peak output 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.
INPUTS
V
Y
*
±2V FS
3nF
INPUT
GND
0.22µF
0.22µF 0.22µF
1
2
3
4
5
6
7
AD539
TOP VIEW
(Not to Scale)
8
0.22µF
+
S
TYP +6V
AT 15µA
–V
S
TYP –6V
AT 15µA
10Ω
10Ω
10Ω 10Ω
V
W
=
2V
–V
X
V
Y
OUTPUT
V
W
*V
X
AND V
Y
INPUTS MAY OPTIONALLY BE TERMINATED;
TYPICALLY BY USING A 50Ω OR 75Ω RESISTOR TO GROUND.
V
X
*
0V TO 3
2
3
7
6
4
AD844
9
16
15
14
13
12
11
10
00897-044
Figure 44. 20 MHz VGA Using the AD539
