ad844sq-883b Analog Devices, Inc., ad844sq-883b Datasheet - Page 13

• Manufacturer Part Number: ad844sq-883b
• Description: 60 Mhz, 2000 V/us Monolithic Op Amp
• Manufacturer: Analog Devices, Inc.

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 AD539
data sheet for full details.) Figure 14 shows a simple multiplier
providing the output:
where V
0 V to 3.2 V (max), and V
± 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
REV. E
Figure 13. DAC Amplifier Full-Scale Transient Response
X
is the “gain control” input, a positive voltage from
X
. The gain at V
V
W
Y
DIGITAL
INPUTS
= –
is the “signal voltage,” nominally
V
X
X
2 V
= 3.16 V is 4 dB.
V
Y
10
11
12
1
2
3
4
5
6
7
8
9
(Not to Scale)
MSB
LSB
AD568
TOP VIEW
REFCOM
THCOM
Figure 12. High Speed DAC Amplifier
ACOM
LCOM
SPAN
SPAN
+15V
–15V
I
I
BPD
OUT
V
R
TH
L
24
23
22
21
20
19
18
17
16
15
14
13
*
*
100pF
*0.22 F
POWER SUPPLY
BYPASS CAPACITORS
–13–
Figure 15 shows the small signal response for a 50 dB gain
control range (V
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
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.
AD844
INPUTS
0 TO 3V
0.22 F
2V FS
GND
V
3nF
V
I/P
X
Y
Figure 14. 20 MHz VGA Using the AD539
Y
for V
0.22 F
10
*V
X
BE TERMINATED – TYPICALLY BY USING
A 50
GROUND
X
X
= 10 mV to 3.16 V). At small values of V
AND V
= 1 V, 2 V, and 3 V. For these results, a load
(Not to Scale)
–5V
R
1
8
OR 75
TOP VIEW
I
AD539
Y
INPUTS MAY OPTIONALLY
*
*
+15V
ANALOG
SUPPLY
GROUND
DIGITAL
SUPPLY
–15V
V
RESISTOR TO GROUND.
OUT
16
10
9
0.22 F
0.22 F
10
AD844
TYP –6V
@15 A
@15 A
TYP+6V
10
AD844
V
+V
–V
W
=
S
S
OUTPUT
V
W
–V
2V
X
V
Y
X
,