Datasheet

AD844
REV. C
–12–
C1258b–5–9/90
PRINTED IN U.S.A.
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 38 shows a simple multiplier
providing the output:
V
W
= –
V
X
V
Y
2V
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 in-
dependent of V
X
. The gain at V
X
= 3.16 V is +4 dB.
Figure 39 shows the small signal response for a 50 dB gain con-
trol 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 40 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.
Figure 38. 20 MHz VGA Using the AD539
Figure 39. VGA AC Response
Figure 40. VGA Transient
Response with V
X
= 1 V, 2 V, and 3 V
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
Cerdip (Q) PackageMini-DIP (N) Package 16-Pin SOIC (R) Package