AD844

REV. C

–12–

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

V

2 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 k

Ω, at which value the

bandwidth of the AD844 is about 22 MHz, and is essentially in-

Figure 39 shows the small signal response for a 50 dB gain con-

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

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

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

Cerdip (Q) Package

Mini-DIP (N) Package

16-Pin SOIC (R) Package