REV. D

AD844

–12–

Figure 13. 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 AD539

data sheet for full details.) Figure 14 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

INPUTS

0 TO 3V

2V FS

3nF

I/P

GND

0.22 F

TOP VIEW

(Not to Scale)

16

1

AD539

89

AD844

10

0.22 F

0.22 F

10

0.22 F

TYP+6V

@15 A

10

TYP+6V

@15 A

10

2V

OUTPUT

BE TERMINATED – TYPICALLY BY USING

A 50

OR 75

RESISTOR TO GROUND.

Figure 14. 20 MHz VGA Using the AD539

Figure 15 shows the small signal response for a 50 dB gain

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

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.

TOP VIEW

(Not to Scale)

24

23

22

21

20

19

18

17

16

15

14

13

1

2

3

4

5

6

7

8

9

10

11

12

AD568

+15V

REFCOM

–15V

ACOM

LCOM

SPAN

SPAN

THCOM

DIGITAL

INPUTS

+15V

–15V

ANALOG

SUPPLY

GROUND

DIGITAL

SUPPLY

GROUND

–5V

100pF

*0.22 F

POWER SUPPLY

BYPASS CAPACITORS

AD844

MSB

LSB

*

*

*

*

Figure 12. High Speed DAC Amplifier