DAC8512

–15–

REV. A

A Digitally Controlled, Ultralow Noise VCA

The circuit in Figure 38 illustrates how the DAC8512 can be

used to control an ultralow noise VCA, using the AD600/

AD602. The AD600/AD602 is a dual, low noise, wideband,

variable gain amplifier based on the X-AMP topology.* Both

channels of the AD600 are wired in parallel to achieve a

wideband VCA which exhibits an RTI (Referred To Input)

noise voltage spectral density of approximately 1 nV/

√Hz. The

output of the VCA requires an AD844 configured in a gain of 4

to account for signal loss due to input and output 50

Ω termina-

tions. As configured, the total gain in the circuit is 40 dB.

Since the output of the DAC8512 is single quadrant, it was nec-

essary to offset the AD600’s gain control voltage so that the gain

of the circuit is 0 dB for zero scale and 40 dB at full scale. This

was achieved by setting C1LO and C2LO to +625 mV using R1

and R2. Next, the output of the DAC8512 was scaled so that

the gain of the AD600 equaled 20 dB when the digital input

function of digital code is shown in Figure 39.

*For more details regarding the AD600 or AD602, please consult the AD600/

AD602 data sheet.

+70

+20

–30

100k

100M

10M

1M

10k

+30

+40

+50

+60

–20

–10

0

+10

FREQUENCY – Hz

4095

3072

2048

1024

0

Figure 39. VCA Frequency Response vs. Digital Code

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

REF

AD600JN

V+

V–

0.1

µF

0.1

µF

6

2

DAC8512FZ

8

7

CS

CLR

1

0.1

µF

V+

1

µF

R6

2.26k

Ω

R7

1k

Ω

0

≤ V

G ≤ 1.25V

5

LD

3

SCLK

4

SDI

R1

619

Ω

R2

4.32k

Ω

V+

+625mV

6

3

2

0.1

µF

0.1

µF

V+

V

OUT

0.01dB/BIT

10

µF

+5V

10

µF

–5V

V+

V–

FB = FAIR RITE

#2743001111

SUPPLY DECOUPLING NETWORK

V–

R5

806

Ω

R3

402

Ω

R4

402

Ω

R4

49.9

Ω

V

IN

AD844

Figure 38. A Digitally Controlled, Ultralow Noise VCA