DAC8512 Datasheet, PDF (15/20 Pages) Analog Devices – % V, Serial Input Complete 12-Bit DAC

R1
R2
619Ω 4.32kΩ
V+
AD600JN
+625mV
1
16
2
15
3
14
V_{I N}
4
5
6
7
8
V+
0.1µF
13
REF
12
11
10
9
0.1µF
V+
V–
0.1µF
R3
402Ω
R4
402Ω
CS
CLR
LD
SCLK
SDI
1
2
6
5 _DAC8512FZ
3
4
7
R6
2.26kΩ
8
1µF
0 ≤ VG ≤ 1.25V
R7
1kΩ
R5
806Ω
V+
0.1µF
2
AD844 ⁶
3
0.1µF
R4
49.9 Ω
V–
SUPPLY DECOUPLING NETWORK
+5V
10µF
FB = FAIR RITE
V+
#2743001111
V–
10µF
–5V
DAC8512
V_OUT
0.01dB/BIT
Figure 38. A Digitally Controlled, Ultralow Noise VCA
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
code equaled 800_H. The frequency response of the VCA as a
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
+60
+50
4095
+40
3072
+30
2048
+20
1024
+10
0
0
–10
–20
–30
10k
100k
1M
10M
FREQUENCY – Hz
100M
Figure 39. VCA Frequency Response vs. Digital Code
REV. A
–15–