AD9826
–18–
TOP VIEW
(Not to Scale)
28
27
26
25
24
23
22
21
20
19
18
17
16
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
AD9826
CDSCLK1
CDSCLK2
ADCCLK
OEB
DRVDD
DRVSS
(MSB) D7
D6
D5
D4
D3
D2
D1
(LSB)D0
AVDD
AVSS
VINR
OFFSET
VING
CML
VINB
CAPT
CAPB
AVSS
AVDD
SLOAD
SCLK
SDATA
0.1F
0.1F
0.1F
0.1F
10F
0.1F
0.1F
0.1F
RED INPUT
GREEN INPUT
BLUE INPUT
CLOCK
INPUTS
0.1F
DATA
INPUTS
5V/3V
5V
0.1F
1.0F
0.1F
0.1F
5V
SERIAL
INTERFACE
Figure 18. Recommended Circuit Configuration, 3-Channel CDS Mode
Programmable Gain Amplifiers
The AD9826 uses one Programmable Gain Amplifier (PGA) for
each channel. Each PGA has a gain range from 1× (0 dB) to
6.0× (15.56 dB), adjustable in 64 steps. Figure 17 shows the
PGA gain as a function of the PGA register code. Although the
gain curve is approximately “linear in dB,” the gain in V/V var-
ies nonlinearly with register code, following the equation:
Gain
G
=
+
⎡
⎣
⎢
⎤
⎦
⎥
60
150
63
63
.
.
–
where G is the decimal value of the gain register contents, and
varies from 0 to 63.
PGA REGISTER VALUE – Decimal
0
0
12
GAIN – dB
4
12
24 36 63
GAIN – V/V
GAIN – dB
GAIN – V/V
48 60
8
16
1.00
2.25
4.75
3.50
6.00
Figure 17. PGA Gain Transfer Function
APPLICATIONS INFORMATION
Circuit and Layout Recommendations
The recommended circuit configuration for 3-Channel CDS
Mode operation is shown in Figure 18. The recommended
input coupling capacitor value is 0.1 μF (see Circuit Operation
section for more details). A single ground plane is recommended
for the AD9826. A separate power supply may be used for
DRVDD, the digital driver supply, but this supply pin should
still be decoupled to the same ground plane as the rest of the
AD9826. The loading of the digital outputs should be mini-
mized, either by using short traces to the digital ASIC, or by
using external digital buffers. To minimize the effect of digital
transients during major output code transitions, the falling edge
of CDSCLK2 should occur coincident with or before the
rising edge of ADCCLK (see Figures 1 through 6 for timing).
All 0.1 μF decoupling capacitors should be located as close as
possible to the AD9826 pins. When operating in 1CH or 2CH
Mode, the unused analog inputs should be grounded.
For 3-Channel SHA Mode, all of the above considerations also
apply, except that the analog input signals are directly connected
to the AD9826 without the use of coupling capacitors. The analog
input signals must already be dc-biased between 0 V and 4 V.
Also, the OFFSET pin should be grounded if the inputs to the
AD9826 are to be referenced to ground, or a dc offset voltage
should be applied to the OFFSET pin in the case where a coarse
offset needs to be removed from the inputs. (See Figure 16 and
the Circuit Operation section for more details.)
REV. B
