AD9058KD AD [Analog Devices], AD9058KD Datasheet - Page 5

• Manufacturer Part Number: AD9058KD
• Description: Dual 8-Bit 50 MSPS A/D Converter
• Manufacturer: AD [Analog Devices]

THEORY OF OPERATION
The AD9058 contains two separate 8-bit analog-to-digital con-
verters (ADCs) on a single silicon die. The two devices can be
operated independently with separate analog inputs, voltage
references and clocks.
In a traditional flash converter, 256 input comparators are required
to make the parallel conversion for 8-bit resolution. This is in
marked contrast to the scheme used in the AD9058, as shown
in Figure 1.
Unlike traditional “flash,” or parallel, converters, each of the two
ADCs in the AD9058 utilizes a patented interpolating archi-
tecture to reduce circuit complexity, die size and input capacitance.
These advantages accrue because, compared to a conventional
flash design, only half the normal number of input comparator
cells is required to accomplish the conversion.
In this unit, each of the two independent ADCs uses only 128
(2
the seven most significant bits (MSBs) is performed by the 128
comparators. The value of the least significant bit (LSB) is
determined by interpolation between adjacent comparators in
the decoding register. A proprietary decoding scheme processes
the comparator outputs and provides an 8-bit code to the output
register of each ADC; the scheme also minimizes error codes.
Analog input range is established by the voltages applied at the
voltage reference inputs (+V
operate from 0 V to +2 V using the internal voltage reference,
or anywhere between –1 V and +2 V using external references.
Input range is limited to 2 V p-p when using external references.
The internal resistor ladder divides the applied voltage reference
into 128 steps, with each step representing two 8-bit quantiza-
tion levels.
7
) comparators to make the conversion. The conversion for
ANALOG
ANALOG
±0.5 V
±0.5 V
IN A
IN B
0.1µF
REF
200
200
–
800
800
2V
and –V
REF
AD9617
–
AD9617
–
+
+
20k
400
AD707
400
). The AD9058 can
–
+
20k
0.1µF
5
5
+2V
0.1µF
ENCODE
40
38
43
3
8
6
2
1
–V
–V
+V
+V
+V
A
COMP
A
AD9058
ENCODE
IN A
IN B
The onboard voltage reference, +V
which has sufficient drive capability for both reference ladders.
It provides a +2 V reference that can drive both ADCs in the
AD9058 for unipolar positive operation (0 V to +2 V).
USING THE AD9058
Refer to Figure 2. Using the internal voltage reference con-
nected to both ADCs as shown reduces the number of external
components required to create a complete data acquisition sys-
tem. The input ranges of the ADCs are positive unipolar in this
configuration, ranging from 0 V to +2 V. Bipolar input signals
are buffered, amplified and offset into the proper input range of
the ADC using a good low distortion amplifier such as the
AD9617 or AD9618.
(J-LEAD)
REF A
REF B
REF A
REF B
10
INT
A
4,19, 21
25, 27, 42
ANALOG IN
–V
D
D
ENCODE
D
D
36
+V
REF
7A
7B
0A
0B
REF
B
(MSB)
(MSB)
(LSB)
(LSB)
+V
–V
S
S
50
18
16
15
17
14
13
12
11
32
5, 9, 22,
24, 37, 41
28
29
30
31
33
34
35
7, 20,
26, 39
0.1µF
74HCT04
128
127
2
1
1N4001
+5V
1k
–
256
5V
INT
, is a bandgap reference
(SEE TEXT)
10pF
CLOCK
CLOCK
8
8
8
AD9058
8