AD9432-105 Analog Devices, AD9432-105 Datasheet - Page 11

• Manufacturer Part Number: AD9432-105
• Description: 12-Bit, 105 MSPS Analog-to-digital Converter
• Manufacturer: Analog Devices

Code
+2047
•
•
0
–1
•
•
–2048
Voltage Reference
A stable and accurate 2.5 V voltage reference is built into the
AD9432 (VREFOUT). In normal operation the internal refer-
ence is used by strapping Pin 45 to Pin 46 and placing a 0.1 F
decoupling capacitor at VREFIN.
The input range can be adjusted by varying the reference voltage
applied to the AD9432. No appreciable degradation in perfor-
mance occurs when the reference is adjusted 5%. The full-scale
range of the ADC tracks reference voltage changes linearly.
Timing
The AD9432 provides latched data outputs, with 10 pipeline
delays. Data outputs are included or available one propagation
delay (t
(see Figure 1). The length of the output data lines and loads
placed on them should be minimized to reduce transients within
the AD9432; these transients can detract from the converter’s
dynamic performance.
The minimum guaranteed conversion rate of the AD9432 is
1 MSPS. At internal clock rates below 1 MSPS, dynamic
performance may degrade. Therefore, input clock rates below
1 MHz should be avoided.
During initial power-up, or whenever the clock to the AD9432
is interrupted, the output data will not be accurate data for 200 ns
or 10 clock cycles, whichever is longer.
Using the AD8138 to Drive the AD9432
A new differential output op amp from Analog Devices, Inc.,
the AD8138, can be used to drive the AD9432 in dc-coupled
applications. The AD8138 was specifically designed for ADC
driver applications. Superior SNR performance is maintained up
to analog frequencies of 30 MHz. The AD8138 op amp provides
single-ended-to-differential conversion, providing for a low-cost
option to transformer coupling for ac applications as well.
The circuit in Figure 10 was breadboarded and the measured
performance is shown in Figures 11 and 12. The figures shown
are for 5 V supplies at the AD8138—performance dropped by
about 1 dB–2 dB with a single 5 V supply at the AD8138.
Figure 11 shows SNR and SINAD for a –1 dBFS analog input
frequency varied from 2 MHz to 40 MHz with an encode rate of
105 MSPS. The measurements are for nominal conditions at
room temperature. Figure 12 shows the second and third har-
monic distortion performance under the same conditions.
Table I. Output Coding (VREF = 2.5 V) (Two’s Complement)
PD
) after the rising edge of the encode command
AIN–AIN (V)
1.000
•
•
0
–0.00049
•
•
–1.000
Digital Output
0111 1111 1111
•
•
0000 0000 0000
1111 1111 1111
•
•
1000 0000 0000
The dc common-mode voltage for the AD8138 outputs can be
adjusted via input V
the AD9432 inputs require.
VIN
–100
25
–70
–80
–90
66
65
64
63
62
61
60
0
0
50
500
500
OCM
H2
to provide the 3 V common-mode voltage
20
20
V
500
AD8138
OCM
10pF
SINAD
10pF
500
SNR
H3
AIN – MHz
AIN – MHz
40
40
50
50
22pF
3k
AD9432
5V
2k
AIN
AIN
AD9432
0.1 F
60
60