AD9240 Analog Devices, AD9240 Datasheet - Page 18

• Manufacturer Part Number: AD9240
• Description: Complete 14-Bit, 10 MSPS Monolithic A/D Converter
• Manufacturer: Analog Devices

Specifications of AD9240

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 10MSPS
• Interface: Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p,2 V p-p,5V p-p,Uni (Vref) x 2,Uni 2.0V,Uni 5.0V
• Adc Architecture: Pipelined
• Pkg Type: QFP

AD9240
Variable Input Span with V
Figure 42 shows an example of the AD9240 configured for an
input span of 2
reference drives the VINB pin thus setting the common-mode
voltage at 2.5 V. The input span can be independently set by a
voltage divider consisting of R1 and R2, which generates the
VREF signal. A1 buffers this resistor network and drives VREF.
Choose this op amp based on accuracy requirements. It is
essential that a minimum of a 10 F capacitor in parallel with a
Figure 42. External Reference, V
Resistor Divider to Make VREF)
Single-Ended Input with 0 to 2
Figure 43 shows an example of an external reference driving
both VINB and VREF. In this case, both the common mode
voltage and input span are directly dependent on the value of
VREF. More specifically, the common-mode voltage is equal to
VREF while the input span is equal to 2 VREF. Thus, the
valid input range extends from 0 to 2 VREF. If, for example,
the REF191, a 2.048 external reference, were selected, the valid
input range extends from 0 V to 4.096 V. In this case, 1 LSB of
the AD9240 corresponds to 0.250 mV. It is essential that a
minimum of a 10 F capacitor in parallel with a 0.1 F low induc-
tance ceramic capacitor decouple the reference output to ground.
Low Cost/Power Reference
The external reference circuit shown in Figure 44 uses a low cost
1.225 V external reference (e.g., AD580 or AD1580) along with an
op amp and transistor. The 2N2222 transistor acts in conjunction
with 1/2 of an OP282 to provide a very low impedance drive for
VINB. The selected op amp need not be a high speed op amp and
may be selected based on cost, power and accuracy.
0.1 F low inductance ceramic capacitor decouple the reference
output to ground.
+5V
2.5V+VREF
2.5V–VREF
0.1 F
+5V
2.5V
0.1 F
Figure 43. Input Range = 0 V to 2
2.5V
REF
2xREF
VREF centered at 2.5 V. An external 2.5 V
VREF
0V
0.1 F
22 F
10 F
CM
R1
R2
= 2.5 V
+5V
0.1 F
0.1 F
VREF Range
CM
A1
= 2.5 V (2.5 V on VINB,
0.1 F
+5V
VINB
SENSE
VINA
VREF
AD9240
VREF
VINA
VINB
VREF
SENSE
AD9240
–18–
Figure 44. External Reference Using the AD1580 and Low
Impedance Buffer
DIGITAL INPUTS AND OUTPUTS
Digital Outputs
The AD9240 output data is presented in positive true straight
binary for all input ranges. Table IV indicates the output data
formats for various input ranges regardless of the selected input
range. A twos complement output data format can be created by
inverting the MSB.
Input (V)
VINA – VINB < –VREF
VINA – VINB = –VREF
VINA – VINB = 0
VINA – VINB = +VREF – 1 LSB 11 1111 1111 1111 0
VINA – VINB
Out Of Range (OTR)
An out-of-range condition exists when the analog input voltage
is beyond the input range of the converter. OTR is a digital
output that is updated along with the data output corresponding
to the particular sampled analog input voltage. Hence, OTR
has the same pipeline delay (latency) as the digital data. It is
LOW when the analog input voltage is within the analog input
range. It is HIGH when the analog input voltage exceeds the
input range as shown in Figure 45. OTR will remain HIGH
until the analog input returns within the input range and an-
other conversion is completed. By logical ANDing OTR with
the MSB and its complement, overrange high or underrange low
conditions can be detected. Table V is a truth table for the over/
underrange circuit in Figure 46 which uses NAND gates. Sys-
tems requiring programmable gain conditioning of the AD9240
input signal can immediately detect an out-of-range condition,
thus eliminating gain selection iterations. Also, OTR can be
used for digital offset and gain calibration.
+5V
OTR
7.5k
0
0
1
1
0
0
1k
3.75V
1.25V
111111 1111 1111
111111 1111 1111
111111 1111 1110
000000 0000 0001
000000 0000 0000
000000 0000 0000
DATA OUTPUTS
AD1580
OP282
1k
Figure 45. Output Data Format
1/2
Condition (V)
Table IV. Output Data Format
+VREF
+5V
0.1 F
1k
OTR
820
10 F
–FS –1/2 LSB
316
–FS+1/2 LSB
–FS
2N2222
0.1 F
10 F
00 0000 0000 0000 1
00 0000 0000 0000 0
10 0000 0000 0000 0
11 1111 1111 1111 1
Digital Output
+5V
0.1 F
1.225V
+FS –1 1/2 LSB
+FS –1/2 LSB
+FS
VINA
VINB
VREF
SENSE
AD9240
REV.
OTR
B