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參數(shù)資料
| 型號： | AD9260EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | High-Speed Oversampling CMOS ADC with 16-Bit Resolution at a 2.5 MHz Output Word Rate |
| 中文描述： | 高速16位分辨率在2.5 MHz的輸出字速率采樣的CMOS模數(shù)轉換器 |
| 文件頁數(shù)： | 23/36頁 |
| 文件大?。?/td> | 572K |
| 代理商： | AD9260EB |
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AD9260
–23–
REV. B
VIN
499
50
+5V
AD9260
VINA
VREF
C
S
100pF
0.1 F
10 F
VINB
C
S
100pF
499
499
499
50
AD8138
Figure 57. AD8138 Single-Ended Differential ADC Driver
The second driver circuit, shown in Figure 58, can provide slightly
enhanced noise performance relative to the AD8138, assuming
low-noise, high-speed op amps are used. This differential op amp
driver circuit is configured to convert and level-shift a 2 V p-p
single-ended, ground-referenced signal to a 4 V p-p differential
signal centered at the common-mode level of the AD9260. The
circuit is based on two op amps that are configured as matched
unity gain difference amplifiers. The single-ended input signal is
applied to opposing inputs of the difference amplifiers, thus
providing differential outputs. The common-mode offset voltage
is applied to the noninverting resistor leg of each difference ampli-
fier providing the required offset voltage. This offset voltage is
derived from the common-mode level (CML) pin of the AD9260
via a low output impedance buffer amplifier capable of driving a
1
μ
F capacitive load. The common-mode offset can be varied
over a 1.8 V to 2.5 V span without any serious degradation in
distortion performance as shown in Figure 44, thus providing
some flexibility in improving output compression distortion from
some
±
5 op amps with limited positive voltage swing.
To protect the AD9260 from an undervoltage fault condition
from op amps specified for
±
5 V operation, two 50
series
resistors and a diode to AGND are inserted between each op
amp output and the AD9260 inputs. The AD9260 will inherently
be protected against any overvoltage condition if the op amps
share the same positive power supply (i.e., AVDD) as the AD9260.
Note, the gain accuracy and common-mode rejection of each dif-
ference amplifier in this driver circuit can be enhanced by using
a matched thin-film resistor network (i.e., Ohmtek ORNA5000F)
for the op amps. Resistor values should be 500
or less to main-
tain the lowest possible noise.
The noise performance of each unity gain differential driver
circuit is limited by its inherent noise gain of two. For unity gain
op amps ONLY, the noise gain can be reduced from two to one
VIN
C
C
100pF
C
100pF
V
CML
-VIN
50
R
50
C
F
R
R
C
F
R
R
R
R
R
50
V
CML
-VIN
0.1 F
1.0 F
AD817
AD9260
50
VINA
VINB
CML
C
C
100pF
Figure 58. DC-Coupled Differential Driver with
Level-Shifting
beyond the input signals passband by adding a shunt capacitor,
C
F
, across each op amp
’
s feedback resistor. This will essentially
establish a low-pass filter which reduces the noise gain to one
beyond the filter
’
s f
–
3 dB
while simultaneously bandlimiting the
input signal to f
–
3 dB
. Note, the pole established by this filter can
also be used as the real pole of an antialiasing filter. Since the
noise contribution of two op amps from the same product family
are typically equal but uncorrelated, the total output-referred
noise of each op amp will add root-sum square leading to a
further 3 dB degradation in the circuit
’
s noise performance.
Further out-of-band noise reduction can be realized with the
addition of single-ended and differential capacitors, C
S
and C
D
.
The distortion and noise performance of the two op amps
within the signal path are critical in achieving the AD9260
’
s
optimum performance. Low noise op amps capable of providing
greater than 85 dB THD at 1 MHz while swinging over a 1 V to
3 V range are a rare commodity, yet should only be considered.
The AD9632 op amp was found to provide superb distortion
performance in this circuit due to its ability to maintain excel-
lent distortion performance over a wide bandwidth while swing-
ing over a 1 V to 3 V range. Since the AD9632 is gain-of-two or
greater stable, the use of the noise reduction shunt capacitors
discussed above was prohibited thus degrading its noise perfor-
mance slightly (1 dB
–
2 dB) when compared to the OPA642.
Note, the majority of the AD9260 test and characterization data
presented in this data sheet was taken using the AD9632 op
amp in this dc coupled driver circuit. This driver circuit is also
provided on the AD9260 evaluation board since the AD8138
was unreleased at that time.
Table IV. Reference Configuration Summary
Reference
Operating Mode
INTERNAL
INTERNAL
INTERNAL
Input Span (VINA
–
VINB)
(V p-p)
1.6
4.0
1.6
≤
SPAN
≤
4.0 and
SPAN = 1.6
×
VREF
1.6
≤
SPAN
≤
4.0
Required VREF
(V)
1
2.5
1
≤
VREF
≤
2.5 and
VREF = (1+R1/R2)
1
≤
VREF
≤
2.5
Connect
SENSE
SENSE
R1
R2
SENSE
VREF
To
VREF
REFCOM
VREF and SENSE
SENSE and REFCOM
AVDD
EXT. REF.
EXTERNAL
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