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參數資料
| 型號： | AD8137YCP-REEL |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 運動控制電子 |
| 英文描述： | Low Cost, Low Power 12-Bit Differential ADC Driver |
| 中文描述： | OP-AMP, 2600 uV OFFSET-MAX, PDSO8 |
| 封裝： | 3 X 3 MM, LFCSP-8 |
| 文件頁數： | 22/24頁 |
| 文件大?。?/td> | 594K |
| 代理商： | AD8137YCP-REEL |
AD8137
Rev. A | Page 22 of 24
AD8137
+
–
8
2
1
6
3
4
0V
2V p-p
R
52.3
5
+
–
V
OCM
1k
1.02k
1k
1k
0.1
μ
F
0.1
μ
F
+5V
–5V
V
IN
SIGNAL
SOURCE
50
0
Figure 66. AD8137 with Terminated Input
The 52.3 termination resistor, R
T
, in parallel with the 1 k
input resistance of the AD8137 circuit, yields an overall input
resistance of 50 that is seen by the signal source. In order to
have matched feedback loops, each loop must have the same R
G
if they have the same R
F
. In the input (upper) loop, R
G
is equal
to the 1 k resistor in series with the (+) input plus the parallel
combination of R
T
and the source resistance of 50 . In the
upper loop, R
G
is therefore equal to 1.03 k. The closest stan-
dard value is 1.02 k and is used for R
G
in the lower loop.
Things get more complicated when it comes to determining the
feedback resistor values. The amplitude of the signal source
generator V
IN
is two times the amplitude of its output signal
when terminated in 50 . Therefore, a 2 V p-p terminated
amplitude is produced by a 4 V p-p amplitude from V
S
. The
Thevenin equivalent circuit of the signal source and R
T
must be
used when calculating the closed-loop gain because R
G
in the
upper loop is split between the 1 k resistor and the Thevenin
resistance looking back toward the source. The Thevenin volt-
age of the signal source is greater than the signal source output
voltage when terminated in 50 because R
T
must always be
greater than 50 . In this case, R
T
is 52.3 and the Thevenin
voltage and resistance are 2.04 V p-p and 25.6 , respectively.
Now the upper input branch can be viewed as a 2.04 V p-p
source in series with 1.03 k. Since this is to be a unity-gain
application, a 2 V p-p differential output is required, and R
F
must therefore be 1.03 k × (2/2.04) = 1.01 k ≈ 1 k. This
example shows that when R
F
and R
G
are large compared to R
T
,
the gain reduction produced by the increase in R
G
is essentially
cancelled by the increase in the Thevenin voltage caused by R
T
being greater than the output resistance of the signal source. In
general, as R
F
and R
G
become smaller in terminated applications,
R
F
needs to be increased to compensate for the increase in R
G
.
When generating the typical performance characteristics data,
the measurements were calibrated to take the effects of the
terminations on closed-loop gain into account.
Power Down
The AD8137 features a PD pin that can be used to minimize the
quiescent current consumed when the device is not being used.
PD is asserted by applying a low logic level to Pin 7. The
threshold between high and low logic levels is nominally 1.1 V
above the negative supply rail. See the Specification tables for
the threshold limits.
DRIVING AN ADC WITH GREATER THAN 12-BIT
PERFORMANCE
Since the AD8137 is suitable for 12-bit systems, it is desirable to
measure the performance of the amplifier in a system with
greater than 12-bit linearity. In particular, the effective number
of bits, ENOB, is most interesting. The AD7687, 16-bit,
250 KSPS ADC’s performance makes it an ideal candidate for
showcasing the 12-bit performance of the AD8137.
For this application, the AD8137 is set in a gain of 2 and driven
single-ended through a 20 kHz band-pass filter, while the output
is taken differentially to the input of the AD7687 (see Figure 67).
This circuit has mismatched R
G
impedances and, therefore, has a
dc offset at the differential output. It is included as a test circuit to
illustrate the performance of the AD8137. Actual application
circuits should have matched feedback networks.
For an AD7687 input range up to 1.82 dBFS, the AD8137 power
supply is a single 5 V applied to V
S+
with V
S
tied to ground. To
increase the AD7687 input range to 0.45 dBFS, the AD8137
supplies are increased to +6 V and 1 V. In both cases, the V
OCM
pin is biased with 2.5 V and the PD pin is left floating. All voltage
supplies are decoupled with 0.1 μF capacitors. Figure 68 and
Figure 69 show the performance of the 1.82 dBFS setup and the
0.45 dBFS setup, respectively.
AD8137
+
–
1nF
1nF
V
OCM
V
S
–
V
S
+
V+
1.0k
1.0k
20kHz
33
33
0
499
499
+2.5
AD7687
GND
V
DD
V
IN
GND
BPF
Figure 67. AD8137 Driving AD7687, 16-Bit 250 KSPS ADC
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