AD8137 Analog Devices, AD8137 Datasheet - Page 22

• Manufacturer Part Number: AD8137
• Description: Low Cost, Low Power 12-Bit Differential ADC Driver
• Manufacturer: Analog Devices

AD8137
The 52.3 Ω termination resistor, R
input resistance of the AD8137 circuit, yields an overall input
resistance of 50 Ω that is seen by the signal source. In order to
have matched feedback loops, each loop must have the same R
if it has the same R
1 kΩ resistor in series with the (+) input plus the parallel
combination of R
upper loop, R
standard value is 1.02 kΩ and is used for R
Things become more complicated when it comes to
determining the feedback resistor values. The amplitude of the
signal source generator V
output signal when terminated in 50 Ω. Therefore, a 2 V p-p
terminated amplitude is produced by a 4 V p-p amplitude from
V
must be used when calculating the closed-loop gain because R
in the upper loop is split between the 1 kΩ resistor and the
Thevenin resistance looking back toward the source. The
Thevenin voltage of the signal source is greater than the signal
source output voltage when terminated in 50 Ω because R
must always be greater than 50 Ω. In this case, R
the Thevenin voltage and resistance are 2.04 V p-p and 25.6 Ω,
respectively.
Now the upper input branch can be viewed as a 2.04 V p-p
source in series with 1.03 kΩ. Since this is to be a unity-gain
application, a 2 V p-p differential output is required, and R
must therefore be 1.03 kΩ × (2/2.04) = 1.01 kΩ ≈ 1 kΩ.
S
. The Thevenin equivalent circuit of the signal source and R
V
IN
SIGNAL
SOURCE
50Ω
G
Figure 66. AD8137 with Terminated Input
is therefore equal to 1.03 kΩ. The closest
T
F
and the source resistance of 50 Ω. In the
. In the input (upper) loop, R
2V p-p
R
52.3Ω
T
IN
is two times the amplitude of its
0V
1.02kΩ
1kΩ
V
OCM
T
, in parallel with the 1 kΩ
0.1µF
0.1µF
8
2
1
AD8137
+
–
3
6
+5V
–5V
G
in the lower loop.
1kΩ
4
1kΩ
5
T
G
is 52.3 Ω and
is equal to the
–
+
T
F
Rev. B | Page 22 of 24
T
G
G
This example shows that when R
to R
essentially cancelled by the increase in the Thevenin voltage
caused by R
signal source. In general, as R
nated applications, R
the increase in R
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 thresh-
old between high and low logic levels is nominally 1.1 V above
the negative supply rail. See the Specification tables (Table 1 to
Table 3) 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 two 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
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
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
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.
T
, the gain reduction produced by the increase in R
T
being greater than the output resistance of the
G
.
F
needs to be increased to compensate for
F
and R
F
S+
and R
with V
G
become smaller in termi-
G
S−
are large compared
tied to ground. To
G
impedances and,
G
is
OCM