AD8137 Analog Devices, AD8137 Datasheet - Page 20

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

AD8137
The input impedance of a conventional inverting op amp
configuration is simply R
because a fraction of the differential output voltage appears at
the summing junctions, V
bootstraps the voltage across the input resistor R
the increased input resistance.
Input Common-Mode Swing Considerations
In some single-ended-to-differential applications when using a
single-supply voltage, attention must be paid to the swing of the
input common-mode voltage, V
Consider the case in Figure 64, where V
about a baseline at ground and V
The input signal to the AD8137 is originating from a source
with a very low output resistance.
The circuit has a differential gain of 1.0 and β = 0.5. V
amplitude of 2.5 V p-p and is swinging about ground. Using the
results in Equation 16, the common-mode voltage at the AD8137’s
inputs, V
1.25 V. The maximum negative excursion of V
0.63 V, which exceeds the lower input common-mode voltage limit.
One way to avoid the input common-mode swing limitation is
to bias V
swinging about a baseline at 2.5 V, and V
low-Z 2.5 V source. V
is swinging about 2.5 V. Using the results in Equation 17, V
is calculated to be equal to V
V
input common-mode voltage limits of the AD8137. Another
benefit seen by this example is that since V
no wasted common-mode current flows. Figure 65 illustrates a
way to provide the low-Z bias voltage. For situations that do not
require a precise reference, a simple voltage divider will suffice
to develop the input voltage to the buffer.
ICM
swings from 1.25 V to 3.75 V, which is well within the
ACM
IN
and V
, is a 1.25 V p-p signal swinging about a baseline of
REF
at midsupply. In this case, V
ICM
G
now has an amplitude of 2.5 V p-p and
AN
, but is higher in Equation 19
+2.5V
GND
–2.5V
and V
ICM
because V
ACM
REFB
AP
.
V
IN
. This voltage partially
is connected to ground.
IN
2.5V
V
REF
REFB
is 5 V p-p swinging
OCM
OCM
ACM
is connected to a
Figure 64. AD8137 Driving AD7450A, 12-Bit A/D Converter
V
V
= V
ACM WITH
REFB
= V
0.1µF
1kΩ
V
1kΩ
in this case is
IN
G
OCM
, leading to
ICM
is 5 V p-p
ACM
= 0
ICM
. Therefore,
8
2
1
has an
= V
AD8137
+
–
3
6
ICM
ACM
Rev. B | Page 20 of 24
1kΩ
1kΩ
4
,
+1.88V
+1.25V
+0.63V
5
50Ω
50Ω
5V
Another way to avoid the input common-mode swing limita-
tion is to use dual power supplies on the AD8137. In this case,
the biasing circuitry is not required.
Bandwidth vs. Closed-Loop Gain
The AD8137’s 3 dB bandwidth will decrease proportionally to
increasing closed-loop gain in the same way as a traditional
voltage feedback operational amplifier. For closed-loop gains
greater than 4, the bandwidth obtained for a specific gain can be
estimated as
or equivalently, β(72 MHz).
This estimate assumes a minimum 90 ° phase margin for the
amplifier loop, a condition approached for gains greater than
four. Lower gains will show more bandwidth than predicted by
the equation due to the peaking produced by the lower phase
margin.
1.0nF
1.0nF
0V TO 5V
f
−
3
dB
0.1µF
V
10µF
V
V
IN
V ,
IN
IN
0.1µF
–
+
O,
0.1µF
dm
+
VDD
GND
=
AD7450A
1kΩ
V
1kΩ
0.1µF
OCM
R
Figure 65. Low-Z Bias Source
G
R
+
V
G
V
8
2
1
REFA
R
REF
AD8137
F
+
–
AD8031
3
6
×
5V
5V
(
REFERENCE
2.5V SHUNT
72
ADR525A
1kΩ
4
1kΩ
5
+
–
MHz
0.1µF
)
2.5kΩ
REFERENCE
TO
AD7450A
V
2.5V SHUNT
REF
ADR525A
10kΩ
(20)