Datasheet

Data Sheet AD8137
Rev. E | Page 25 of 32
04771-0-018
GND
V
REF
V
REFA
ADR525A
2.5V SHUNT
REFERENCE
AD7450A
V
IN
+
V
IN
–
VDD
AD8137
+
–
8
V
REFB
2.5V
2
1
6
3
4
5
V
OCM
1kΩ
1kΩ 1kΩ
2.5kΩ
1kΩ
5V
50Ω
50Ω
V
IN
1.0nF
1.0nF
0.1µF
0.1µF
+1.88V
+1.25VV
ACM WITH
V
REFB
= 0
+0.63V
+2.5V
GND
–2.5V
Figure 65. AD8137 Driving AD7450A, 12-Bit ADC
The input impedance of a conventional inverting op amp
configuration is simply R
G
; however, it is higher in Equation 19
because a fraction of the differential output voltage appears at
the summing junctions, V
AN
and V
AP
. This voltage partially
bootstraps the voltage across the input resistor R
G
, leading to
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
ACM
.
Consider the case in Figure 65, where V
IN
is 5 V p-p swinging
about a baseline at ground and V
REFB
is connected to ground.
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
ICM
has an
amplitude of 2.5 V p-p and is swinging about ground. Using the
results in Equation 16, the common-mode voltage at the inputs of
the AD8137, V
ACM
, is a 1.25 V p-p signal swinging about a baseline
of 1.25 V. The maximum negative excursion of V
ACM
in this case is
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
IN
and V
REF
at midsupply. In this case, V
IN
is 5 V p-p
swinging about a baseline at 2.5 V, and V
REF
is connected to a
low-Z 2.5 V source. V
ICM
now has an amplitude of 2.5 V p-p and
is swinging about 2.5 V. Using the results in Equation 17, V
ACM
is calculated to be equal to V
ICM
because V
OCM
= V
ICM
. Therefore,
V
ICM
swings from 1.25 V to 3.75 V, which is well within the input
common-mode voltage limits of the AD8137. Another benefit
seen by this example is that because V
OCM
= V
ACM
= V
ICM
, no
wasted common-mode current flows. Figure 66 illustrates a way
to provide the low-Z bias voltage. For situations that do not
require a precise reference, a simple voltage divider suffices to
develop the input voltage to the buffer.
04771-0-019
V
IN
0V TO 5V
AD8137
+
–
8
2
1
6
3
4
5
V
OCM
1kΩ
1kΩ
5V
1kΩ 1kΩ
10kΩ
0.1µF
0.1µF
0.1µF
10µF
+
AD8031
+
–
0.1µF
5V
ADR525A
2.5V SHUNT
REFERENCE
TO
AD7450A
V
REF
Figure 66. Low-Z Bias Source
Another way to avoid the input common-mode swing limitation
is to use dual power supplies on the AD8137. In this case, the
biasing circuitry is not required.
Bandwidth vs. Closed-Loop Gain
The 3 dB bandwidth of the AD8137 decreases 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
)MHz72(,
3
×
+
=
−
F
G
G
dmO,dB
RR
R
Vf
(20)
or equivalently, β(72 MHz).
This estimate assumes a minimum 90° phase margin for the
amplifier loop, a condition approached for gains greater than 4.
Lower gains show more bandwidth than predicted by the equation
due to the peaking produced by the lower phase margin.