ada4930-1 Analog Devices, Inc., ada4930-1 Datasheet - Page 19

• Manufacturer Part Number: ada4930-1
• Description: Ultralow Noise Drivers For Low Voltage Adcs Ada4930-1/ada4930-2
• Manufacturer: Analog Devices, Inc.

MINIMUM R
Due to the wide bandwidth of the ADA4930-1/ADA4930-2, the
value of R
to provide sufficient damping in the amplifier front end. In the
terminated case, R
source and load terminations.
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The V
the total supply voltage above −V
The input impedance of the V
on the internal bias, the output common-mode voltage is within
about 100 mV of the expected value.
In cases where accurate control of the output common-mode
level is required, it is recommended that an external source or
resistor divider be used with source resistance less than 100 Ω.
The output common-mode offset listed in the Specifications
section assumes that the V
voltage source.
It is also possible to connect the V
voltage (V
to ensure that the output has sufficient drive capability. The
input impedance of the V
multiple ADA4930-1/ADA4930-2 devices share one reference
output, it is recommended that a buffer be used.
CALCULATING THE INPUT IMPEDANCE FOR AN
APPLICATION CIRCUIT
The effective input impedance depends on whether the signal
source is single-ended or differential. For a balanced differential
input signal, as shown in Figure 44, the input impedance (R
between the inputs (+D
Figure 44. ADA4930-1/ADA4930-2 Configured for Balanced (Differential) Inputs
OCM
G
pin of the ADA4930-1/ADA4930-2 is biased at 3/10 of
CM
must be greater than or equal to 301 Ω at unity gain
+D
–D
) output of an ADC. However, care must be taken
IN
IN
G
VALUE
G
R
R
includes the Thevenin resistance of the
G
G
IN
V
OCM
OCM
OCM
+IN
and −D
–IN
ADA4930
input is driven by a low impedance
pin is approximately 10 kΩ. If
OCM
+V
S
R
R
with an internal voltage divider.
F
S
F
IN
pin is 8.4 kΩ. When relying
OCM
) is R
input to a common-mode
IN, dm
V
OUT, dm
= 2 × R
G
.
IN, dm
Rev. A | Page 19 of 28
)
For an unbalanced single-ended input signal, as shown in
Figure 45, the input impedance is
where:
β1 =
β2 =
For a balanced system where R
the equations simplify to
The input impedance of the circuit is effectively higher than it
would be for a conventional op amp connected as an inverter
because a fraction of the differential output voltage appears at
the inputs as a common-mode signal, partially bootstrapping
the voltage across the input resistor R
voltage at the amplifier input terminals can be easily determined
by noting that the voltage at the inverting input is equal to the
noninverting output voltage divided down by the voltage divider
formed by R
terminals due to negative voltage feedback and is in phase
with the input signal, thus reducing the effective voltage across
R
G1
Figure 45. ADA4930-1/ADA4930-2 with Unbalanced (Single-Ended) Input
, partially bootstrapping it.
R
β1
R
R
IN,SE
G1
G2
=
R
R
R
+
IN, SE
= R
+
G1
β2
G
2
R
R
F2
=
F1
F
G1
2
and R
R
V
β1
R
R
G
OCM
β1
G1
G2
R
(
+
β2
G
G2
+
R
. This voltage is present at both input
β2
+
F
) 1
and
ADA4930
R
R
ADA4930-1/ADA4930-2
F1
F2
+V
–V
R
G1
S
S
IN,SE
= R
=
G2
G1
⎛
⎜
⎜
⎜
⎜
⎝
= R
1
. The common-mode
−
R
2(
G
L
and R
R
R
V
G
G
R
OUT, dm
+
F
R
F1
F
= R
)
⎞
⎟
⎟
⎟
⎟
⎠
F2
= R
F
,