ADA4940-1 AD [Analog Devices], ADA4940-1 Datasheet - Page 25

• Manufacturer Part Number: ADA4940-1
• Description: Ultralow Power, Low Distortion
• Manufacturer: AD [Analog Devices]

Data Sheet
INPUT AND OUTPUT CAPACITIVE AC COUPLING
Although the
coupled applications, it is nonetheless possible to use it in ac-
coupled circuits. Input ac coupling capacitors can be inserted
between the source and R
of the dc common-mode feedback current and causes the
ADA4940-1/ADA4940-2
equal the dc output common-mode voltage. These ac coupling
capacitors must be placed in both loops to keep the feedback
factors matched. Output ac coupling capacitors can be placed in
series between each output and its respective load.
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The V
biased at a voltage approximately equal to the midsupply point,
[(+V
output common-mode voltage that is within approximately
100 mV of the expected value.
In cases where more accurate control of the output common-mode
level is required, it is recommended that an external source, or
resistor divider (10 kΩ or greater resistors), be used. The output
common-mode offset listed in the Specifications section assumes
that the V
It is also possible to connect the V
level (CML) output of an ADC. However, care must be taken to
ensure that the output has sufficient drive capability. The input
impedance of the V
DISABLE PIN
The
be used to minimize the quiescent current consumed when the
device is not being used. DISABLE is asserted by applying a low
logic level to the DISABLE pin. The threshold between high and
low logic levels is nominally 1.4 V above the negative supply rail.
See Table 5 and Table 8 for the threshold limits.
The DISABLE pin features an internal pull-up network that
enables the amplifier for normal operation. The ADA4940-1/
ADA4940-2
external connection is required) and does not require an
external pull-up resistor to ensure normal on operation (see
Figure 70). When the
output is high impedance. Note that the outputs are tied to the
inputs through the feedback resistors and to the source using the
gain resistors. In addition, there are back-to-back diodes on the
input pins that limit the differential voltage to 1.2 V.
ADA4940-1/ADA4940-2
S
) + (−V
OCM
OCM
pin of the
DISABLE pin can be left floating (that is, no
S
input is driven by a low impedance voltage source.
ADA4940-1/ADA4940-2
)]/2. Relying on this internal bias results in an
OCM
ADA4940-1/ADA4940-2
ADA4940-1/ADA4940-2
pin is approximately 250 kΩ.
G
dc input common-mode voltage to
. This ac coupling blocks the flow
feature a DISABLE pin that can
OCM
input to a common-mode
is best suited to dc-
is internally
is disabled, the
Rev. B | Page 25 of 32
DRIVING A CAPACITIVE LOAD
A purely capacitive load reacts with the bond wire and pin
inductance of the ADA4940-1/ADA4940-2, resulting in high
frequency ringing in the transient response and loss of phase
margin. One way to minimize this effect is to place a resistor in
series with each output to buffer the load capacitance. The resistor
and load capacitance form a first-order, low-pass filter; therefore,
the resistor value should be as small as possible. In some cases,
the ADCs require small series resistors to be added on their inputs.
Figure 71 illustrates the capacitive load vs. the series resistance
required to maintain a minimum 45° of phase margin.
DISABLE
120
100
80
60
40
20
0
5
Figure 71. Capacitive Load vs. Series Resistance (LFSCP)
10
Figure 70. DISABLE Pin Circuit
LOAD CAPACITANCE (pF)
ADA4940-1/ADA4940-2
V
IN
+V
–V
R3
R1
S
S
0.1µF
100
R4
R2
+IN
–IN
V OCM
–FB
+FB
+2.5V
–2.5V
–OUT
+OUT
BIAS CURRENT
AMPLIFIER
R
R S
S
C L
C L
1000