AD8139 AD [Analog Devices], AD8139 Datasheet - Page 20

• Manufacturer Part Number: AD8139
• Description: Low Noise Rail-to-Rail Differential ADC Driver
• Manufacturer: AD [Analog Devices]

AD8139
Using the β notation, Equation 15 can be written as
or equivalently,
where V
For proper operation, the voltages at V
within their respective linear ranges.
Calculating Input Impedance
The input impedance of the circuit in Figure 57 will depend on
whether the amplifier is being driven by a single-ended or a
differential signal source. For balanced differential input signals,
the differential input impedance ( R
V
ICM
V
V
R
=
ACM
ACM
IN,
V
ICM
dm
IP
=
=
is the common-mode voltage of the input signal, i.e.,
+
=
2
β
V
V
V
ICM
2
IN
R
OCM
G
+
.
β
+
(
V
(
1
OCM
+2.5V
GND
–2.5V
−
β
)
V
−
ICM
V
ICM
V
V
WITH V
2.5V
IN
REF
)
IN, dm
0.1µF
200Ω
200Ω
AN
V
REF
) is simply
OCM
V
ACM
and V
Figure 59. AD8139 Driving AD7674, 18-Bit, 800 kSPS A/D Converter
= 0
2
1
8
AD8139
+
–
AP
3
6
must stay
324Ω
324Ω
4
+1.7V
+0.95V
+0.2V
5
15Ω
15Ω
(16)
(17)
(18)
Rev. A | Page 20 of 24
5V
2.7nF
2.7nF
For a single-ended signal (for example, when V
and the input signal drives V
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 59, where V
about a baseline at ground and V
IN–
IN+
0.1µF
DGND AGND REFGND REF REFBUFIN PDBUF
R
IN
AVDD
=
1
−
( 2
20Ω
R
R
AD7674
G
G
R
+
F
0.1µF
R
DVDD
F
)
47µF
G
AN
, but it is higher in Equation 19
and V
IP
), the input impedance becomes
ACM
0.1µF
REF
REFERENCE
AP
.
ADR431
is connected to ground.
. This voltage partially
2.5V
IN
is 5 V p-p swinging
IN
G
is grounded
, leading to
(19)