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AD8139ACP-R2 Datasheet(PDF) 20 Page - Analog Devices

Part # AD8139ACP-R2
Description  Low Noise Rail-to-Rail Differential ADC Driver
PDF  24 Pages
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

AD8139ACP-R2 Datasheet(HTML) 20 Page - Analog Devices

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AD8139
Rev. A | Page 20 of 24
Using the β notation, Equation 15 can be written as
( )
ICM
OCM
ACM
V
V
V
β
−
+
β
=
1
(16)
or equivalently,
(
)
ICM
OCM
ICM
ACM
V
V
V
V
−
β
+
=
(17)
where VICM is the common-mode voltage of the input signal, i.e.,
2
IN
IP
ICM
V
V
V
+
=
.
For proper operation, the voltages at VAN and VAP must stay
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 (RIN, dm) is simply
G
dm
IN,
R
R
2
=
(18)
For a single-ended signal (for example, when VIN is grounded
and the input signal drives VIP), the input impedance becomes
)
(
2
1
F
G
F
G
IN
R
R
R
R
R
+
−
=
(19)
The input impedance of a conventional inverting op amp
configuration is simply RG, but it is higher in Equation 19
because a fraction of the differential output voltage appears at
the summing junctions, VAN and VAP. This voltage partially
bootstraps the voltage across the input resistor RG, 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, VACM.
Consider the case in Figure 59, where VIN is 5 V p-p swinging
about a baseline at ground and VREF is connected to ground.
DGND AGND REFGND REF REFBUFIN PDBUF
ADR431
2.5V
REFERENCE
47
µF
AD7674
IN+
IN–
AVDD
DVDD
AD8139
+
–
8
VREF
2.5V
2
1
6
3
4
5
VOCM
200
Ω
200
Ω
324
Ω
324
Ω
5V
20
Ω
15
Ω
15
Ω
VIN
2.7nF
2.7nF
0.1
µF
0.1
µF
0.1
µF
0.1
µF
+1.7V
+0.95V
VACM
WITH VREF = 0
+0.2V
+2.5V
GND
–2.5V
Figure 59. AD8139 Driving AD7674, 18-Bit, 800 kSPS A/D Converter



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