AD8004 Analog Devices, AD8004 Datasheet - Page 11

• Manufacturer Part Number: AD8004
• Manufacturer: Analog Devices

Specifications of AD8004

• -3db Bandwidth: 250MHz
• Slew Rate: 3kV/µs
• Vos: 1mV
• Ib: 35µA
• # Opamps Per Pkg: 4
• Input Noise (nv/rthz): 1.5nV/rtHz
• Vcc-vee: 4V to 12V
• Isy Per Amplifier: 4.25mA
• Packages: DIP,SOIC

DRIVING A SINGLE-SUPPLY A/D CONVERTER
New CMOS A/D converters are placing greater demands on the
amplifiers that drive them. Higher resolutions, faster conversion
rates, and input switching irregularities require superior settling
characteristics. In addition, these devices run off a single +5 V
supply and consume little power, so good single-supply operation
with low power consumption is very important. The AD8004 is
well positioned for driving this new class of A/D converters.
Figure 12 shows a circuit that uses an AD8004 to drive an
AD876, a single supply, 10-bit, 20 MSPS A/D converter that
requires only 140 mW. Using the AD8004 for level shifting and
driving, the A/D exhibits no degradation in performance com-
pared to when it is driven from a signal generator.
The analog input of the AD876 spans 2 V centered at about
2.6 V. The resistor network and bias voltages provide the level
shifting and gain required to convert the 0 V to 1 V input signal
to a 3.6 V to 1.6 V range that the AD876 wants to see.
Biasing the noninverting input of the AD8004 at 1.6 V dc forces
the inverting input to be at 1.6 V dc for linear operation of the
amplifier. When the input is at 0 V, there is 3.2 mA flowing out
of the summing junction via R1 (1.6 V/499 Ω). R3 has a current
of 1.2 mA flowing into the summing junction (3.6 V – 1.6 V)/
1.65 kΩ. The difference of these two currents (2 mA) must flow
REV. C
Figure 10. Frequency Response vs. Capacitive Loading,
R
L
= 100 Ω Output
–10
–12
–14
–10
–12
–14
–2
–4
–6
–8
Figure 11. Flatness with 10 pF Capacitive Load
–2
–4
–6
–8
2
0
2
0
1
1
R
G = +2
R
V
R
V
IN
IN
L
L
F
G = +2, R
5V
5V
G = –2, R
= 100
= 1k
= 1.2k
= 50mV rms
= 50mV
S
S
F
F
= 1.10k
= 698
FREQUENCY – MHz
FREQUENCY – MHz
10
10
40
40
C
C
L
L
= 10pF
= 10pF
C
C
100
100
L
L
= 0
= 0
C
L
= 10pF
C
L
= 0
500
500
–2
–4
–6
–8
–10
–12
–14
2
0
–11–
through R2. This current flows toward the summing junction
and requires that the output be 2 V higher than the summing
junction or at 3.6 V.
When the input is at 1 V, there is 1.2 mA flowing into the sum-
ming junction through R3 and 1.2 mA flowing out through R1.
These currents balance and leave no current to flow through
R2. Thus the output is at the same potential as the inverting
input or 1.6 V.
The input of the AD876 has a series MOSFET switch that turns
on and off at the sampling rate. This MOSFET is connected to
a hold capacitor internal to the device. The on impedance of the
MOSFET is about 50 Ω, while the hold capacitor is about 5 pF.
In a worst case condition, the input voltage to the AD876 will
change by a full-scale value (2 V) in one sampling cycle. When
the input MOSFET turns on, the output of the op amp will be
connected to the charged hold capacitor through the series
resistance of the MOSFET. Without any other series resistance,
the instantaneous current that flows would be 40 mA. This
would cause settling problems for the op amp.
The series 100 Ω resistor limits the current that flows instanta-
neously after the MOSFET turns on to about 13 mA. This
resistor cannot be made too large or the high frequency perfor-
mance will be affected.
The sampling MOSFET of the AD876 is closed for only half of
each cycle or for 25 ns. Approximately seven time constants are
required for settling to 10 bits. The series 100 Ω resistor along
with the 50 Ω on resistance and the hold capacitor, create a
750 ps time constant. These values leave a comfortable margin
for settling. Obtaining the same results with the op amp A/D
combination as compared to driving with a signal generator
indicates that the op amp is settling fast enough.
Overall the AD8004 provides adequate buffering for the AD876
A/D converter without introducing distortion greater than that
of the A/D converter by itself.
LAYOUT CONSIDERATIONS
The specified high speed performance of the AD8004 requires
careful attention to board layout and component selection.
Table I shows the recommended component values for the
AD8004 and Figures 14–16 show the layout for the AD8004
evaluation boards (14-lead DIP and SOIC). Proper R
techniques and low parasitic component selection are mandatory.
1V
0V
3.6V
V
IN
Figure 12. AD8004 Driving the AD876
0.1 F
50
0.1 F
1.65k
499k
R3
R1
1.6V
1k
R2
AD8004
1/4
+5V
0.1 F
3.6V
1.6V
100
AD8004
F
10 F
design
AD876
+3.6V
+1.6V
REFT
REFB