AD900 Analog Devices, Inc., AD900 Datasheet - Page 8

• Manufacturer Part Number: AD900
• Description: Highspeed 6-bit Aidconverter
• Manufacturer: Analog Devices, Inc.

ANALOG DEVICES fAX-ON-DEMAND HOTLINE
APPL YING THE AD9006/AD9016
op amp.
amount of current available at the reference connections must be
limited.
of a wide bandwidth, low distortion op amp such as the Analog
Setting Reference
Th~ AD9006/AD9016
Care should ~ taken to assure that these references are driven
from stable..low impedance sources. Reference connections
should ~ capacitively coupled to ground to reduce interference
generated by noise and/or digital switching.
Resistance between the reference connections and the point at
which the first comparator threshold is connected causes offset
errors. These errors, called "top and bottom of the ladder
offsets," can be nulled oUt using the + VSENSE and
connections. These sense lines are intended for connection only
to high impedance (low current) nodes such as the input of an
Applying a voltage greater than l.IV across the internal resistor
ladder will cause current densities to exceed rated values and
may cause permanent damage to the AD9006lAD9016.The
Driving the Analog Input
Careful design and layout of the AD9006/AD9016 have resulted
in a typical input capacitance oC8pF (9.5pF max). This is low
in comparison to most flash converters, but it is still a signifi-
cant load at high input frequencies and must be taken into ac-
count when choosing a drive amplifier.
DC-coupled applications require the performance characteristics
ANALOG
SOURCE
ret~rc:n..:es:
across th~ internal resistor ladder (nominally 8011)and deter-
mine the analog input range of the converter.
REV.A
RMATCH
+ Y REF and
Levels
requires that the user provide two voltage
--
V
REF'
REFERENCE
VooT=2.SV
VO~TAGE
AD1403
+v.
These two voltages are applied
8.
- Pag" 23
Ru....
-v.
R..
R.
Figure 2. Analpg Input Circuits
---
R,
Figure 1. Reference Circuit
V
1011
R.
SENSE
ANA~OG
INPUT
1010
AD9016
AD9006/
-9-
The midpoint of the comparator reference ladder (MIDSCALE
analog input ranges because integral nonlinearity increases under
One method of nulling the offset errors is shown in Figure 1-
The Analog Devices ADI403 voltage reference supplies a
of current drawn directly from the up amp; resistors at the base
and emilter of the transistor stabilize its operation.
essentially the same way, using Rz to adjust the reference ladder
voltage; and using feedback from the - VSENSE connection to
null any offset between the reference and the threshold of the
VREF)is shown tied to ground in Figure I. This allows the user
[0 adjust the voltage reference for minimum integral nonlinear-
ity. This featUre becomes important in applications with reduced
these conditions.
stable 2.5V reference for the circuit, and RI.1MITdetermines
the range over which the reference can be adjusted. RI adjusts
the voltage at the top of the internal reference ladder through
the AD64lflN3904
line causes the op amp to compensate for offset which appears at
the top comparator threshold. The transistor limits the amount
Voltage at the bottom of the reference ladder is controlled in
bottom comparator.
may be better served by using a low distortion gain block for
Figure 2 illustrates possible connections for both approaches.
Regardless of which driving circuit is selected Corthe applica-
tion, the overall dynamic performance of the amplifier is en-
hanced by inserting a small series resistor between the output of
the amplifier and the analog input of the converter.
Devices AD96 I 1. AC-coupled applications at high frequencies
the driver.
+SV
10
ANALOG
SOURCE
2NJ904
2NJ906
15001
10
combination. Feedback from the + VSENSE
AD9006! AD9016
+vR..
+v......
-v......
-v...,
MIDSCALE v....
AD9006/AD9016
b.
ANA~OG
INPUT
AD9016
AD9006/