AD7661
Rev. 0 | Page 27 of 28
APPLICATION HINTS
BIPOLAR AND WIDER INPUT RANGES
In some applications, it is desirable to use a bipolar or wider
analog input range such as ±10 V, ±5 V, or 0 V to 5 V. Although
the AD7661 has only one unipolar range, simple modifications
of input driver circuitry allow bipolar and wider input ranges to
be used without any performance degradation. Figure 45 shows
a connection diagram that allows this. Component values
required and resulting full-scale ranges are shown in Table 8.
When desired, accurate gain and offset can be calibrated by
acquiring a ground and voltage reference using an analog
multiplexer (U2), as shown in Figure 45.
U1
NALOG
INPUT
R2
R3 R4
100nF
R1
U2
C
REF
IN
INGND
REF
REFGND
AD7661
03033-0-038
C
F
Figure 45. Using the AD7661 in 16-Bit Bipolar and/or Wider Input Ranges
Table 8. Component Values and Input Ranges
Input Range R1 (Ω) R2 (kΩ) R3 (kΩ) R4 (kΩ)
±10 V 500 4 2.5 2
±5 V 500 2 2.5 1.67
0 V to –5 V 500 1 None 0
LAYOUT
The AD7661 has very good immunity to noise on the power
supplies. However, care should still be taken with regard to
grounding layout.
The printed circuit board that houses the AD7661 should be
designed so the analog and digital sections are separated and
confined to certain areas of the board. This facilitates the use of
ground planes that can be separated easily. Digital and analog
ground planes should be joined in only one place, preferably
underneath the AD7661, or as close as possible to the AD7661.
If the AD7661 is in a system where multiple devices require
analog-to-digital ground connections, the connection should
still be made at one point only, a star ground point that should
be established as close as possible to the AD7661.
Running digital lines under the device should be avoided since
these will couple noise onto the die. The analog ground plane
should be allowed to run under the AD7661 to avoid noise
coupling. Fast switching signals like
CNVST
or clocks should be
shielded with digital ground to avoid radiating noise to other
sections of the board, and should never run near analog signal
paths. Crossover of digital and analog signals should be avoided.
Traces on different but close layers of the board should run at
right angles to each other. This will reduce the effect of crosstalk
through the board.
The power supply lines to the AD7661 should use as large a
trace as possible to provide low impedance paths and reduce the
effect of glitches on the power supply lines. Good decoupling is
also important to lower the supply’s impedance presented to the
AD7661 and to reduce the magnitude of the supply spikes.
Decoupling ceramic capacitors, typically 100 nF, should be
placed on each power supply pin—AVDD, DVDD, and
OVDD—close to, and ideally right up against these pins and
their corresponding ground pins. Additionally, low ESR 10 µF
capacitors should be located near the ADC to further reduce
low frequency ripple.
The DVDD supply of the AD7661 can be a separate supply or
can come from the analog supply AVDD or the digital interface
supply OVDD. When the system digital supply is noisy or when
fast switching digital signals are present, if no separate supply is
available, the user should connect DVDD to AVDD through an
RC filter (see Figure 26) and the system supply to OVDD and
the remaining digital circuitry. When DVDD is powered from
the system supply, it is useful to insert a bead to further reduce
high frequency spikes.
The AD7661 has five different ground pins: INGND, REFGND,
AGND, DGND, and OGND. INGND is used to sense the analog
input signal. REFGND senses the reference voltage and, because
it carries pulsed currents, should be a low impedance return to
the reference. AGND is the ground to which most internal ADC
analog signals are referenced; it must be connected with the
least resistance to the analog ground plane. DGND must be tied
to the analog or digital ground plane depending on the
configuration. OGND is connected to the digital system
ground.
EVALUATING THE AD7661’S PERFORMANCE
A recommended layout for the AD7661 is outlined in the
EVAL-AD7661 evaluation board for the AD7661. The
evaluation board package includes a fully assembled and tested
evaluation board, documentation, and software for controlling
the board from a PC via the EVAL-CONTROL BRD2.
