AD627B Analog Devices, AD627B Datasheet - Page 14

• Manufacturer Part Number: AD627B
• Description: Micropower/ Single and Dual Supply Rail-to-Rail Instrumentation Amplifier
• Manufacturer: Analog Devices

AD627
In dc-coupled resistive bridge applications, providing this path
is generally not necessary as the bias current simply flows from
the bridge supply, through the bridge and into the amplifier.
However, if the impedance that the two inputs see are large, and
differ by a large amount (>10 k ), the offset current of the
input stage will cause dc errors compatible with the input offset
voltage of the amplifier.
Figure 41a. Ground Returns for Bias Currents with Trans-
former Coupled Inputs
Figure 41b. Ground Returns for Bias Currents with Ther-
mocouple Inputs
Figure 41c. Ground Returns for Bias Currents with AC
Coupled Inputs
Figure 42. Optimal Grounding Practice for a Bipolar Supply Environment with Separate Analog and Digital Supplies
100k
–INPUT
+INPUT
–INPUT
+INPUT
100k
–INPUT
+INPUT
R
R
G
G
R
G
AD627
AD627
–V
–V
+V
AD627
+V
S
S
–V
S
S
+V
S
REFERENCE
REFERENCE
S
REFERENCE
LOAD
LOAD
AD627
LOAD
0.1 F
TO POWER
SUPPLY
GROUND
TO POWER
SUPPLY
GROUND
V
V
OUT
OUT
0.1 F
V
TO POWER
SUPPLY
GROUND
OUT
ANALOG POWER SUPPLY
+5V
–5V
V
V
–14–
IN1
IN2
GND
V
Layout and Grounding
The use of ground planes is recommended to minimize the
impedance of ground returns (and hence the size of dc errors).
In order to isolate low level analog signals from a noisy digital
environment, many data-acquisition components have separate
analog and digital ground returns (Figure 42). All ground pins
from mixed signal components such as analog-to-digital converters
should be returned through the “high quality” analog ground
plane. Digital ground lines of mixed signal components should
also be returned through the analog ground plane. This may
seem to break the rule of keeping analog and digital grounds
separate. However, in general, there is also a requirement to
keep the voltage difference between digital and analog grounds
on a converter as small as possible (typically <0.3 V). The
increased noise, caused by the converter’s digital return currents
flowing through the analog ground plane, will generally be negli-
gible. Maximum isolation between analog and digital is achieved
by connecting the ground planes back at the supplies.
If there is only a single power supply available, it must be shared
by both digital and analog circuitry. Figure 43 shows the how to
minimize interference between the digital and analog circuitry.
As in the previous case, separate analog and digital ground
planes should be used (reasonably thick traces can be used as an
alternative to a digital ground plane). These ground planes
should be connected at the power supply’s ground pin. Separate
traces (or power planes) should be run from the power supply to
the supply pins of the digital and analog circuits. Ideally each
device should have its own power supply trace, but these can be
shared by a number of devices as long as a single trace is not
used to route current to both digital and analog circuitry.
INPUT PROTECTION
As shown in the simplified schematic (Figure 32), both the
inverting and noninverting inputs are clamped to the positive
and negative supplies by ESD diodes. In addition to this a 2 k
series resistor on each input provides current limiting in the
event of an overvoltage. These ESD diodes can tolerate a maxi-
mum continuous current of 10 mA. So an overvoltage, (that is
the amount by which input voltage exceeds the supply voltage),
of 20 V can be tolerated. This is true for all gains, and for
power on and off. This last case is particularly important since
the signal source and amplifier may be powered separately.
If the overvoltage is expected to exceed 20 V, additional external
series resistors current limiting resistors should be used to keep
the diode current to below 10 mA.
DD
ADC
0.1 F
AGND DGND
AD7892-2
12
DIGITAL POWER SUPPLY
AGND
GND
PROCESSOR
0.1 F
+5V
V
DD
REV. A