AD627

–14–

REV. A

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

Ω), the offset current of the

input stage will cause dc errors compatible with the input offset

voltage of the amplifier.

LOAD

TO POWER

SUPPLY

GROUND

–INPUT

+INPUT

AD627

REFERENCE

Figure 41a. Ground Returns for Bias Currents with Trans-

former Coupled Inputs

LOAD

TO POWER

SUPPLY

GROUND

–INPUT

+INPUT

AD627

REFERENCE

Figure 41b. Ground Returns for Bias Currents with Ther-

mocouple Inputs

LOAD

TO POWER

SUPPLY

GROUND

–INPUT

+INPUT

AD627

REFERENCE

100k

100k

Figure 41c. Ground Returns for Bias Currents with AC

Coupled Inputs

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

Ω

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.

DIGITAL POWER SUPPLY

0.1 F

AGND DGND

AD7892-2

ADC

12

AGND

PROCESSOR

0.1 F

0.1 F

0.1 F

AD627

ANALOG POWER SUPPLY

+5V

–5V

GND

GND

+5V

Figure 42. Optimal Grounding Practice for a Bipolar Supply Environment with Separate Analog and Digital Supplies