AD625

REV. D

–11–

Offset voltage and offset voltage drift each have two compo-

nents: input and output. Input offset is that component of offset

that is generated at the input stage. Measured at the output it is

directly proportional to gain, i.e., input offset as measured at the

output at G = 100 is 100 times greater than that measured at

G = 1. Output offset is generated at the output and is constant

for all gains.

The input offset and drift are multiplied by the gain, while the

output terms are independent of gain, therefore, input errors

dominate at high gains and output errors dominate at low gains.

The output offset voltage (and drift) is normally specified at

G = 1 (where input effects are insignificant), while input offset

(and drift) is given at a high gain (where output effects are negli-

gible). All input-related parameters are specified referred to the

input (RTI) which is to say that the effect on the output is “G”

times larger. Offset voltage vs. power supply is also specified as

an RTI error.

By separating these errors, one can evaluate the total error inde-

pendent of the gain. For a given gain, both errors can be com-

bined to give a total error referred to the input (RTI) or output

(RTO) by the following formula:

Total Error RTI = input error + (output error/gain)

Total Error RTO = (Gain

× input error) + output error

The AD625 provides for both input and output offset voltage

adjustment. This simplifies nulling in very high precision appli-

cations and minimizes offset voltage effects in switched gain

applications. In such applications the input offset is adjusted

first at the highest programmed gain, then the output offset is

adjusted at G = 1. If only a single null is desired, the input offset

null should be used. The most additional drift when using only

the input offset null is 0.9

µV/°C, RTO.

COMMON-MODE REJECTION

Common-mode rejection is a measure of the change in output

voltage when both inputs are changed by equal amounts. These

specifications are usually given for a full-range input voltage

change and a specified source imbalance.

In an instrumentation amplifier, degradation of common-mode

rejection is caused by a differential phase shift due to differences

in distributed stray capacitances. In many applications shielded

cables are used to minimize noise. This technique can create

AD625

SENSE

REFERENCE

AD711

+INPUT

–INPUT

100

Figure 32. Common-Mode Shield Driver

common-mode rejection errors unless the shield is properly

driven. Figures 32 and 33 show active data guards which are

configured to improve ac common-mode rejection by “boot-

strapping” the capacitances of the input cabling, thus minimiz-

ing differential phase shift.

AD625

AD712

100

100

SENSE

REFERENCE

–INPUT

+INPUT

Figure 33. Differential Shield Driver

GROUNDING

In order to isolate low level analog signals from a noisy digital

environment, many data-acquisition components have two or

more ground pins. These grounds must eventually be tied to-

gether at one point. It would be convenient to use a single

ground line, however, current through ground wires and pc runs

of the circuit card can cause hundreds of millivolts of error.

Therefore, separate ground returns should be provided to mini-

mize the current flow from the sensitive points to the system

ground (see Figure 34). Since the AD625 output voltage is

developed with respect to the potential on the reference termi-

nal, it can solve many grounding problems.

AD625

AD7502

AD583

SAMPLE

AND

HOLD

HOLD

CAP

INPUT

SIGNAL

STATUS

ANALOG

OUT

DIGITAL

COMMON

ANALOG POWER

GROUND

AD574A

A/D

CONVERTER

Figure 34. Basic Grounding Practice for a Data Acquisition System