AD693

REV. A

–12–

Error it is necessary to add an error of only (5 – 2)

× V

error budget. Note that span error may by reduced to zero with

the span trim, leaving only the offset and nonlinearity of the

AD693.

EXAMPLE I

The AD693 is configured as a 4-20mA loop powered transmitter

with a 60 mV FS input. The inputs are driven by a differential

voltage at 2 V common mode with a 300

Ω balanced source

resistance. A 24 V loop supply is used with a 500

Ω metering

resistance. (See Table IV below.)

Trimming the offset and span for your application will remove

all span and offset errors except the nonlinearity of the AD693.

Table IV. Example 1

OFFSET ERRORS

Already included in the TUE spec .

0.0

µV

PSRR

PSRR = 5.6

µV/V; (|24 V – 24 V| + [| 500 Ω – 250 Ω × 4 mA]) × 5.6 µV/V =5.6 µV

Ω I

CMRR

CMRR = 30

µV/V; |2 V –3.1 V| × 30 µV/V =

33.0

µV

IOS

Ω; 300 Ω × 3 nA =

0.9

µV

Total Additional Error at 4 mA

39.5

As % of full scale; (39.5

µV × 0.2666 A/V)/20 mA × 100% =

0.053 % of FS

SPAN ERRORS

Already included in the TUE spec

0.0

µV

PSRR = 5.6

µV/V; (|500 Ω – 250 Ω| × 16 mA) × 5.6 µV/V =

22.4

µV

Ω, I

39.6

µV

Ω × 2 × 20 nA

12.0

µV

from Figure 2)

Already included in the TUE

0.0

µV

Total Additional Span Error at Full Scale

74.0

µV + 74.0 µV =

113.5

µV

As % of Full Scale; (113.5

µV × 0.2666A V)/20 mA × 100% =

0.151% of FS

0.651% of FS

and an input common-mode voltage of 3.1 V. The expressions

below calculate errors due to deviations from these nominal

conditions.

The total error at zero consists only of offset errors. The total

error at full scale consists of the offset errors plus the span

errors. Adding the above errors in this manner may result in an

error as large as 0.8% of full scale, however, as a rule, the

AD693 performs better as the span and offset errors do not tend

to add worst case. The specification “Total Unadjusted Error,”

(TUE), reflects this and gives the maximum error as a % of full

scale for any point in the transfer function when the device is

operated in one of its preset spans, with no external trims. The

TUE is less than the error you would get by adding the span

and offset errors worst case.

Thus, an alternative way of calculating the total error is to start

with the TUE and add to it those errors that result from

operation of the AD693 with a load resistance, loop supply

voltage, or common-mode input voltage different than specified.

(See Example 1 below.)

ERROR BUDGET FOR SPANS LESS THAN 30 mV

An accommodation must be made to include the input voltage

offset of the signal amplifier when the span is adjusted to less

than 30 mV. The TUE and the Zero Current Error include the

input offset voltage contribution of the signal amplifier in a gain

of 2. As the input offset voltage is multiplied by the gain of the

signal amplifier, one must include the additional error when the

signal amplifier is set to gains greater than 2.

For example, the 300K span thermocouple application discussed

previously requires a 12.207 mV input span; the signal amplifier

must be adjusted to a gain of approximately 5. The loop trans-

conductance is now 1.333 A/V, (5

× 0.2666 A/V). Calculate the

total error by substituting the new values for the transconductance

and span into the equations in Table III as was done in Example

× V

2

× V

E-20A

20-Terminal Leadless Chip Carrier

D-20

20-Lead Side Brazed Ceramic DIP

Q-20

20-Lead Cerdip

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).