AD694 查看數據表（PDF） - Analog Devices

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AD694

4–20 mA Transmitter

Analog Devices 

AD694

Figure 6 will give an approximately linear adjustment of the

4 mA offset within fixed limits. To find the proper resistor val-

ues, first select X, the desired range of adjustment as a fraction

of 4 mA. Substitute this value in the appropriate formula below

along with the chosen reference output voltage (VREF = 2 V or

10 V usually), to determine the resistor values required.

RP = 180 Ω (1/X – 4.5)

RF = 500 Ω [(VREF / 1.22 V) – 0.18 – 0.82X][1/X – 4.5]

These formulae take into account the ± 10% internal resistor

tolerance and ensure a minimum adjustment range for the 4 mA

offset. For example, assume the 2 V reference option has been

selected. Choosing X = 0.05, gives an adjustment range of ± 5%

of the 4 mA offset.

RP = 180 Ω (1/0.05 – 4.5) = 2.79 kΩ

RF = 500 Ω [(2 V / 1.22) – 0.18 – 0.82 × 0.05][1/0.05 – 4.5]

= 10.99 kΩ

These can be rounded down to more convenient values of

2.5 kΩ and 9.76 kΩ. In general, if the value of RP is rounded

down slightly, the value of RF should be rounded down propor-

tionately, and vice versa. This helps to keep the adjustment

range symmetrical.

Figure 6. Optional 4 mA Zero Adjustment

ADJUSTING SPAN FOR 10 V FS

When the AD694 is configured with a 10 V input full-scale the

span maybe adjusted using the network shown in Figure 7. This

scheme allows an approximately linear adjustment of the span

above or below the nominal value. The span adjustment does

not interact with the 4 mA offset. To select RS and RT, choose

Figure 7. Span Adjustment, 10 V Full Scale

X, the desired adjustment range as a fraction of the span. Sub-

stitute this value in the appropriate formula below.

RT = 1.8 kΩ ((1 – X)/X)

RS = 9 kΩ [1 – 0.2 (1 + X)( 1 – X )] / 2X

These formulae take into account the ± 10% absolute resistor

tolerance of the internal span resistors and ensure a minimum

adjustment range of the span. For example, choosing the adjust-

ment range to be ± 2%, or 0.02 gives:

RT = 1.8 kΩ ((1 – 0.02) / 0.02) = 88.2 kΩ.

RS = 9 kΩ [1 – 0.2 (1 + 0.02)( 1 – 0.02 )] / (2 × 0.02) =

175.5 kΩ

These values can be rounded up to the more convenient values

of 100 kΩ and 198 kΩ. In general, if RT is rounded up, then the

value of RS should be rounded up proportionally, and vice versa.

ADJUSTING SPAN FOR 2 V FS

The precalibrated 2 V full-scale range requires a different ad-

justment scheme due to the single supply nature of the AD694.

Figure 8 shows an adjustment scheme that allows an approxi-

mately linear adjustment of the 2 V span plus or minus the

nominal value. The span adjustment does not affect the value of

the 4 mA offset current.

To find the proper resistor values first select X, the desired

range of adjustment as a fraction of the output span. Substitute

this value into the following formulae:

RA = 2 × X × RB where RB is greater than 5 K

RC = (2.75 kΩ × X)/(1 – 0.275X)

These formulae take into account the ± 10% absolute tolerance

of the internal span resistors and ensure a minimum adjustment

range.

For example, choosing the adjustment range to be ± 320 µA of

FS or, ± 2%, let X = 0.02. Thus:

Setting RB = 10 K, then RA = 2(.02) × 10 kΩ = 400 Ω

RC = (2.75 kΩ × 0.02)/ (1 – 0.275 × (0.02)) = 55.3 Ω

The value of RC can be rounded to the more convenient value of

49.9 Ω. In general, if RA is rounded up, then RC should be

rounded up proportionally, and vice versa; rounding up will in-

crease the range of adjustment.

–8–

REV. B

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