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 (V
REF
= 2 V or
10 V usually), to determine the resistor values required.
R
P
= 180
Ω
(1/X – 4.5)
R
F
= 500
Ω
[(V
REF
/ 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.
R
P
= 180
Ω
(1/0.05 – 4.5) = 2.79 k
Ω
R
F
= 500
Ω
[(2 V / 1.22) – 0.18 – 0.82
×
0.05][1/0.05 – 4.5]
= 10.99 k
Ω
These can be rounded down to more convenient values of
2.5 kΩ and 9.76 kΩ. In general, if the value of R
P
is rounded
down slightly, the value of R
F
should be rounded down propor-
tionately and vice versa. This helps to keep the adjustment range
symmetrical.
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.
R
T
= 1.8 k
Ω
((1 – X)/X)
R
S
= 9 k
Ω
[1 – 0.2 (1 + X)( 1 – X )] / 2X
These formulae take into account the
±
10% absolute resistor
tolerance of the internal span resistors and ensures a minimum
adjustment range of the span. For example, choosing the adjust-
ment range to be
±
2%, or 0.02 gives:
R
T
= 1.8 k
Ω
((1 – 0.02) / 0.02) = 88.2 k
Ω
.
R
S
= 9 k
Ω
[1 – 0.2 (1 + 0.02)( 1 – 0.02 )] / (2
×
0.02) =
175.5 k
Ω
These values can be rounded up to the more convenient values
of 100 kΩ and 198 kΩ. In general, if R
T
is rounded up, then the
value of R
S
should be rounded up proportionally and vice versa.
ADJUSTING SPAN FOR 2 V FS
Figure 6. Optional 4 mA Zero Adjustment
ADJUSTING SPAN FOR 10 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:
R
A
= 2
×
X
×
R
B
where R
B
is greater than 5 K
R
C
= (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 R
B
= 10 K, then R
A
= 2(.02)
×
10 kΩ = 400
Ω
R
C
= (2.75 kΩ
×
0.02)/ (1 – 0.275
×
(0.02)) = 55.3
Ω
The value of R
C
can be rounded to the more convenient values
of 49.9
Ω.
In general, if R
A
is rounded up, then R
C
should be
rounded up proportionally and vice versa; rounding up will in-
crease the range of adjustment.
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 R
S
and R
T
), choose
–8–
REV. A
AD [ ANALOG DEVICES ]
