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SBOA103C
Jan 2006
PGA309 Quick Start
System Reference Guide
92
4 10
3
−
⋅
0.5
R1
2.5
R2
+
100
⋅
Equation 1: Vin = 0.5V, Iout = 4mA
R1 12500.
R2
R2 62500.
−
⋅
Equation 2: solve Equation 1 for R1
20 10
3
−
⋅
4.5
R1
2.5
R2
+
100
⋅
Equation 3: Vin=4.5V, Iout = 20mA
Substitute equation 2 into
equation 3 and solve for R2
R2 125 10
3
⋅
Substitute the value of R2 into
equation 2 to solve for R1
R1 25 10
3
⋅
For the spreadsheet you need to break the function into "extra gain" and "extra
offset" as shown below.
Iout
100
R1
Vin
⋅
100
R2
Vref
⋅
+
Iout
100
25 10
3
⋅
Vin
⋅
100
125 10
3
⋅
2.5
⋅
+
Iout
ExtraGain
(
) Vin
⋅
ExtraOffset
(
) 2.5
⋅
+
Iout
4
10
3
−
×
(
)
Vin
⋅
800
10
6
−
×
(
)
2.5
⋅
+
This calculation shows how
the example configuration
shown on the previous page
can be used to take the 0.5V
to 4.5V output of the PGA309
and re-scale it to 4mA to
20mA.
This equation must be broken
down into an “ExtraGain” and
“ExtraOffset” factor for the
spreadsheet.
PGA309 With Output Scaling
(4mA to 20mA)