Mantracourt Electronics Limited
– ICA User Manual
11
Chapter 3 Calculating Offset and Gain Resistor Values
Calculating the offset resistor values
The ICA1H and 2H can be used in a pseudo ‘bipolar ±’ mode by fitting a resistor ‘R
offs
’ as shown in Figure 2.2. This will
shift the output voltage to a known level (V
offs
) when the input is zero (0 mV/V) allowing both tension and compression
of the load cell to be measured.
Use the following formula for the ICA1H and 2H to calculate the value of R
offs
in kΩ.
𝑅
𝑜𝑓𝑓𝑠
=
148
(𝑉
𝑜𝑓𝑓𝑠
− 0.1)
The gain will also need to be changed to account for smaller changes in output resulting from a given input change.
This means that the total mV/V will change:
For the ICA1H:
𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 mV/V =
10V
𝑏𝑖𝑝𝑜𝑙𝑎𝑟 𝑜𝑢𝑡𝑝𝑢𝑡 𝑐ℎ𝑎𝑛𝑔𝑒
× 𝑙𝑜𝑎𝑑 𝑐𝑒𝑙𝑙 mV/V
For the ICA2H:
𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 mV/V =
5V
𝑏𝑖𝑝𝑜𝑙𝑎𝑟 𝑜𝑢𝑡𝑝𝑢𝑡 𝑐ℎ𝑎𝑛𝑔𝑒
× 𝑙𝑜𝑎𝑑 𝑐𝑒𝑙𝑙 mV/V
Calculating the gain resistor values
The ICA range of strain gauge amplifiers are supplied un-calibrated but optimised for a sensitivity of 2.5 mV/V. To
accommodate other sensitivities the gain resistor ‘R
gain
’ shown in the connection diagrams above can be changed
according to the following formulae.
N.B. a high quality component e.g. 1 % 10-15 ppm/°C metal film resistor should be used for optimum performance. It
may be necessary to use a value from the less common E96 series to optimise the trim range.
𝐺𝑎𝑖𝑛 = (
Gain constant
𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑚𝑉/𝑉
) 𝑅
𝑔𝑎𝑖𝑛
= (
Resistor constant
(𝐺𝑎𝑖𝑛 − 1)
) Ω
Where:
Model
Output range
Gain constant
Resistor constant
ICA1H
0.1 to 10.1 V
372
40000
ICA2H
0.1 to 5.1 V
186
40000
ICA3H
± 10 V
372
40000
ICA4H
4 to 20 mA
160
63200
ICA6H
± 10 V
372
40000
The ICA4H has two surface mount gain resistors. These do not need to be removed if the gain is to be increased (a
lower mV/V input level). However, they need to be taken into account and an extra calculation stage is required:
𝑅 = (
1
(1/𝑅
𝑔𝑎𝑖𝑛
) − 0.001
) Ω
