User`s manual

PK2100
3-10 s I/O Configurations
x
1
=
1
a
y
- a
0
.
If the negative input x
2
is tied to ground, the equation becomes
y = a
1
× (x
1
+ a
0
)
or, solving for x
1
,
This equation returns the input voltage, given the output. The function
up_higain(1) returns this value (with Jumper H7 removed). If the
output of the operational amplifier is out of range (x
1
input above 1volt, y
output above 10volts), the value of a direct measurement of x
1
is returned.
This is less accurate by a factor equal to the gain ratio. Solving this
equation for (x
1
- x
2
) in terms of y and x
1
yields
() .1
1
1
01
1
1
1
21
b
a
ax
b
a
b
y
xx
×−×








−−=−
The function up_higain(2) returns this value (with Jumper H7 re-
moved). This is the difference between inputs x
1
and x
2
in terms of y and
x
1
, both of which can be measured directly. The accuracy with which the
difference voltage is measured is approximately 10 times greater than for
the regular universal input channels: 1 millivolt instead of 10millivolts.
Change resistor R5, and possibly R11, to change the gain of the high-gain
input. These 47-kΩ resistors are factory-installed for a gain of 10 (or 11 if
Jumper H7 is removed). A smaller resistor will increase the gain. For
example, changing both the R5 and R11 resistors to 10kΩ will increase
the gain to 47 (or 48 with Jumper H7 removed). When differential inputs
are desired, it is preferable to operate with H7 removed since the gain
difference between the positive and negative inputs will be exactly 1 and
will not depend on a balance between resistors, making the output 5 volts
when both differential inputs are 5 volts. As the gain is increased, it
becomes necessary to use an operational amplifier with a more stable
offset voltage than the LM324, which has considerable drift over tempera-
ture. The Linear Technology LM1014 is suitable for gains up to 100 or
more. The negative input has a low input impedance compared to the
positive input when H7 is removed. If R5 is decreased to increase the
gain, this impedance becomes even lower. When a bridge is used, the
finite impedance of the negative input will change the gain slightly.