CPC5712
3.3.2 Verify Resistor Selection
Using the selected standard 1% resistor values, and
back calculating to the threshold voltages produces
these results:
Once the resistor values are chosen it is necessary to
back calculate the nominal detector thresholds.
• VL1 = 3.00280V
• VH1 = 5.00467V
• VL2 = 11.9662V
• VH2 = 14.9690V
To do this the following equations are provided for two
variables:
RΣ = RIN1 + RIN2 + RDIFF
As can be seen, the error from using standard value
resistors is less than 0.1% for VL1 and VH1 and is less
than 0.3% for VL2 and VH2.
RREF = R1 + R2 + R3 + R4 + R5
where R∑ is the sum of the resistive interface network
and RREF is the sum of the resistor divider network on
the reference voltage output.
3.4 High Voltage Detection Designs
Designs that require higher detection levels greater
than approximately 17V will necessitate a different
voltage divider ratio to accommodate the operational
range of the CPC5712’s internal circuitry. Changes to
the input resistor divider network are restricted to the
high impedance resistors from the tip and ring leads to
the IN+ and IN- inputs. Changing the differential input
resistor value from 806kΩ is not recommended as this
will introduce offset errors. The degree of offset error
caused by changing this component’s value is not
measured and therefore not calculable.
The following values are also needed to perform the
threshold calculations. They are:
• VREF = 1.5V
• RIN1 = RIN2 = 2 x 10MΩ = 20MΩ
• RDIFF = 806kΩ
• G=2.5 (Single ended gain of input amplifier)
which gives:
• R∑ = 40.806MΩ and
• RREF = 270.1kΩ
The threshold equations are:
The design procedure for higher voltage detect levels
is the same as presented above. Remember to begin
with the equations shown in Section 2.5 “Detector
Threshold Operation” on page 6 and use the
updated value for the “A” term based on the new input
resistor values.
VREF ⋅ RΣ ⋅ R1
G ⋅ RDIFF ⋅ RREF
----------------------------------------
1.
2.
3.
4.
VL1
VH1
VL2
VH2
=
=
=
=
VREF ⋅ RΣ ⋅ (R1 + R2)
----------------------------------------------------
G ⋅ RDIFF ⋅ RREF
VREF ⋅ RΣ ⋅ (R1 + R2 + R3)
------------------------------------------------------------------
G ⋅ RDIFF ⋅ RREF
VREF ⋅ RΣ ⋅ (R1 + R2 + R3 + R4)
-------------------------------------------------------------------------------
G ⋅ RDIFF ⋅ RREF
R01
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