Tacho are general DC/AC generators which provides a fixed voltage on the terminals, based on the shaft speed connected to the armature. It multiplies by back EMF constant with the Speed. Tacho having 5mV/RPM gives 10V at 2000RPM and -10V on the other direction with same speed.
Available solution on the fly is adding an EXT ADC which can read negative input voltages. In needs additional circuitry like external reference, Supply source and Filtering. EXT ADC will be interfaced on Parallel Port or Serial communication which forces additional SW development.
Overview of Proposed Solution
On Mixed signal ECU, having different sources of supply induces noise on sensitive signals and on analog signals which are transduced from physical parameters have an impact over the results. To avoid these noise issues and eliminate EXT ADC as described, proposed solution is shown in figure 3.
It’s a 5 staged circuit with 3 chips and includes a DC/DC converter for OpAmp Power supply.
Circuit Design Considerations
To interface the sensor data to ADC of microcontroller, following data shall be taken into consideration.
- Range of the Signal - To Add gain
- Max Voltage of the Signal - To select the OpAmp
- Accuracy of the Signal - To select the OpAmp
- Noise level on the Signal - To Design Filter
Steps involved in Design
A RC Low pass filter is the best discrete filter to eliminate the noise, and which is economical despite cost, easy to construct, easy in stocking and easy to modify for change in parameters.
Filter Design Steps
- Fix the Capacitor value in µF
- Calculate the Resistor Value from
R = 1/ (2 x π x F x C)
R = Resistance in Ohms
F = Noise Frequency level to be suppressed in Hz
C = Capacitance in Farads
Figure 4 are the results of filter simulation. Filter is designed to allow a frequency less than 1kHz and above will be suppressed.
All discrete components shall be used with least tolerance (1%) for best performance.
Most of the Controller ADC are capable to take input voltages up to 5V/3.3V or 2.5V. To match the input signal level to VAdc Max, signal shall be reduced with gain factor. Gain of the circuit shall be calculated to reduce the given max signal voltage to Max ADC Input. Dual OpAmp which is configured both in inverted mode can reduce additional components and reduces the gain as needed.
Gain = Vmax of ADC/ Vmax of Input Signal
Gain = Ri/Rf
Rf = Feedback Resistor
Ri = Input Resistor
DC-DC Regulator Selection
As Operational amplifier requires negative supply sources to convert the signal positive and negative, on board non isolated DC-DC inverted regulator is needed. As the circuit is having single OpAmp it won’t take much current, hence designing a flyback, buck-boost is much space occupied and using isolated supplies is expensive. A simple CMOS capacitor switch inverter supply is better in performance and pin compatible.
Signal2 is nothing but the reduced gain multiplied input signal. Signal2 can be compared to 0V and output of comparator can be used to select the analog switch. Comparator with a pull up provides a digital LOW or HIGH on input signal from +Ve to -Ve.
Switching the +Ve Signal Always
Signal1 is inverted and reduced Gain, Signal2 is inverted of Signal1. Both signals are connected to SPDT analog switch which is controlled with output of comparator to select signal as output. On input signal -Ve Comparator provides HIGH as output and NO of the analog switch connects to COM, on +Ve LOW as output and NC of the analog switch connects to COM.