Waveshaping circuit

Abstract: A waveshaping circuit for producing a bus output voltage signal having a substantially sinusoidal rising transition from a low voltage level to a high voltage level in response to the rising edge of a data input signal and a substantially sinusoidal falling transition from the high voltage level to the low voltage level in response to the falling edge of said data input signal uses AC coupling to control the waveshaping. This allows the circuit to operate with a large ground offset voltage difference between circuit ground and bus ground. An exponential current source (26) provides current to a regulator bus driver (28) which charges and discharges a capacitor (30) in response to the rising edge or falling edge on the data input signal. The current increases exponentially in response to the rising or falling edge on the data input signal and then decreases exponentially when the bus output signal reaches one-half of its intended full voltage swing or falls below one-half the full voltage swing to produce respectively the sinusoidal rising and falling voltages at the bus output. The regulator bus driver (28) regulates the voltage at the bus output (16) when the voltage reaches the intended full voltage. In practice, the regulator bus driver (28) creates it own regulated high voltage level over supply variations from 9-16 volts.

Description: This invention relates to a waveshaping circuit. A number of circuits have been developed for the purpose of driving a serial data bus without distorting the data contained in the logic input signals while eliminating high frequency noise components associated with the substantially instantaneous rise and fall times of the leading and trailing edges of the logic signals. One such circuit is disclosed in US-A-4,593,206 and provides symmetrical rising and falling edges which have a predetermined slope. This circuit, however, does not meet the stringent requirements of noise reduction, ground offset and propagation delay of anticipated motor vehicle communication buses. EP-A-0250036 and US-A-4216393 disclose waveshaping circuits in accordance with the preamble of claim 1. With the foregoing in mind, it is an object of the present invention to provide an improved waveshaping circuit. A waveshaping circuit in accordance with the present invention is characterised by the features specified in the characterising portion of claim 1. The invention can provide a circuit which generates only a relatively small amount of noise in the AM broadcasting band. The invention can also provide a waveshaping circuit which operates with a delay of less than twenty microseconds from the time the input goes high to the time the bus reaches a recognised high state. According to another aspect of the present invention, there is provided a bus driver as defined in claim 8. According to another aspect of the present invention, there is provided a waveshaping circuit for producing a bus output voltage which is substantially insensitive to ground offsets as defined in claim 9. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a block diagram of an embodiment of waveshaping circuit; Figure 2 shows various waveforms generated in the waveshaping circuit of Figure 1; Figure 3 is a detailed schematic diagram of an exponential current source of the embodiment shown in Figure 1; Figure 4 shows various waveforms associated with the current source of Figure 3; Figure 5 is a detailed schematic diagram of a regulator bus driver of the embodiment of Figure 1; and Figure 6 shows various waveforms associated with the regulator bus driver of Figure 5. Referring to Figure 1, a logic level drive signal designated DATAIN is applied as one input to an Exclusive OR gate 10 which controls the gate electrode of a NMOS transistor 12. The other input to the gate 10 is designated LOAD and is obtained from the output of a comparator 14. The comparator 14 switches when the output voltage on the bus 16 reaches one half of its maximum intended value. The switch point is established by the voltage divider resistors 18 and 20. The transistor 12 is connected in series with and controls a current source 22 which provides 95 microamps of current. A current source 24 which provides 5 microamps of current is connected in parallel with the series connected transistor 12 and source 22. An exponential current source 26 is serially connected to the parallel branches containing the sources 22 and 24 and thus receives 5 microamps when the transistor 12 is off and 100 microamps when the transistor 12 is on. The exponential current source 26 supplies current to a regulator/bus driver block 28 which also responds to DATAIN. The driver block 28 integrates the current from the source 26 via a capacitor 30, and provides an output voltage waveform on the bus 16. The output designated BUS has substantially sinusoidal leading and trailing edges in response to the substantially instantaneous rising and falling edges of the DATAIN logic drive signal as shown in Figure 2. It will be noted in Figure 2 that when DATAIN switches high while LOAD is low, the output of the gate 10 goes high turning on the transistor 12. Thus, the input to the exponential current source 26 is switched from 5 microamps to 100 microamps and the output of the current source 26 begins to rise exponentially. The current out of the source 26 is integrated by the capacitor 30 to produce the rising edge of the voltage waveform BUS (Figure 2e). When the output of the driver 28 reaches midsupply, in the example shown 3.5 volts, the output of comparator 14 switches high. With both inputs to the gate 10 being high the output goes low turning off transistor 12. The input to the source 26 drops to 5 microamps and the output of the source 26 starts to decrease exponentially toward 5 microamps. The integration of the output of the source 26 continues but at a reduced rate, creating a voltage waveform on BUS which is shaped like a sinusoid. The same shaping occurs on the falling edge of DATAIN producing a falling edge shaped like a sinusoid. These sinusoidal rising and falling edges generate relatively small amounts of harmonic noise. Referring now to Figure 3 the exponential current source 26 is shown to include NMOS transistors 22 and 24 which function as the current sources 22 and 24 of Figure 1. PMOS transistors 42 and 44 function as a current mirror. The NMOS transistor 12 acts as a switch as previously indicated. A diode string 31 includes diode-connected NPN transistors 32-38 connected between the transistor 42 and ground. A two-stage operational amplifier 46, comprises a first stage including NPN input transistors 48 and 50, PMOS load transistors 56 and 58, and current source 52. The second stage includes PMOS drive transistor 66 and current source load transistor 64. An integrating capacitor 60 and a zero cancelling resistor 62 are connected between the collector of transistor 50 and the base of transistor 48. A linear-to-exponential converter generally designated 67 comprises a transistor 68 and a diode string 70 which includes diode-connected NPN transistors 72-76. The transistor 68 converts the linearly changing voltage designated VB, at its base, into an exponentially changing current at its collector. A current mirror comprising PMOS transistors 80 and 82 is connected to the transistor 68. A bias string reference comprising an NMOS transistor 84 converts the current from PMOS transistor 82 into a reference voltage for biasing other NMOS transistors in the bus driver 28 connected to conductor 86. A current source 88 is connected to a diode-connected NMOS transistor 90 and sets up the bias voltages for the current sources 22, 24,52 and 64. It will be apparent that other biasing means may be provided as required, as appreciated by the skilled person. The exponential current source 26 operates in the following manner, with reference to the circuit of Figure 3 and the waveforms of Figure 4. When INPUT is low, the transistor 12 is turned off. Therefore the current sourced from transistor 44 is limited to the current being sunk by transistor 24 which in this example is 5 microamps. The current in transistor 44 is reflected in transistor 42 and flows through diode string 31 creating a reference voltage at node VA. In a steady state condition, the voltage at node VA is recreated at node VB by the two-stage operational amplifier 46. The voltage at node VB is converted back into a current by the linear-to-exponential converter 67. The current from the linear-to-exponential converter 67 is sourced from transistor 82 which in turn is reflected in transistor 80. The current from transistor 80 is converted to a bias reference voltage designated EXPBIAS by transistor 84 and applied to the regulator/bus driver via conductor 86. When INPUT goes high (Figure 4a), the transistor 12 turns on and the current sourced from transistor 44 is the current sunk by transistors 22 and 24 which totals 100 microamps. This current is reflected by transistor 42 into the diode string 31 which causes the voltage at node VA to change immediately to a higher value (Figures 4b and 4c). The amplifier 46 tries to change the voltage at node VB to match the voltage at node VA. However, the voltage change at node VB is controlled by the discharging of capacitor 60. Since node voltage VB is lower than node voltage VA, the transistor 50 conducts all of the current being sunk by transistor 52. The current from transistor 50 is used to discharge capacitor 60. The rate of discharge is controlled by the size of the capacitor 60 and the size of the current source transistor 52. This linearly discharging capacitor creates a linearly increasing voltage at node VB (Figure 4d). The linearly increasing voltage at node VB, when applied to the linear-to-exponential converter 67 creates an exponentially increasing current (Figure 4e). The linear-to-exponential converter 67 operates in the following manner. The voltage at node VB induces a current in transistor 68 which is given by: I = Is * exp VB/4*Vt, where I is the current flow in transistor 68, Is is the saturation current of the transistor 68 which is a constant for a given process, VB is the node voltage VB, Vt is the thermal voltage which is a constant at a given temperature. This current is sourced from the collector of transistor 68. A linearly increasing voltage at node VB causes an exponentially increasing current at the collector of transistor 68. The current from the collector of transistor 68 is reflected by the current mirror 78 to the transistor 84 to create the bias reference voltage EXPBIAS. When INPUT goes low, the transistor 12 turns off and the current sourced from transistor 44 is reflected into the diode string which causes the voltage at node VA to change immediately to a lower value. The amplifier 46 tries to change the voltage at node VB to match the voltage at node VA. However the voltage change at node VB is controlled by the charging of capacitor 60. Since node voltage VB is higher than node voltage VA, the transistor 48 conducts all of the current being sunk by transistor 52. The current from transistor 48 is sourced from load transistor 56. The current sourced from load transistor 58 is reflected in load transistor 56. The current from load transistor 56 is used to charge capacitor 60. The rate of charge is controlled by the size of the capacitor 60 and the size of the current source transistor 52. This linearly charging capacitor creates a linearly decreasing voltage at node VB. The linearly decreasing voltage at node VB, when applied to the linear to exponential converter 67 creates an exponentially decreasing current at the collector of transistor 68. The current from the collector of transistor 68 is reflected by the current mirror to the transistor 84 to create the bias reference voltage EXPBIAS applied to the conductor 86. Referring now to Figure 5, a detailed schematic diagram of the regulator bus driver 28 is shown. The regulator bus driver is connected between a supply line 100 designated BATTERY and ground and includes a first subsection comprising a two-stage differential amplifier generally designated 102 which is connected with an NMOS transistor current source 103 controlled by the exponential current source 26 via conductor 86. The amplifier 102 includes a first stage comprising NMOS input transistors 104 and 106, and PMOS load transistors 108 and 110. The second stage is connected to the output of the first stage at node 112 and includes a PMOS drive transistor 114 and a current source load 116 which keeps the amplifier 102 balanced at the regulated voltage. A current source 118 and a PMOS transistor 120 is connected between BATTERY and a node 122 and ensures that turn off will occur smoothly without unwanted voltage spikes. The gate of transistor 120 is connected to and controlled by the input signal DATAIN. DATAIN is also connected to a PMOS transistor 126 through an inverter 124. A voltage source 128 comprising an NPN transistor 130, a PMOS transistor 132, and a current source 134 is shorted by the transistor 126 when DATAIN goes high in order to minimize turn on delay at node BUS as will be explained more fully hereinafter. A feedback voltage is applied from node 142 to the input of the amplifier 102 at the node 122 from the junction of resistors 138 and 140 which are connected to the emitter of a transistor 136 and follows the voltage at nodes 142. The integrating capacitor 30 is connected between the node 142 and 144. An emitter follower output stage comprising an NPN transistor 146 and a resistor 148 are also connected to the junction 142. Loss of ground protection is provided by a PMOS transistor 150 which has its gate connected to ground. The resistor 152 and capacitor 154 connected between the node BUS and a node designated BUS GROUND represent the load presented to the regulator bus driver 28 by the communication bus. The voltage level of BUS GROUND can be +/- 2 volts different from the ground reference for the regulator bus driver 28, and still permit accurate communications on the bus. This ground offset insensitivity exists because the exponential current from the source 26 is converted by the capacitor 30 into a voltage and imposed onto the node BUS. Since the current-to-voltage conversion accomplished by the capacitor 30 only controls the rate of change of the voltage and is insensitive to the value of the starting voltage, the voltage shaping can occur from whatever the starting voltage on BUS GROUND. The regulator bus driver 28 operates in the following manner with reference to the circuit of Figure 5 and the waveforms of Figure 6. When DATAIN changes to a high level (Figure 6a), the transistor 120 turns off, and the current source 118 no longer supplies current into node 122. This causes the voltage at node 122 (Figure 6b) to drop from 10 volts to five sevenths of the voltage on node BUS as determined by the divider resistors 138 and 140. Since DATAIN has just changed from a low level, node BUS will be at ground, and therefore the voltage at node 122 will drop from 10 volts to 0 volts. Since the voltage at node 122 is at 0 volts, the transistor 104 will turn off and the current from current source 103 will flow through transistor 106 (Figures 6c and 6d). Any current from transistor 104 is pulled through transistor 108 which establishes the voltage at the gate of transistor 110 which in turn determines the amount of current which will flow out of transistor 110. Thus, the current flow in transistor 110 is equal to the current flow in transistor 104 and is also zero at this time. With DATAIN high, the gate of transistor 126 is low and this transistor conducts shunting the path through transistors 130 and 132. Since there is no current flow out of transistor 110, all of the current from current source 103 flows through transistors 114 and 150, capacitor 30, transistor 126, and transistor 106, as indicated in Figure 6f, causing the voltage across the capacitor 30 to increase. As node 112 is a high gain node, a small change in the voltage at node 112 causes a big increase in current into node 142, and the node 142 has high current gain into node BUS. When the voltage across capacitor 30 changes, the voltage change occurs at node 142 and, because of the emitter follower configuration, also occurs at node BUS as shown in Figure 6g. The rate of change of the voltage (dV/dt) at node BUS is determined by the ratio of the current source 103 to the capacitor 30. The voltage on node BUS continues to increase as indicated in Figure 6g, until it reaches a value of 7 volts. When the voltage on node BUS reaches 7 volts, the voltage at node 122 reaches 5 volts which is equal to the value of the voltage at node REF. Under this condition the input transistors 104 and 106 are balanced and the circuit 28 acts as a voltage regulator holding the voltage at node BUS at a constant value of 7 volts. Thus, the capacitor 30 performs the dual function of both integrating the rising and falling edges and stabilizing the regulated voltage. When DATAIN changes to a low level, the transistor 120 turns on allowing current source 118 to flow into node 122 which creates a voltage of 10 volts. This high voltage value at node node 122 causes transistor 104 to conduct all of the current from current source 103 (Figure 6e). With DATAIN low, transistor 126 is off and thus permits current to flow through transistors 130 and 132. The current from transistor 104 is reflected in transistor 110 by transistor 108. Transistor 106 is not conducting any current because the voltage at node 122 is much higher than the voltage at node REF. Since there is no current flow in transistor 106, the current from current source 103 is sourced from transistor 110. Thus current is sourced into capacitor 30 from transistor 110 through transistors 130 and 132 (Figure 6f). This causes the voltage at node BUS to decrease at a rate determined by the ratio of the current source 103 to the capacitor 30. As previously indicated, turn on delay of the output transistor 146 is minimized by placing the voltage source 128 in series with the integrating capacitor 30 when DATAIN goes to a low state and short circuiting this voltage source when DATAIN goes to a high state. The voltage source 128 is created by pulling a current from source 134 through diode connected transistors 130 and 132. The value of the voltage source 128 equals the voltage required to turn on the output transistor 146. The output can turn on when the base of the transistor 146 is a VBE higher than BUS, and the gate of the transistor 114 is a threshold voltage (VTH) lower than BATTERY. While DATAIN is low, transistor 126 is off and the voltage across the capacitor 30 is equal to the voltage at node 112 minus the voltage at node 142 minus the voltage across the voltage source 128. The voltage on node 112 equals the supply voltage BATTERY, the voltage on node 142 equals BUS, and the voltage across the voltage source 128 equals a VBE plus a VTH. When DATAIN goes high, the transistor 126 is turned on and the voltage source 128 is shorted out. The voltage across the capacitor 30 can not change instantly, therefore the voltage between node 112 and node 142 is equal to BATTERY minus BUS minus a VBE minus a VTH. This makes the voltage at node 112 equal to BATTERY minus a VTH, and the voltage at node 142 equal to BUS plus a VBE. The voltage on node 112 causes transistor 114 to be turned on, and the voltage on node 142 causes transistor 146 to be turned on. This permits the voltage at node BUS to start changing with minimal delay. As previously indicated, the voltage at node BUS is being regulated and accordingly the circuit has no turn off delay. As a result of the regulation, the transistor 114 is turned on just enough to provide the base drive to the transistor 146 necessary to meet the load requirements. When DATAIN goes low, the current source 118 is dumped into node 122 causing the voltage on node 122 to rise. This causes transistor 104 to conduct more current than transistor 106. Since the current in transistor 104 is reflected in transistor 110 by transistor 108, the increased current conduction in transistor 104 causes the voltage on node node 112 to increase. Since transistor 114 was turned on just enough to hold node BUS at the desired voltage, when the voltage on node 112 increases, the transistor 114 can no longer provide enough base drive to transistor 146 to keep the voltage at node BUS at the regulated value. Therefore the voltage on node BUS immediately starts to fall when DATAIN goes low.

Claims: A waveshaping circuit for producing a bus output voltage having a substantially sinusoidal rising transition from a low voltage level to a high voltage level in response to the rising edge of a data input signal and a substantially sinusoidal falling transition from a high voltage level to a low voltage level in response to the falling edge of a data input signal; comprising a current source (26) including a voltage generating means (31) for converting input current levels to voltage levels; and an integrator (30) connected to the current source (26) for providing the bus output voltage; characterised by current supply means (22,24); by logic circuit means (10,12,14) coupled to the current supply means for switching the current supply means to provide a first current level in response to the rising and falling edges of the data input signal and a second current level lower than the first current level when the bus output voltage exceeds a predetermined value while the data input is high or the bus output voltage drops below the predetermined value while the data input signal is low; and in that the current source (26) provides an exponentially increasing current in response to the first current level and an exponentially decreasing current in response to the second current level and comprises a controlled slew rate amplifier (46) including an input node and an output node, the voltage generating means (31) which converts the input current levels to voltage levels at the input node, the amplifier being adapted to produce a substantially linearly increasing output voltage in response to the first current level and a substantially linearly decreasing output voltage in response to the second current level, and a linear-to exponential converter (67) for converting the substantially linearly increasing output voltage to an exponentially increasing current and the substantially linearly decreasing output voltage to an exponentially decreasing current. A waveshaping circuit according to claim 1, comprising a bus driver (28) for supplying the output current of the current source to the integrator. A waveshaping circuit according to claim 2, comprising logic means (10,14) responsive to the data input signal and the bus output signal for producing a first logic level input signal on the rising and falling edges of the data input signal and a second logic level signal when the bus output signal exceeds the predetermined value while the date input signal is at the high voltage level or when the bus output signal falls below the predetermined value while the data input signal is at the low voltage level; wherein the current source (26) comprises a voltage generating stage (30,44) adapted to produce first and second voltage levels in response to the first and second current levels, the amplifier (46) is adapted to produce a substantially linearly increasing voltage in response to the application of the first voltage level, and a substantially linearly decreasing voltage in response to the application of the second voltage level, and the linear-to-exponential converter (67) produces an exponentially increasing current in response to the substantially linearly increasing voltage and an exponentially decreasing current in response to the substantially linearly decreasing voltage; wherein the bus driver (28) comprises a differential amplifier (102) including a reference node (REF) connected to a reference voltage and a feedback node (122) responsive to the output bus voltage and the data input signal, the differential amplifier being responsive to the output of the current source to provide a charging current to the integrator in response to the rising edge of the data input signal and to provide a discharging current to the integrator in response to the falling edge of the data input signal, and an output stage (146) adapted to apply the voltage across the integrator to an output node, the differential amplifier being adapted to regulate the voltage at the output node to a value related to the reference voltage. A waveshaping circuit according to claim 3, wherein the bus driver (28) comprises a voltage source (128) substantially equal to the voltage required to actuate the output stage and a switch (126) connected in parallel to the voltage source and responsive to the data input signal for connecting the voltage source in series with the integrator when the data input signal changes to a low state and for shunting the voltage source when the data input signal changes to a high state. A waveshaping circuit according to any one of claims 1 to 4, wherein the predetermined voltage is substantially midway between the high and low voltage levels. A waveshaping circuit according to any one of claims 1 to 5, wherein the amplifier (46) of the current source (26) comprises a first stage including first and second input transistors (48,50), first and second load transistors (56,58) and a current source (52) ; a second stage comprising a drive transistor (66) and a current source load (64); a capacitor (60) ; and a zero-cancelling resistor (62) connected between the collector of the first input transistor (50) and the base of the second input transistor (48). A waveshaping circuit as claimed in claim 2, wherein the bus driver comprises the integrator (30); and a differential amplifier (102) including a reference node (REF) connected to a reference voltage and a feedback node (122) responsive to the bus output signal and data input signal; the differential amplifier being adapted to provide an exponentially increasing charging current to the integrator in response to the rising edge of a data input signal until the bus output signal reaches a voltage level substantially midway between the low voltage level and the high voltage level, and to provide an exponentially decreasing charging current to the integrator between the midway voltage level and the high voltage level, and thereafter to regulate the bus output signal to the high voltage level the differential amplifier being adapted to provide an exponentially increasing discharging current to the integrator in response to the falling edge of a data input signal until the bus output signal falls to substantially the midway voltage level and to provide an exponentially decreasing discharging current to the integrator until the bus output signal reaches substantially the lower voltage level.

Inventors: SUMME RICHARD AUGUSTINE, SUMME, RICHARD AUGUSTINE
Applicants: DELCO ELECTRONICS CORP, DELCO ELECTRONICS CORPORATION
Application year: 1991
Main classification: H03K5
IPC codes: H03K4, H04L25, H03K19, H03K5