Background Of The Invention The present invention relates generally to an apparatus and method for providing a feedback signal representative of the current through a load driven by a ^" current amplifier. The present invention has particular application in constructing a feedback signal represen¬ tative of the torque producing current through a multi¬ phase brushless DC motor driven by a pulse width modulated (PWM) amplifier, but is not limited thereto.

For explanation purposes only, the following discus¬ sion relates to the current sensing problem in multi-phase brushless DC motors and a solution for it. It will be understood, however, that the herein described invention is not limited in application to brushless DC motors.

Brushless DC motors are known. In such motors, a plurality of fixed stator windings and rotatable magnetic poles are provided. Motor rotation results by energizing ^* each stator winding in a proper sequence. A proper se¬ quence is one wherein a stator winding is energized when a torque producing magnetic pole is aligned with it. Gener¬ ally, logic circuitry is provided which energizes the stator windings in the proper sequence. This logic cir¬ cuitry requires as an input a drive signal which alternates between two states, such as a PWM signal. In order to

determine the proper duty cycle of the PWM signal for a given load and a desired speed, the circuit generating th PWM signal requires a feedback signal containing informa¬ tion relating to both the instantaneous polarity and magnitude of the torque producing current through the stator windings.

Prior art current sensing schemes for providing feedback information to the PWM signal generation circuit generally sense the current in each individual motor phas The signals obtained from each motor-phase are summed together to construct a waveform representative of the magnitude of the torque producing current. This is usually accomplished by placing a sense resistor in seri with each motor phase, amplifying the voltage developed across each resistor and summing the amplified outputs together in the proper sequence. Summing the outputs together in the proper sequence is usually achieved by supplying the signal from each sense resistor to a separ solid state switch; the switch outputs are tied together in common and are opened and closed in the proper sequen under control of external logic. The external logic re¬ quires as an input the signal from an angular position sensor located within the motor. This sensor provides information relative to the angular- position of the rota ing magnetic poles.

The signal provided by t e above described circuit is representative of the instantaneous magnitude of the torque producing current, but because o -the current in¬ duced by the motor's inductance, it is not necessarily representative of the correct polarity of the torque pro ducing currant.

To obtain a signal which is representative of both the instantaneous magnitude and polarity of the torque producing current, the prior art requires the use of add tional circuitry. This additional circuitry is responsi to the PWM signal to pass the above described feedback signal in an inverted for when the PWM signal is in one

state and in a non-inverted form when the PWM signal is in the other state.

It will be appreciated from the foregoing discussion that prior art current sensing schemes have two major draw¬ backs. First, in multi-phase loads, each phase requires a sense resistor and accompanying circuitry for providing a signal indicative of the magnitude of the instantaneous current through that phase. Second, circuitry is required for summing each of the phase signals in the proper sequence and for correcting the polarity of the signal ob- - tained from the summation. In multi-phase brushless DC motors, this has required the use of an angular position sensor associated with the motor. Thus, prior art circuits for constructing a feedback signal representative of both 'the instantaneous magnitude and polarity of the torque producing motor current are cumbersome, expensive, and require complex logic.

It is therefore desirable to provide a circuit for generating a current signal continuously representative of both the instantaneous polarity and magnitude of the curren through a load which is simple and requires few components. Such a signal can typically be used for feedback purposes but is not limited to such applications.

It is also desirable to provide a circuit for gener¬ ating a current feedback signal continuously representative of both the instantaneous polarity and magnitude of the current through a multi-phase load which does not require a sense resistor for each phase and which does not require that multiple sense signals from the phases be summed toget

It is further desirable to provide a circuit for gen¬ erating a current feedback signal continuously representati of both the instantaneous polarity and magnitude of the torque producing current in a multi-phase brushless DC motor which requires only a single sense resistor and does not require an angular position sensor associated with the motor to reproduce the signal and yet is simple, reliable, and inexpensive.

Summary Of The Invention

Apparatus for providing a current signal continuously representative of both the instantaneous magnitude and polarity of the current through a load driven by a current amplifier which regulates the current through the load according to the duty cycle of a drive signal having first and second states comprises sensing means for providing a sense signal representative of the magnitude of the current through the load and polarity correction means responsive to the drive signal for supplying the sense signal in an inverted form when the drive signal is in the first state and for supplying the sense signal in a non-inverted form when the drive signal is in the second state. The apparatu requires only a single sensing means (resistor) regardless of the number of phases in the load and hence does not require circuitry for summing individual phase signals together. As applied to brushless DC motors, the apparatus does not require an angular position sensor associated with the motor to produce a feedback signal which is representative of both the magnitude and polarity of the - torque producing current. A feature of the present inven¬ tion is that the polarity correction means is directly coupled to the sensing means, as opposed to the hereinbefor described prior art devices which require an intermediate switching network to sum the load currents from each phase .

In a preferred embodiment, the current amplifier is a full wave commutation bridge circuit receiving a source of electric current from a power supply and the drive signal is a pulse width modulation (PWM) signal. The load is connected across the bridge circuit. The sensing means comprises a single sense resistor connecting the bridge circuit to the return path of the power supply such that the current through the load flows through the sense resistor. The voltage developed across the sense resistor is a sense signal representative of the magnitude of the current through all phases of the load.

In one embodiment, the polarity correction means com¬ prises switch means and differential input amplifier means wherein the switch means is responsive to the PWM signal to supply the sense signal to the inverting input of the differential input amplifier means when the PWM signal is in a first state and to supply the sense signal to the non-inverting input of the differential input amplifier when the PWM signal is in a second state. The output of the differential amplifier means thereby supplies the sense signal in an inverted form when the PWM signal is in the first state and in a non-inverted form when the PWM signal is in the second state. The output signal from the differential amplifier means is an analog current feedback signal.

In another embodiment, the polarity correction means comprises first and second switch means and first and second differential amplifier means and the sense signal is supplied to the inverting input of the first differen¬ tial input amplifier means and to the non-inverting input of the second differential amplifier means. The first differential amplifier means is effective to supply the ^• sense signal in an inverted form. The second differential input amplifier means is effective to supply the sense signal in a non-inverted form. The input of the first switch means is operatively connected to the output of the first differential input amplifier means and is respon¬ sive to pass the output of the first differential amplifier means only when the PWM signal is in the first state. The second switch means is operatively connected to the output of the second differential amplifier means and is respon¬ sive to pass the output of the second differential ampli¬ fier means only when the PWM signal is in the second state. The output of the first and second switch means are tied together in common. The output signal from the first and second switch means is the current feedback signal.

In still another embodiment, an intentional dead time is introduced into the PWM drive signal to prevent the

bridge circuit from shorting during transition times of the PWM signal. Differential input amplifier means and switching means are responsive to first and second switch¬ ing signals derived from the PWM signal for providing the sense signal in an inverted form when the first switching signal is in an active state and for providing the sense signal in a non-inverted form when the second switch signal is in an active state. A time delay is interposed between subsequent occurrences of the active states of the first and second switching signals and the time delay substan¬ tially corresponds to the dead time introduced into the PWM drive signal . Buffer means are provided at the output of the differential input amplifier and switching means to hold during the time delay period the last output provided by the differential input amplifier and switching means before the time delay occurred.

As applied to multiphase loads, an advantage of- the present invention is that only a single sense resistor is required , regardless of the number of phases in the load and the circuitry for generating the current feedback signal is the same regardless of the number of phases in the load. As applied to multi-phase brushless DC motors, an additional advantage of the present invention is that an angular position sensor is not required to generate a feedback signal representative of both the polarity and magnitude of the load current. A further advantage is that the intermediate switching network for summing each of the phase load currents together required by the prior art devices is eliminated. Still a further advantage is that the feedback signal is an analog signal which is continuous ly representative of both the instantaneous magnitude and polarity of the load current.

Brief Description Of The Drawings

Figure 1 illustrates a full wave commutation bridge driving a single phase load.

Figure 2 illustrates the waveforms of the PWM drive signal, the sense signal and the actual motor load current.

Figure 3 illustrates a current feedback circuit according to one embodiment of the invention.

•Figure 4 illustrates a current feedback circuit according to another embodiment of the invention.

Figure 5 illustrates a full wave bridge circuit driving a three phase load and the circuitry for deriving ■the first and second switching signals from the PWM drive signal.

Figure 6 illustrates a circuit for providing a cur¬ rent feedback signal according to another embodiment of the invention.

Figure 7 illustrates a circuit for generating a current feedback signal according to still another embodi¬ ment of the invention.

Figures 8A and 8B illustrate various timing diagrams associated with the practice of the present invention. Detailed Description Of The Drawings

Referring now to the drawings wherein like numerals represent like elements, there is shown in Figure 1 a current amplifier embodied as a full wave commutation bridge circuit 10 driving a motor load M. Full wave commu tation bridge circuit comprises transistors Si, S2, S3 and S4 and associated diodes CR1 , CR2, CR3, and CR4. Motor load M is connected across bridge circuit 10 as shown. One side of bridge circuit 10 is connected to a source of electrical power V+. A sense resistor 12 connects the other side of bridge circuit 10 to the return path of the power supply V+. A capacitor C is typically, but not necessarily, connected across the power supply terminals as shown.

The base of transistors SI and S4 receive a PWM drive signal. The base of transistors S2 and S3 receive the inverted PWM drive signal, herein designated as PWM. It will be appreciated that when the PWM signal is in a high state (logic 1), transistors Si and S4 will become conduc¬ tive and that transistors S2 and S3 will be non-conductive. Current flows from the power supply V+ through transistor

Sl, motor load M, transisitor S4, sense resistor 12, and through the return path to the power supply. As shown in Figure 2, during the time that the PWM signal is in the high or logic 1 state (i.e., between time tg and tτ_) the motor current 1^ s increasing at a finite rate due to the inductance of motor load M. During this time the voltage V _R across sense resistor 12 is positive and increasing as shown in Figure 2. When the PWM signal changes to the low (logic 0) state at time tτ_, transistors SI and S4 become non-conductive. The PWM signal becomes high (logic 1) and transistors S2 and S3 saturate and become conductive. However, the inductance of motor M pre¬ vents the current IM. from reversing instantly. Instead, the motor current decays in magnitude via the path defined by diode CR3, capacitor C, sense resistor 12 and diode CR2. This is shown in Figure 2 between times tι_ and t • During this time, the voltage V _R across sense resistor 12 is ^" of negative polarity and decreasing magnitude as shown in Figure 2. At time t2#- PWM again becomes high (logic 1), transistors S2 and S3 become nonconductive and transistors SI and S4 become conductive. The motor current 1^ becomes positive and increases as shown in Figure 2 between times t and t3. It will be appreciated that the speed and torque of motor M, are regulated according to the duty cycle of the PWM drive signal.

The actual motor current I[ is shown in Figure 2. It can be seen that, given only the PWM signal and the sense signal VR, an accurate representation of both the magnitude and polarity of the motor current I _M can be reconstructed without using an angular position sensor. This is achieved by passing the sense signal VR unchanged when the PWM signal is high (logic 1) and inverting the sense signal VR when the PWM signal is low (logic 0).

Figure 3 illustrates one embodiment of a circuit which utilizes the PWM signal and the sense signal VR to construct a waveform representative of both the polarity and magnitude of the torque producing motor current 1^.

Referring to Figure 3, circuit 22 comprises switch means 50, 52, 54 and 56 and differential input amplifier means 24. As shown, switches 50 and 56 are responsive to ^' the PWM signal and switches 52 and 54 are responsive to the PWM signal. Thus, when the PWM signal is active (logic 1), switches 50 and 56 will be closed and switches 52 and 54 will be open. Alternatively, when the PWM signal is active (logic 1), switches 52 and 54 will be closed and switches 50 and 56 will be open.

When the PWM signal is active (logic 1), differential input amplifier 24 is configured as an inverting amplifier and therefore the sense signal VR is applied to the invert¬ ing input of differential amplifier 24. The signal Vg at the output Vo of differential input amplifier 24 is an inverted form of the sense signal VR.

When the PWM signal is active (logic 1), differential amplifier 24 is configured as a non-inverting amplifier. The sense signal VR is applied to the non-inverting input of differential input amplifier 24. The signal Vg at the output of differential input amplifier 24 is a non-inverted form of the sense signal VR.

Referring back to Figure 2, it will be appreciated that the output signal V of differential input amplifier 24 corresponds to the motor current waveform Ijj. It will be seen therefore, that the circuit of Figure 3 provides an analog current signal which is continuously representative of both the instantaneous magnitude and polarity of the torque producing motor current 1^.

Referring now to Figure 4, there is shown a second embodiment of a circuit which utilizes the PWM drive signal and the sense signal VR to provide a current signal representative of both the instantaneous magnitude and polarity of the torque producing motor current I _M .

Circuit 26 comprises first and second differential input amplifier means 28 and 30 and first and second switch means 58 and 68. As shown, differential input amplifier

28 receives the sense signal VR on its inverting input

(and therefore inverts the sense signal VR) and differen¬ tial input amplifier 30 receives the sense signal V _R on its non-inverting input (and therefore passes the sense signal VR non-inverted) . The output of amplifier 28 is supplied to the input of a switch 58. The output of differential input amplifier 30 is supplied to the input of a switch 68. The outputs of switches 58 and 68 are tied together in common as shown. Switch 58 closes when the PWM signal is active (logic 1) and switch 68 closes when the PWM signal is active (logic 1).

An inverted form of the sense signal VR is passed through switch 58 and supplied as the output signal Vg when the PWM signal is active (logic 1). A non-inverted form of the sense signal V _R is passed through switch 68 and supplied as the output signal Vg when the PWM signal is active (logic 1) .

It will be appreciated that the output signal V ₀ substantially corresponds to the motor current I _M shown in Figure 2. The output VQ is therefore an analog current signal which is continuously representative of both the instantaneous polarity and magnitude of the torque produc¬ ing motor current l _M .

Referring now to Figure 5, a full wave commutation bridge 32 driving a three phase brushless DC motor is shown. Full wave commutation bridge 32 comprises transis¬ tors S1-S6 and associated diodes CR1-CR6. One side of the bridge circuit 32 is connected to a power supply V+. The other side of the bridge circuit is connected via a sense resistor 36 to the return path of- the power supply.

A PWM signal is applied to a logic circuit 34. Logi circuit 34 also receives clock pulses from a clock 42. Logic circuit 34 applies the PWM signal to the transistors S1-S6 in the proper sequence to insure rotation of brush¬ less DC motor 35 in well-known manner. A typical energiz tion sequence for transistors S1-S6 is shown in the fol¬ lowing table.

Displacement Current Flow "On" Transistors

It will be appreciated that the motor current 1^ through any of the phases φA, φB, and φC of brushless DC motor 35 will also flow through sense resistor 36. For example, when transistors Si and S4 are turned on, current flows from power supply V+, through transistor SI, through phases φB and φA, through transistor S4 and sense resistor 36. The voltage VR developed across sense resistor 36 is representative of the instantaneous motor current Irø through all phases of the motor M.

The sense signal VR is applied to a double buffer amplifier circuit comprising differential input amplifiers 38 and 40. The buffer amplifiers remove any ground dif¬ ferential error and provide sufficient gain to sense signal VR to preserve the signal to noise ratio prior to 'subse¬ quent processing. Thus, as used in connection with this embodiment, the sensing means includes not only the resistor 36, but the amplifiers 38, 40, and the sensing means is still directly coupled to the polarity correction means, as will become evident hereinafter.

The previous discussion has been based on the premise that the transistors comprising the bridge circuit will - instantaneously turn off when the PWM or PW drive signal changes to its inactive state (i.e. changes from logic 1 to logic 0). However, in practical circuits utilizing transistor type bridge circuits, instantaeous turn-off of the transistors is not realizable. For example, referring to Figure 1, when the PWM signal changes from logic 1 to logic 0, transistor Si does not turn fully off before transistor S2 turns on. This is because a finite amount

of time is required for transistor SI to come out of satur¬ ation after the PWM drive signal changes to its inactive state (logic 0). This results in simultaneous conduction of transistors SI and S2. Since transistors Si and S2 form a series connection across the power supply V+, their simultaneous conduction results in excessive current and the destruction of transistors of Si and S2.

To avoid this problem, an intentional "dead time" is introduced into the PWM and PWM signals driving the bridge circuit, via logic circuit 34. Referring to Figure 8A, it will be explained how a dead time is introduced into the PWM and PWM signals which are applied to transistors S1-S4 in the bridge circuit 10 in Figure 1.

A dead time signal 95 is generated by logic circuit 34. As shown, the dead time signal 95 is a train of pulses wherein each pulse is approximately two clock periods in duration. Circuitry in logic circuit 34 is responsive to dead time signal 95 to provide a time ^' delay between the occurrence of the active state of drive signal 93 for transistors SI and S4 and the active state of drive signal 94 for transistors S2 and S3. The duration of the time delay is sufficient to insure that all transistors are "off" before allowing a transistor pair in the bridge circuit to turn "on". It will be noted that the duration of the time delay is substantially equal to the duration of the pulse in the dead time signal 95.

Due to the time delay introduced into transistor drive signals 93 and 94, the PWM and PWM signals can no longer be used to directly control the circuitry of Figures 3 and 4. Additional circuitry which will compensate for the time delay introduced into the transistor drive signals is required.

Embodiments of the invention which incorporate the inventive principles described above and which contain cir¬ cuitry to compensate for the time delay are now described .

Referring again to Figure 5, there is shown a four stage serial shift register 44. Shift register 44 receives

a source of clock pulses from clock 42 via its clock input CLK. The PWM signal is applied to the input of the first stage Dl of shift register 44. The output of the last stage Q4 of shift register 44 is applied to one input of an AND gate 46 and to one input of a NOR gate 48. The out¬ put of an intermediate stage, herein- shown as the output of the first stage, Ql , is applied to the other input of AND gate 46 and to the other input of NOR gate 48. For reference purposes, the output of the intermediate stage Ql is labelled SPWM and the output of the last stage Q4 is labelled DPWM.

For purposes of the following explanation, it will be assumed that the CLK input of shift register 44 is respon¬ sive to the negative edge of the clock pulses supplied by clock 42. Thus, data in shift register 44 is clocked by one stage upon the occurrence of a negative going clock pulse edge. As shown in Figure 8A, the occurrence of the SPWM signal 96 substantially coincides with the occurrence the PWM signal 92 except as herein noted. As shown, the PWM signal 92 goes high at time t _n -l. At time t _n , the negative-going edge of a clock pulse 91 coincides with ^' the high state of the PWM signal 92 an c oeks a "1" into the first stage of the shift register 44 thereby causing the SPWM signal 96 to become high. The PWM signal 92 remains high until time t _n +3 at which time it becomes low. A short tine thereafter, at time t _n +4, which coincides with the first occurrence of a negative-going clock pulse edge after the PWM signal goes low, a "0" is clocked into the first stage of the shift register 44 and the SPWM signal 96 goes low. This process repeats itself, as shown in Figure 8A.

As will be understood, the signal DPWM, which is the output of the last stage Q4 of shift register 44, will be delayed by three clock periods relative to the SPWM signal 96. This is illustrated in Figure 8A. As shown, the occurrence of the DPWM signal 97 coincides with the occur¬ rence of the negative-going edge of the third clock pulse

after the SPWM signal 96 occurred. Thus, the DPWM signal 97 becomes high at time t _n τ2 and goes low three clock periods after the SPWM signal goes low at time t _n +6.

The logic circuit comprising AND gate 46 and NOR gate 48 combines the SPWM and DPWM signals to supply first and second switching signals Yl and Y2. As shown in Figure 8A, the Yl switching signal 98 substantially coincides with the SI, S4 drive signal 93 except that it's occurence is delayed by one clock period. Similarly, the Y2 switch¬ ing signal 99 substantially coincides with the S2, S3 drive signal 94 except that it's occurrence is also delayed by one clock period. Switching signals Yl and Y2 are active high, i.e., each is active when it is a logic 1.

It will be appreciated from an examination of Figure 8A that there is a time delay between the occur¬ rence of the active state of the Yl switching signal 98 and the occurrence of the active state of the Y2 switching- signal 99. This time delay substantially corresponds to the duration of the pulses in the dead time signal 95 except that it exceeds the duration of the pulses in dead time signal by one clock period. Switching signals Yl and Y2 may be applied to circuitry similar to that shown in Figures 3 and 4 for converting the sense signal VR into an analog current feedback signal which is continuously repre¬ sentative of both the magnitude and polarity of the torque producing motor current 1^.

Turning now to Figure 6, there is shown a circuit which is responsive to the switching signals Yl and Y2 and to the sense signal VR for providing an analog current feedback signal VQ.

Circuit 67 comprises first and second differential input amplifiers 62 and 64 and first and second switches 66 and 68. Differential input amplifier 62 receives the sense signal VR on its inverting input and is effective to pass the sense signal VR in an inverted form. Differ¬ ential input amplifier 64 receives the sense signal VR on its non-inverting input and is effective to pass the sense

signal VR in a non-inverted form. The output of amplifier 62 is connected to the input of switch 66, Switch 66 is responsive to switch signal Y2 such that switch 66 closes when switch signal Y2 is active (logic 1) and opens when it is inactive (logic 0). The output of amplifier 64 is provided to the input of switch 68. . Switch 68 is respon¬ sive to switch signal Yl such that switch 68 closes when switch signal Yl is active (logic 1) and opens when it is inactive (logic 0). The outputs of switches 66 and 68 are tied together in common as shown.

When switch signal Yl is active, the sense signal VR is passed in a non-inverted form through switch 68. When switch signal Y2 is active, the sense signal VR is passed in an inverted form through switch 66. Assuming that "circuitry comprising amplifier 70 and holding capacitor 72 were not provided, the signal V ' 101 shown in Figure 8B would result. The "gaps" 102, 103, 104 and 105 which occur in the V _Q ' signal 101 are directly attributable to the time delay interposed between subsequent occurrences of the active states of switching signals Yl and Y2 and are indirectly attributable to the time delay introduced into the transistor drive signals. Circuitry comprising amplifier 70 and holding capacitor 72 is provided to cor¬ rect the V ₀ ' signal. Holding capacitor 72 holds during the time delay the last value supplied by switches 66 and 68 before the time delay occurred. At all other times, holding capacitor 72 and therefore amplifier 70 follow the signal Vg'. The addition of amplifier 70 and holding capacitor 72 results in a corrected signal VQ 102 which is shown in Figure 8B. As can be seen, the output signal VQ substantially corresponds to the actual current wave¬ form I _M 103.

Another circuit which uses the Yl and Y2 switching signals and the sense signal VR and generates a current signal is shown in Figure 7.

Circuit 75 comprises differential input amplifier 84 and first switch means 76, 82 and second switch means 78,

80. A third switch means 86 is also provided for purposes which will become evident hereinafter.

Switches 76 and 82 are responsive to the second switch signal Y2 and close when switch signal Y2 is active (logic 1) and open when it is in inactive (logic 0) . Switches 78 and 80 are responsive to the switch signal Yl and close when switch signal Yl is active (logic 1) and open when it is inactive (logic 0). It will be seen that when switch signal Y2 is active, the sense signal VR will be applied to the inverting input of amplifier 84. Ampli¬ fier 84 will thereby be effective to pass the sense signal VR in an inverted form to the input of switch 86. Alterna¬ tively, when switch signal Yl is active, the sense signal VR is applied to the non-inverting input of amplifier 84. Amplifier 84 is thereby effective to pass the sense signal VR in a non-inverted form to the input of switch 86. Tt will be appreciated that the signal VQ' appearing at the input of switch 86 corresponds to the signal V _Q ' 101 as shown in Figure 8B. Again, the "gaps" 102, 103, 104 and 105 are directly attributable to the time delay interposed between subsequent occurrences of the Yl and Y2 switching signals and are indirectly attributable to the time delay introduced into the transistor drive signals. Circuitry comprising switch.86, amplifier 88 and holding capacitor 90 are provided to correct the signal V _Q ' 101 and to obtain a corrected Vo signal 102.

Switching signals Yl and Y2 are supplied to the in¬ puts of an OR gate 74. The output of OR gate 74 drives switch 86. It will be appreciated that switch 86 closes when either switching signal Yl or Y2 is active (logic 1) and is open when both of the switching signals are inactive (logic 0). Stated otherwise, the switch 86 is open during the time delay period. The holding capacitor 90 holds during the time that switch 86 is open the last value supplied by amplifier 84 before switch 86 opened; that is, holding capacitor 90 holds during the time delay the last value supplied by amplifier 84 before the time delay

occurre . When switch 86 is closed, ^' the output of ampli¬ fier 88 follows the signal provided by amplifier 84. It will be appreciated that the signal Vo provided by the amplifier 88 corresponds to the signal VQ 102 shown in Figure 8B. As seen, signal VQ 102 accurately represents the motor current I _M . 103. Thus, signal V ₀ 102 is an analog -current feedback signal continuously representative of both the instantaneous polarity and magnitude of the torque producing motor current.

The functions performed by the circuits of Figures 6 and 7 are summarized at the bottom of Figure 8B. As shown, when the Yl switching signal 98 is active, the sense signal VR is passed in a non-inverted form, i.e., the gain of the circuit is +1. When the Y2 switching signal 99 is active, the sense signal VR is passed in an inverted form, i.e., the gain of the circuit is -1. During the time delay period, the circuitry holds the last value provided before the time delay occurred.

The present invention may be embodied in other spe¬ cific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.