Signal Isolating and Sampling Circuit

1. Field Of The Invention

The present invention relates to signal sampling circuits. More specifically, the present invention is directed to a signal sampling circuit having signal isolating means for conductively isolating a signal input circuit from a signal output circuit.

2. Description Of The Prior Art

A prior art sampling circuit with a signal isolating capability used a first transformer to couple a signal input circuit to a signal output circuit. Switches were provided on the primary and secondary sides of the transformer and were synchronously operated to sample an input signal. A second transformer is used to couple a switch energizing signal to the sampling switches on the primary and secondary sides of the first transformer. Such a prior art circuit requires shielding between the transformers and is uneconomical because of the added cost of a second transformer. Another prior art circuit used a photocoupler to directly transfer an input signal from an input circuit to an output circuit. This prior art circuit required a conversion of the input signal to a pulse-width modulated signal before transmission by the photocoupler, and a demodulation after transmission by the photocoupler. Thus, this prior art circuit is more complicated than the aforesaid prior art circuit and even less economical as a result of the expensive converter circuits and their isolated power supplies.

An object of the present invention is to provide an improved signal isolating and sampling circuit for overcoming the aforesaid prior art problems.

In accomplishing this and other objects, there has been provided, in accordance with the present invention, a signal isolating and sampling circuit having a transformer for coupling a signal input circuit to a signal output circuit while providing conductive isolation therebetween. A photocoupler is arranged to supply a switch selecting signal to a sampling switch provided on the signal input circuit side of the transformer. A switch energizing pulse for driving the sampling switch is supplied through the transformer from the secondary side of the transformer between signal sampling operations.

A better understanding of the present invention may be had when the following description is read in connection with the accompanying drawings, in which:

FIG. 1 is a block diagram of a prior art transformer coupled signal isolating and sampling circuit,

FIG. 2 is a block diagram of a prior art photocoupler signal isolating and sampling circuit,

FIG. 3 is a block diagram of a signal isolating and sampling circuit embodying the present invention,

FIG. 4(a) - 4(d) are timing diagrams illustrating the operation of the circuit shown in FIG. 3, and

FIG. 5 is a detailed schematic illustration of a signal isolating and sampling circuit also embodying the present invention.

A transformer coupled prior art isolator and sampling circuit is shown in FIG. 1 in which "IN" designates an input terminal adapted to receive an input signal and "OUT" an output terminal. FILER designates a signal filter comprising an RC network T.sub.1 is a signal transmission transformer, and SW.sub.1 and SW.sub.2 are first and second switches provided on the primary and secondary sides respectively on the signal transmission transformer T.sub.1. Switches SW.sub.1 and SW.sub.2 are arranged to operate synchronously. CONTROL 1 is a first control circuit for controlling the first switch SW.sub.1, and CONTROL 2 is a second control circuit for controlling the second switch SW.sub.2. A control transformer T.sub.2 is interposed between the first and second control circuits CONTROL 1 and CONTROL 2 for synchronizing the operation of the first switch SW.sub.1 with the second switch SW.sub.2. With the isolator of this construction, the input signal applied to input terminal "IN" enters into the primary side of the signal transmission transformer T.sub.1 via the first switch SW.sub.1 and the signal induced on the secondary side of the transformer T.sub.1 is sent to the output terminal "OUT" via the second switch SW.sub.2.

With such a prior art isolator and sampling circuit, however, there is an electromagnetic coupling between the input signal transmission transformer T.sub.1 and switch operating transformer T.sub.2 for controlling the first switches SW.sub.1 and SW.sub.2, and it is difficult in practice to effectively shield both transformers to neutralize this inter-transformer coupling. Moreover, such a signal isolator and coupling circuit is not economical because it requires a second transformer T.sub.2 for controlling the switches SW.sub.1 and SW.sub.2.

A prior art circuit using a photocoupler as shown in FIG. 2 has been proposed to obviate the transformer problems described above. In FIG. 2, "IN" represents an input terminal, "OUT" an output terminal, Pd is a luminous, i.e., light emitting, element and Pt a light receiving element in a photocoupler PC. CONVERTER 1 represents an analogue-pulse width converter and CONVERTER 2 a pulse width-analogue converter. With this prior art isolator circuit, an analogue signal impressed upon input terminal "IN" is converted into a pulse width by the analogue-pulse width converter CONVERTER and the output thereof is applied to the pulse width analogue converter CONVERTER 2 through the photocoupler PC to be converted into an analogue signal. Thus, an analogue signal corresponding to the input signal appears on the output termnal "OUT."

The isolator shown in FIG. 2, however, is even more complicated than that shown in FIG. 1 and requires relatively expensive converters for converting an analogue signal into a pulse width and vice versa. Moreover it is necessary to use an isolated (or floating) power source for the converters thus rendering the actual circuit even more complicated and uneconomical.

FIG. 3 is a block diagram showing the basic construction of one embodiment of the signal isolating and sampling circuit of this invention in which "IN" represents an input terminal connected to receive an analogue signal, "OUT" is an output terminal, "T" is a transformer for transmitting the input signal and for supplying energy to energize a signal sampling switch on the primary side, and SW.sub.1 and SW.sub.2 are switches provided on the primary and secondary sides respectively of the transformer "T." Input switch SW.sub.1 is a sampling switch for sampling the input signal to send it to the output "OUT" through the transformer "T," and output switch SW.sub.2 is a switch for sampling the input signal sent from the sampling switch SW.sub.1. These switches are arranged to operate synchronously. Output sampling switch SW.sub.2 not only operates to sample the input signal but also prevents a switch energizing signal supplied through a supply switch SW.sub.4, described later, from appearing on the output terminal "OUT." Control switch SW.sub.3 is a switch for controlling the ON-OFF operation of the sampling switch SW.sub.1 in response to the output of a photocoupler PC to be described later. The supply SW.sub.4 switch is a supply switch provided on the secondary side of transformer T and is operated to store energy for operating switch SW.sub.1 in a capacitor C via transformer T. Source ES represents a power source circuit provided on the secondary side of transformer T for supplying a switch energizing pulse via switch SW.sub.4. A rectifying network RS is a rectifying and smoothing circuit that rectifies and flattens the energizing pulse induced on the primary side of transformer T from the source ES. A photocoupler PC supplying a control signal for driving the sampling switch SW.sub.1 on the primary side of transformer T is controlled by an external signal applied to a control signal terminal CONTROL. When the output signal from the photocoupler PC closes the supply switch SW.sub.3, sampling switch SW.sub.1 on the primary side of the transformer T is driven by the output of the rectifying and smoothing circuit RS stored in capacitor C to transmit the input signal to the primary side of transformer T. Since output switch SW.sub.2 closes with switch SW.sub.1, the input signal is coupled through the transformer T and is applied to the output terminals "OUT."

FIG. 4 shows a time chart to explain the operation of FIG. 3 by showing the operating sequences of respective switches in FIG. 3.

In response to the external control signal CONTROL, the photocoupler PC operates to turn "ON," i.e., close, switch SW.sub.3 by its output as shown in FIG. 4a. The closing of switch SW.sub.3 enables the charge stored in the capacitor C to be sent to the input sampling switch SW.sub.1 to turn it "ON" as shown in FIG. 4b. As the input sampling switch SW.sub.1 turns "ON," the input signal applied to the input terminal "IN" appears on the secondary side of transformer T, and then appears on the output terminals "OUT" when output sampling switch SW.sub.2 is turned "ON" as shown by FIG. 4c. At this time, supply switch SW.sub.4 is in its "OFF," i.e., open, state. After the input signal has been transmitted to the output terminals "OUT" and after the switches SW.sub.1, SW.sub.2 and SW.sub.3 have been turned "OFF," the supply switch SW.sub.4 is turned "ON" as shown in FIG. 4d to supply a pulse from pulse source ES via switch SW.sub.4 the transformer T and the rectifying network RS to recharge the capacitor C. Thereafter, the above cycle of operation is repeated.

Fig. 5 is a detailed schematic diagram showing a specific construction of one embodiment of the signal isolator and sampler of this invention. In FIG. 5, elements corresponding to those shown in FIG. 3 are designated by the same reference symbols. A diode D comprising the rectifying and smoothing circuits RS and functions to prevent the charge stored on the capacitor C from flowing back to the input terminals "IN" when the sampling switch SW.sub.1 is closed. A luminous element Pd and a light receiving element Pt constitute a photocoupler PC for driving the sampling switch SW.sub.1 on the primary side of transformer T. A first field effect transistor FET 1 comprises the sampling switch SW.sub.1. A second field effect transistor FET 2 comprises the sampling switch SW.sub.2. A resistor R1 together with the light receiving element Pt of the photocoupler PC constitutes switch SW.sub.3 that controls the "ON" and "OFF" operation of the sampling switch SW.sub.1. One end of the receiving element of the light receiving element Pt is connected to the junction between the diode D and one side of the capacitor C whereas the other end thereof is connected to one end of the resistor R.sub.1 and to the first gate electrode of the first field effect transistor FET 1 comprising the sampling switch SW.sub.1. The other end of resistor R.sub.1 is connected to the other side capacitor C, to one terminal of the primary of transformer T and one of the input terminals "IN." A PNP type transistor Tr and a resistor R2 constitute source switch SW.sub.4. The collector electrode of transistor Tr is connected to the junction between the secondary of transformer T, the drain electrode and the second gate electrode of the field effect transistor FET.sub.2 comprising the sampling switch SW.sub.2. The emitter electrode of transistor Tr is connected to the base electrode of transistor Tr through resistor R.sub.2 and to the anode electrode of the luminous element Pd of the photocoupler PC via resistor R.sub.3. The cathode electrode of the luminous element Pd is connected to terminal CONTROL to which is applied an external control signal.

In the isolator and sampler circuit of the present invention constructed as above described, the switch SW.sub.3 circuit is turned "ON" by the external control signal applied through the photocoupler PC to control the ON-OFF timing of the sampling switch SW.sub.1 provided on the primary side of transformer T. Specifically, the charge stored in capacitor C is applied to the first gate electrode of the first field effect transistor FET 1 comprising the sampling switch SW.sub.1 thereby rendering conductive the path between the drain and source electrodes of the first field effect transistor FET 1. When the first field effect transistor FET.sub.1 conducts, the sampled input signal will appear on the secondary side of the transformer T so that the input signal will be sent to the output terminals "OUT" when the second field effect transistor FET.sub.2 comprising the sampling switch SW.sub.2 is turned "ON." A control signal to the gate electrode of the second field effect transistor FET.sub.2 can either be applied in synchronism with the control signal applied to the control input terminal CONTROL or delayed with respect thereto. The only criteria to be observed is that the timing of the switch circuits shown in FIG. 4 should also be followed in FIG. 5.

After transferring, i.e., sampling, of the input signal is completed, i.e., switches SW.sub.1, SW.sub.2 and SW.sub.3 are open, the transistor Tr comprising the source switch SW.sub.4 is turned "ON" so as to supply a supply pulse through transformer T. The supply pulse induced on the primary side of transformer T is rectified by the diode D and smoothed as shown by arrows to recharge capacitor C. Thereafter the above sampling cycle is repeated.

As can be clearly noted from the foregoing description, the primary side sampling switch SW.sub.1 of the isolator and sampler circuit of this invention is included in an input line while the transformer T is connected to perform transfer of the supply pulse and the transfer of the input signal at different times so that the function of the isolator and sampler circuit can be provided by a single transformer thereby eliminating electromagnetic interference between a plurality of transformers. Moreover the photoconductor circuit is used without expensive converter circuits. Since the construction is extremely simple, it is possible to fabricate the isolator and sampler circuit of the present invention as a compact unit and to decrease the cost.

Accordingly, it may be seen that there has been provided, in accordance with the present invention, an improved signal isolator and sampling circuit having a relatively simple and inexpensive construction.