A RECONFIGURABLE VOLTAGE CONTROLLED CURRENT-VOLTAGE SOURCE DEVICE

AND DIGITAL-TO-ANALOGUE CONVERTER COMPRISING IT

FIELD OF THE INVENTION

The present invention is enclosed in the area of voltage / voltage and voltage / current conversion, in particular with reconfigurable properties, and more specifically applied to digital-to-analogue conversion.

PRIOR ART

Several types of digital-to-analogue converters (DAC) exist in the art, having in common the fact that such DAC either has a current or voltage output, but not both.

Nevertheless, there are integrated circuits that combine DACs and analogue circuitry providing current and voltage outputs.

In these circuits, the current and voltage outputs are assigned to different pins and use different analogue blocks for each output, thereby increasing complexity, layout area and power consumption.

Moreover, the current output of these DACs is usually unidirectional, which reduces the range of applications where the DAC can be used.

To improve the versatility, flexibility and solve the drawbacks above mentioned, the present invention provides a simple and reliable solution for the reconfiguration of the output, thereby providing an output current or an output voltage in the same output pin, as well as bipolar output voltages and bidirectional currents.

SUMMARY OF THE INVENTION

It is an object of the present invention a reconfigurable voltage controlled current-voltage source device comprising two signal inputs, a single signal output, five resistors, four switches, a differential amplifier and two secondary amplifiers. In particular:

- the five resistors consist of first, second, third, fourth and fifth resistor;

- the two signal inputs consist of a first and a second signal input, wherein:

o the first signal input is connected to the inverting input of the differential amplifier through the third resistor r ₃ , and

o the second signal input is connected to the non-inverting input of the differential amplifier through the first resistor r _x ;

- the output of the differential amplifier is connected to the single signal output through the fifth resistor r _2¾ ;

- the two secondary amplifiers consist of:

o a first secondary amplifier in which:

■ an output is connected to the inverting input of the differential amplifier through the fourth resistor r ₄ , and

^■ it is part of the negative feedback loop of the differential amplifier;

o a second secondary amplifier in which:

■ an output is connected to the non-inverting input of the differential amplifier through the second resistor r _2a , and

^■ it is part of the positive feedback loop of the differential amplifier;

- the four switches are so arranged that: o when a first switch and a third switch are locked and a second switch and a fourth switch unlocked, the voltage controlled current- voltage source operates in current source mode, and

o when a second switch and a fourth switch are locked and a first switch and a third switch unlocked, the voltage controlled current-voltage source operates in voltage source mode.

The reconfigurable current-voltage source device of the present invention thereby provides a bidirectional output, i.e., it has the ability to source or sink a current from a load, such load being connected to the referred single signal output. Therefore, it enables the usage of a single circuitry, providing a single output. The differential amplifier feeds a load connected to the single signal output, thereby having a similar functionality to that of the Howland based circuits or of the voltage controlled voltage source. The secondary amplifiers specifically provide mitigation of the effects of the resistances of the switches on the circuit performance. Furthermore, they work as voltage buffers and improve the isolation between the single output and the two inputs, facilitating the resistance matching between the two feedback paths (positive and negative feedback paths). Furthermore, the operation mode of the reconfigurable voltage controlled current-voltage source is selected by the proper combination of the switches states (unlocked/open or locked/closed). The actual topology allows the hot switching between the two operation modes.

In a preferred configuration of the device of the present invention, the switches are specifically arranged so that:

- the first switch connects an input of the first secondary amplifier with the output of the differential amplifier,

- the second switch connects an input of the first secondary amplifier with the single signal output, - the third switch connects an input of the second secondary amplifier with the single signal output, and

- the fourth switch connects an input of the second secondary amplifier with a voltage reference, such voltage reference preferably consisting of the ground.

Such configuration is that of Figure 1, in such case the secondary amplifiers being connected in a voltage follower configuration, thereby consisting of an enhanced version of that, more general, above described.

Thus, when working in controlled current source mode, the first switch and the third switch are locked/closed and the second switch and the fourth switch unlocked/open (Figure 2, in such case the secondary amplifiers being connected in a voltage follower configuration).

When in controlled voltage source mode the second switch and the fourth switch are locked/closed and the first switch and the third switch are unlocked/open (Figure 3, in such case the secondary amplifiers being connected in a voltage follower configuration).

In an enhanced and preferred embodiment of the device of the present invention, the secondary amplifiers are connected in a voltage follower configuration, the first secondary amplifier thereby providing a first voltage follower and the second secondary amplifier thereby providing a second voltage follower. Such configuration is presented in Figure 1. Preferably, for the voltage follower configuration and with regard to the above described arrangement of the switches:

- the first switch specifically connects the non-inverting input of the first voltage follower with the output of the differential amplifier, - the second switch specifically connects the non-inverting input of the first voltage follower with the single signal output,

- the third switch specifically connects the non-inverting input of the second voltage follower with the single signal output, and

- the fourth switch specifically connects the non-inverting input of the second voltage follower with a voltage reference, such voltage reference preferably consisting of the ground.

In an inventive aspect of the circuit of the present invention, the first, second, third and fourth resistors respect the following relation:

± ^r ₌ ^r

r ₃ r _x ^'

Such relation balances the transconductances relative to each of the inputs, providing the best possible performance.

It is also an object of the present invention a reconfigurable digital-to- analogue converter comprising digital-to-analogue conversion circuitry, an output block and a single analogue output, the output block comprising the voltage controlled current-voltage source device according to any of the described configurations and the single output consisting of the single signal output of such device. Such DAC provides output reconfigurable, providing a single output and a single output block/driver, unlike those known in the art. Preferably, it further comprises a control unit, such control unit being configured to control the locking/unlocking of any of the four switches. Such inventive aspect of the DAC of the present invention therefore provides for automatic reconfigurable of the DAC and, accordingly, of the device of the present invention.

It is yet an object of the present invention an apparatus for reconfigurable digital-to-analogue conversion comprising the digital-to-analogue converter according to any of the described configurations and a microcontroller (or microprocessor), such microcontroller being configured to instruct the control unit to control the locking/unlocking of any of the four switches. Such apparatus preferably consists of a building automation equipment, an environment monitoring system, an industry process controller, a field sensor, a medical equipment or a communication electronic system.

DESCRIPTION OF FIGURES

Figure 1 - representation of an embodiment of the circuit of the present invention. The circuit has two signal inputs vi and v ₂ , which can be used as a differential input, one single signal output (through the resistance n) and comprises i) the differential amplifier (Opi), ii) the two secondary amplifiers, in such case operational amplifiers each arranged in a voltage follower fashion (O _p2 and O _p3 ), iii) four switches (5 _wl , S _w2 , S _w3 and 5 _w4 ) and iv) five resistors (ri, r _2a , r ₃ , r ₄ , and r ₂ b). The load of the circuit is represented in Figure 1 by the load resistor n. The reconfigurable voltage controlled current-voltage source that is able to operate in two operation modes, one working as controlled current source and the other as a controlled voltage source.

Figure 2 - representation of the circuit of Figure 1 in controlled current source mode, in which the first and third switches 5 _wl and 5 _w3 are locked/closed and the second and fourth switches S _w2 and 5 _w4 are unlocked/open. Passage of current ii on a load n, representing the single signal output, is shown.

Figure 3 - representation of the circuit of Figure 1 in controlled voltage source mode, in which the first and third switches 5 _wl and 5 _w3 are unlocked/open and the second and fourth switches are locked/closed S _w2 and 5 _w4 .

DETAILED DESCRIPTION The more general and inventive aspects of the device, DAC and apparatus of the present invention are described in the Summary of the invention. Such configurations are detailed below in accordance with other advantageous and/or preferred embodiments of implementation of the present invention.

In several possible embodiments of the device of the present invention, the switches consist of mechanical switches, electrical controlled mechanical switches or solid state (semiconductor) switches. The switches can be developed in any technology that allows the implementation of switches or circuits that have similar function/operation of a switch. To achieve a reconfigurable voltage controlled current-voltage source device with an ideal performance the open resistance of each switch should tend to infinite and the closed resistance of each switch should tend to zero. In practice, the performance rises as the ratio between the open and closed resistances of the switch increases.

In additional possible embodiments of the device of the present invention, the differential amplifier and the secondary amplifiers consist of operational amplifiers or transconductance amplifiers. These amplifiers can be developed/implemented in any technology and take any form and topology.

In an embodiment of the device of the present invention, each of the secondary amplifiers provides a unitary gain.

In a further embodiment of the device of the present invention, the two signal inputs consist of voltage signal inputs and, optionally, are arranged forming a balanced differential input. Alternatively, the two signal inputs consist of current signal inputs, r ₄ = r _2a and r ₃ = r _x = 0, thereby providing an optimal performance with a current signal input. In yet another embodiment of the device of the present invention, it further comprises a control unit, such unit being configured to control the locking/unlocking of any of the four switches, preferably the control unit being specifically configured to alter the state of the four switches so that the first and third switches have a same state, which is opposite to that of the second and fourth switches, which thereby also have a same state. The device is therefore autonomous as regards its reconfiguration abilities.

In an embodiment of the converter of the present invention comprising a control unit, such control unit is specifically configured to alter the state of the four switches so that the first and third switches have a same state, which is opposite to that of the second and fourth switches, which thereby also have a same state. Preferably, the control unit is specifically configured to:

- lock the first switch and the third switch and unlock the second switch and the fourth switch, thereby configuring the single output as a current output, and - lock the second switch and the fourth switch and unlock the first switch and the third switch, thereby configuring the single output as a voltage output.

An embodiment of the device of the present invention is subsequently described.

The circuit topology of the present invention is independent of the technology used to develop and implement the reconfigurable voltage controlled current-voltage source device (and all its internal blocks/circuits) and, for that reason, can be implemented with discrete and / or integrated blocks / components available in any technology. The circuit topology, which is based on the enhanced Howland current source and on the voltage differential amplifier, allows the converter to operate as a controlled voltage source or as a controlled current source with a configurable functionality. The developed converter has two inputs, which can be used as differential input, and a bidirectional output, i.e., it has the ability to source or sink a current from the load. All these features provide a distinct versatility when compared to others (some available in the market), due to the many applications that can take advantage using it. For instance, the presented circuit can constitute the microcontrollers DAC output driver. In this application, where the pin count is important, the output type could be chosen or altered by software. I n the test and measurement area, the presented circuit can be part of controllable voltage / current sources to characterize the performance of discrete and integrated circuits. Nevertheless, there are many other possibilities that one can figure out. Figure 1 presents the circuit of the present invention. As can be seen, the circuit has two signal inputs vi and v ₂ , which can be used as a differential input, one single signal output and comprises i) a differential amplifier (O _pl ), ii) the two secondary amplifiers, in such case operational amplifiers each arranged in a voltage follower fashion (O _p2 and O _p3 ), iii) four switches (5 _wl , S _w2 , S _w3 and 5 _w4 ) and iv) five resistors (ri, r _2a , r ₃ , r ₄ , and r ₂ b). The load of the circuit is represented in Figure 1 by the load resistor n. The reconfigurable voltage controlled current-voltage source is able to operate in two operation modes, one working as controlled current source and the other as a controlled voltage source. Besides that, the actual topology allows the hot switching between the two operation modes. The operation mode is selected by the proper combination of the states of the four switches of the circuit represented in Figure 1. When working in current source mode, the first and third switches 5 _wl and 5 _w3 are closed and the second and fourth switches S _w2 and 5 _w4 open. On the other hand, when the reconfigurable voltage controlled current-voltage source is operating in voltage source mode, the second and fourth switches S _w2 and 5 _w4 are closed and the first and third switches S _w ^ and 5 _w3 are open. All the switches can be implemented using discrete and / or integrated technologies and can be mechanical switches, electrical controlled mechanical switches or solid state (semiconductor) switches or any other form. To achieve a reconfigurable voltage controlled current-voltage source device with an ideal performance the open resistance of each switch should tend to infinite and the closed resistance of each switch should tend to zero. In practice, the performance rises as the ratio between the open and closed resistances of the switch increases.

The circuit has three amplifiers (e.g. OPAMPs): the differential amplifier O _pl (herein referred as the main amplifier) and the first O _p2 and second O _p3 secondary amplifiers, operational amplifiers arranged in a voltage follower configuration. The amplifiers can be implemented in any available technology, be discrete or integrated and have any topology. The open loop gain, gain-bandwidth product, stability (gain and phase margin), slew-rate, distortion levels (total harmonic / intermodulation distortion), input referred voltage / current noise, input (common and differential) and output impedance, input offset voltage, offset drift, input common-mode voltage range, common-mode rejection ratio, output voltage / current swing and power supply rejection ration of the amplifiers are examples of important parameters that should be considered when implementing a reconfigurable voltage controlled current-voltage source device.

The functionality of the main amplifier, O _pl , is similar to the one of the enhanced Howland circuit or to the one of the voltage differential amplifier, depending on the selected operation mode. Thus, O _pl is of paramount importance to the performance of the circuit of the present invention. It is this amplifier that feeds the load of the reconfigurable voltage controlled current-voltage source. Thus, the current driving characteristics of the reconfigurable voltage controlled current-voltage source device is limited by the main amplifier O _pl . The amplifiers O _p2 and O _p3 , can be connected in any topology as long as the gain of these amplifiers is equal to a predetermined value. In Figure 1, each secondary amplifier is implemented with one OPAMP configured / connected as a voltage follower or voltage buffer. Thus, both feedback amplifiers of the Figure 1 have a unity gain. The O _p2 and O _p3 amplifiers are used to improve the isolation between the output and the inputs, to facilitate the resistance matching between the two feedback paths (positive and negative feedback paths) and to mitigate the effects of the resistances of the switches on the circuit performance. Depending on the type of switches used in the reconfigurable voltage controlled current-voltage source, the input impedances of O _p2 and O _p3 can have a nefarious impact on the performance of the circuit of the present invention if their input impedance values are lower than a given minimum value.

The circuit of the present invention uses five resistors (ri, r _2a , r ₃ , r ₄ , and r ₂ b). These resistors can be of any type and implemented with any available technology. The resistor characteristics, like its resistance accuracy, resistance frequency dependency (associated with parasitic elements), resistance nonlinearity, resistance temperature coefficient and power rating, are important to the design of the circuit of the present invention since the attainable performance is also dependent on the resistors. The resistors of the circuit of the present invention can take any resistance value. To achieve a reconfigurable controlled current-voltage source device with an ideal performance, the resistors ri, r _2a , r ₃ and r ₄ should verify the following equation

The resistor r ₂ is also important to the performance of the circuit of the present invention since the circuit transconductance is dependent of r ₂ b.

Yet another embodiment of the device of the present invention is subsequently described. Figure 1 shows the two main operation modes of the circuit of the present invention, one working as current source and the other as voltage source. The operating mode is select by the proper combination of the four switches. When working in current source mode, the switches 5 _wl and 5 _w3 are locked and S _w2 and 5 _w4 unlocked, when in voltage source mode the S _w2 and 5 _w4 are locked and 5 _wl and 5 _w3 are unlocked. The circuit has three OAMPs and the O _pl can be considered as the main OPAM. Its functionality is similar to as in Howland based circuit or as in the voltage to voltage converter, depending of the operation mode is selected. The O _p3 is used to improve the resistance ratio between the two branches of the circuit (one providing positive feedback and other providing negative feedback), buffering the voltage at the positive branch and consequently isolating the current. The O _p2 cuts-off the current that flows in the negative branch making possible the merging of the tree circuits. The two secondary amplifiers O _p2 and O _p3 configured as voltage buffers are used to mitigate the parasitic resistances caused by the switches presence in the circuit. This circuit has two OAMPs working exclusively as voltage buffers and one implementing the circuit operation selected by switches and at same time feeding the load.

Another embodiment of the device of the present invention is subsequently described.

As referred, the circuit provides two distinct operation modes. These operation modes are achieved by using the appropriated topology that is achieved by the proper conjugation of four switches. This circuit has two main functionalities with same output, more, it can perform a warm reconfiguration without a physical change of output or input connectors. Therefore, it can be used in two operation modes:

• Voltage controlled current source;

· Voltage controlled voltage source.

This reconfigurable capacity of the proposed final circuit it is one of its best characteristics and is not present in prior art. Each of the chosen operation mode was based on specific parameters such as linearity, output resistance and the possibility of controlling the output from an input voltage (controlled source) in conjugation with a differential input. The presented proposal circuit gathers these ideal characteristics in order to provide directly the supply to a DUT or to be the output block of a DAC.

In Figure 2 is shown the current mode operation mode and in Figure 3 the voltage source operation mode. For simplicity, the switch control circuit is not shown.

As will be clear to one skilled in the art, the present invention should not be limited to the embodiments described herein, and a number of changes are possible which remain within the scope of the present invention.

The preferred embodiments shown above are combinable, in the different possible forms, being herein avoided the repetition all such combinations.